Choosing Your First Telescope for Astronomy: Complete Essentials.

 You can get an instrument capable of giving years of good astronomical views for

around a week's wages... There are big pitfalls with certain telescope designs. I will tell you how to avoid these... Please read on!

"There is a very good reason that so many disgruntled people are selling their tiny telescopes ...

                                          ...It's because tiny telescopes are no good for astronomy!"


I don't like the term "beginners' telescope", it implies that those who are new to astronomy don't get to use a 'proper telescope'. 

However, the use of a telescope is something that is learned, and to that end, a lighter, easier telescope to manage,

and one that will show everything but not be too pricey, could be described as a 'beginners' telescope, I suppose. 


So, what would be a great telescope to start you off into this wonderful hobby? Read on!



Sooner or later, if you're interested in astronomy, you'll decide you want a telescope... If you buy yourself the wrong one, you may end up giving up on astronomy because your expectations have been let down. My advice will save you throwing away money and will get you a telescope that you will be able to keep using for many years without great expense.


Any telescope is better than no telescope at all, but very small telescopes have very limited applications in astronomy and will soon have you wanting to see more. 


Allow me to guide you towards your first 'scope with an explanation of the considerations you need to make while choosing.  

With qualifications in astronomy, having been a director of an astronomical company, and with over forty years experience in observational astronomy, I hope you will trust my informed guidance in this matter.

I recommend the beginner starts by using one of only TWO types of telescope and I'll give you a guide on the SIZE that you need to buy.

Size is the most important consideration: Especially for the beginner.

In this guide I have explained to the best of my ability with many pictorial examples why this is the case.


 In this guide: 20+ Photographs / 30+ Illustrations / 2 Videos






The two types of telescope I am going to recommend are: 'Newtonian Reflector' and 'Astronomical Refractor'. I will generally refer to them as Newtonian and Refractor, to save confusion between reflector and refractor.


These two (left) are impressive looking instruments, with their equatorial mountings and slow motion controls, but they are too small to be of any real use in amateur astronomy.  You will need something slightly larger to see what you will read about and want to see. 


Although, technically, the 120mm Newtonian should be at least equivalent to an 80mm refractor (See 'Resolution' section later) this model has a spherically figured mirror (see 'Parabolisation' section later) and would not produce images as good as an 80mm achromatic refractor. The smallest Newtonian I recommend is a parabolic 130mm  (Search "130P" or see Newtonian Reflector section below).

The reason I recommend only the refractor and the Newtonian telescope types is that these are the two best suited to the beginner. This is the case  because they are the simplest and therefore have the largest aperture for your money available - So you can get into the right SIZE of telescope with the minimum outlay.  Please read on to get information on the size of telescope you need to see the things you want to see.


There is also an information section covering other types of telescope that you may have come across, for those that are interested.  However, the beginner is recommended to get either a refractor or a Newtonian, dependendent on your interest.


Broadly speaking:



Recommended Sizes:




Refractors & Maksutov telescopes:


Smaller than 60mm = useless!


60mm = Absolute minimum size: Will show some detail on the planets.


70mm = Better than starter size for good views: Starts to show more detail on the planets.


80mm = Good size for detailed views of the planets: Starts to show detail in the detail!


90mm = Excellent detail level on the planets and good views of all objects.


100mm and larger = Superb planetary detail and excellent views of all objects.




Recommended Sizes:

It is with the Newtonian reflector that we have the most chance of buying something truly awful. Please heed my advice:





If Newtonian reflecting telescopes do not have a parabolic main mirror any size is virtually useless, giving blurry smudged images!  I've heard experienced amateur astronomers defending their f10 spherical telescopes, but don't get one!  Buy yourself something that works well, without compromise!


DO NOT BUY spherical mirror Newtonians.


Smaller than 100mm = not enough light grasp or resolution (Very unlikely to be parabolic) = useless!


100mm Parabolic = Absolute minimum size: Will show some detail on the planets.


114mm Parabolic = Good size for reasonable views of the planets.


130mm Parabolic = Good views of detail on the planets: Minimum size for serious astronomy.


150mm Parabolic = Excellent detail and light gathering power for all objects.


200mm and larger Parabolic = Superb planetary detail and excellent views of all objects.


DO NOT BUY Newtonians with built in barlow lenses (Catadioptric / Bird-Jones designs) They will be awful.



General Thoughts:

If you're interested in general planetary views, the moon or double-stars, then the minimum size for a great experience would be the long focus 70mm refractor or larger (A 90mm is not much more money and gives much better image quality and detail!)


If you're really interested in general views and would like to look at everything, (comets, nebulae, galaxies,

globular clusters,variable stars, star clouds and planets.) then the 130mm+ Parabolic Newtonian would be the minimum size you would want, and the 150mm Parabolic Newtonian is not much more expensive for better views of everything!


If you were only interested in Deep Sky, even a 'Dobsonian' would fill the requirement.

But, you still need one with a parabolic primary mirror of at least 130mm diameter.


You can see a good number of astronomical objects with smaller instruments - Any optical aid is better than none - But, if you are setting yourself up for a good look around the sky, and to see if amateur astronomy is for you, why start off on the minimum?  Good telescopes are available at very reasonable prices and you'll get a lot more satisfaction out of your instrument and move further on in the hobby before you have to 'upgrade' if you get a decent instrument to begin with!


Please read on to discover which type and size you need.  I have called this 'Complete Essentials' because you really NEED to be aware of all these things when considering which telescope is the right one for you.



As you learn more about the things you're looking at you'll get an idea of the direction you would like to go and decide what your favourite types of astronomical object are.  You may already know!  General viewing / Planets / Deep Sky / Comets...


Different areas of astronomy require different equipment. You'll read about needing a big Newtonian for Deep Sky objects, and the planets are best seen through a large long focal ratio refractor or Maksutov. These are the kinds of considerations you will learn to apply to your future optical equipment planning.  IF you already know (I did at 13 years old!) you can get the right telescope NOW!

Also, as you read more, talk to people and learn about astronomy, you'll come across other types of telescope.  Some of these are very well suited to specialisms.  In particular and worth a mention here is the Maksutov catadioptric telescope.  Absolute wonder on the planets!  However, I would still not recommend a beginner to start here - Get some experience with the simpler telescope types before overcomplicating it for yourself!


For now, take it from an expert, that you will do best to start out with a refractor or a parabolic Newtonian...


Let me give you some guidance on these two types of telescope and their friends and allow me to tell you a little about why these two types are the most suitable for you to begin your astronomy journey.


Here we go...



The refractor is the telescope with the big lens at the front and you look in the small end!

You'll need one with a front lens (Object Glass) of AT LEAST 2.75" or 70mm to see anything useful!

(If you can get an 80 or 90mm... All the better, for not much more money!)


An equatorial mounting with slow motions is ideal - A manual Alt-Azimuth mounting is very difficult to use on astronomical objects.


Refractors are particularly good if you're interested in the planets, double-stars or the moon (Though they can be used for any subject with experience and varying results). They show great detail for their size and when used on the relatively bright planets, the objects are easy to locate, even given the small field of view.


 If you're buying one of these make sure it's an 'astronomical refractor'



Above: My Phenix 127mm (5") f10 Astronomical Refractor - A very nice bit of kit that cost me £90 second hand. (Bargains are out there once you know what you're looking for).  This telescope was just about the best refractor I've ever used. Beautiful images of stars and great detail on the planets too.



The newcomer to telescope use will find the small field of view and high focal ratio of a refractor make it difficult to find 'Deep Sky Objects'. A beginner who wants to spend a lot of time looking at Galaxies and Gas Clouds would be best to get a Newtonian Reflector (See below).


Refractors are robust and can stand a good deal of handling without going out of alignment. They do need longer than Newtonian reflectors to cool down to ambient temperature for the best image quality (Because they are closed tube optical systems). The refractor is the telescope type that transmits the most light for its size and reveals the best detail. All other types have some sort of obstruction in the light path and this deteriorates from the quality of the image.


All Refractors suffer slightly from chromatic abberation or 'false colour' but on most objects this is minimal (Even achromatic telescopes give excellent images when fitted with qualtiy optics).  Some refractors have object lenses made from very expensive 'Extra-Low Dispersion' (ED) glass. These produce much better images, but be prepared, they are nearly five times as much to buy new!  (See Chromatic Abberation section later.) ED lenses give a better view, but is the view five times better?  I'm not convinced.


Although I recommend 70mm as the smallest size refractor telescope - There are some very nice 80mm and 90mm telescopes that are not that much more expensive and are so much better! - These represent an excellent, rather than a 'bargain basement', start to astronomy.


A SkyWatcher 90mm (3.5") f10 achromatic, equatorially mounted refractor:  New price in UK is about £225 (Oct 2022)

If the 90mm is still too expensive and you really can't manage to afford one - A Bresser 70mm SkyLux or LYRA achromatic, equatorially mounted refractor might have to suffice (if you really can't manage the extra to get a really great telescope!) as an 'entry level telescope'. New price in UK is about £165 (April 2020).


My advice, though: Please SAVE UP for the 90mm! If you have the will to wait, you can get a superb telescope.


You wouldn't buy a pair of shoes that are the wrong size - Why do it with your first telescope?



















The Refractor uses a large lens, the object glass, to collect and focus the light to a point. The image at this point is examined by the use of an eyepiece.  Different eyepieces magnify the image differing amounts.


The parts of a refractor are there to hold the object glass and the eyepiece in precisely the correct relative positions no matter where you point the telescope.  It can be seen that the light rays pass uninterrupted down the tube, passing through the holes in the baffles (often called 'stops') and arriving at a focus near the eyepiece.  This is a closed system with nothing in the way of the light to detract from the image.


The Dew Cap stops moisture forming on the cold glass.

The Cell, holds the lens and accurately directs the light path down the centre of the tube.

The light-tight tube holds everything in place.

The baffles help to stop internal reflections. (Not all refractors have more than one).

The focuser allows precise movement of the draw tube in and out of the main tube.

The Draw Tube holds the eyepiece.

The eyepiece magnifies the image formed by the Object Glass.




This is the simplest type of reflector. It uses a mirror to focus the light instead of a lens and the observer looks into the side of the tube instead of up it!

Technically, a 100mm Newtonian could be approximately equal to an 80mm refractor - But it would have to be parabolic - Many smaller telescopes (Up to 150mm) are NOT parabolic and this detracts from the qualtiy of the image, giving poor results. To have the best start in amateur astronomy you'll need one with a parabolic mirror AT LEAST 5" or 130mm in diameter. A telescope with a 6" (150mm) mirror would be all the better!  I will assume you want more than the bare minimum and will continue this guide refering to the 6" 150mm Newtonian as your ideal choice.


150mm Newtonians are great all-rounders.  The short focal length ones (f4 = 600mm focal length to f6 900mm focal length) are especially good for 'deep sky' (Galaxies, Nebulae, Star Clusters). Focal ratios of f6 to f8 are good for planetary views too! Newtonians over f8 (1200mm/150mm) are very cumbersome. Newtonian reflectors are not recommended for study of the Sun.


An equatorial mounting with slow motions is ideal - A manual Alt-Azimuth mounting is very difficult to use on astronomical objects. Though a Dobson mounted Newtonian is good for Deep Sky.  If you don't want to be bothered with learning your way about the sky, you could consider a Go-To telescope - But you have to be reasonably technically minded. Personally, I would recommend the equatorial mount


Above: My 150P on EQ3-2. A great all-rounder telescope. Good for Deep Sky objects but also reveals good detail on the planets.
A very nice bit of kit that cost me £100 (Bargains are out there once you know what you're looking for).



Mirrors can go out of alignment with rough handling - Nevertheless, in my experience it takes quite a big jolt to upset the optical settings - Bearing this in mind it's advised that you be careful when setting up or carrying!



Newtonian (And ALL other reflectors) have obstructions in their light-paths and so suffer from deterioration of the image. In the case of Newtonians, they also have open tubes and suffer at times from tube currents. However, they are great value for money and give some excellent views on the best nights.


I recommend at least a 130mm parabolic telescope - These represent an excellent, rather than bargain basement, start to astronomy.



A 150mm (6") f5 parabolic, equatorially mounted Newtonian Reflector New price in UK is about £430 (Oct 2022)

If the 6" is too expensive and you really can't manage to afford one - the 130mm (5.1") parabolic, equatorially mounted Newtonian may have to suffice! 


A SkyWatcher 130P = New price in UK is around £260 (Oct 2022).


If you can't afford that - Please SAVE UP! Don't buy anything smaller than a 130mm parabolic Newtonian!


You wouldn't buy a pair of shoes that are the wrong size - Why do it with your first telescope?



The Newtonian uses a mirror to focus the light by reflection. (Ideally this mirror should have a Parabolic figure). The light would pass out of the tube but for the secondary flat mirror that directs the light out through a hole in the tube to an eyepiece.  Different eyepieces magnify the image different amounts.





The parts of a Newtonian Reflector are there to hold the Primary  and Secondary Mirrors and the eyepiece in precisely the correct relative positions no matter where you point the 'scope.


It can be seen that the light rays are interrupted as they pass up and down the tube, passing through the vanes of the spider and being reflected a second time by the secondary mirror (Usually between 15% and 25% of the primary mirror diameter depending on focal ratio), finally arriving at a focus near the eyepiece.  This is an open optical system with air currents and obstructions along the way of the light path which detracts from the quality of the image.  Because of this interruption in the light path, to get the same detail in a reflecting telescope you need a slightly bigger size to counteract the effects of the interrupted light path.


The telescope Tube holds everything in place.  This needn't be a solid tube, it can be a lattice or framework, as long as the optical components are correctly positioned relative to each other.  The only thing to watch out for is stray light getting into the eyepiece, to avoid this on skeleton and lattice tube Newotnians, there is usually a box or tube section where the focuser is fitted.

The Cell holds the parabolic Primary Mirror and has adjustments that direct the light precisely down the center of the tube toward the secondary mirror.

The Secondary Mirror is perfectly flat and directs the light at 90 degrees out of the tube to the correct point near the eyepiece.

The Spider holds the secondary mirror mount, which itself can be precisely adjusted to bring the light to the correct focus point.

The focuser allows precise movement of the Draw Tube.

The draw tube holds the Eyepiece in the correct position to view the image.


You can imagine, therefore, that the Newtonian is a slightly more delicate instrument than the bomb-proof refractor and should be handled more carefully and the alignment of the mirrors checked periodically.  Aligning the mirrors correctly is called 'collimation', and makes sure the column of light passes down the very center of the optics.




DO NOT invest in a "Catadipotric-Newtonian" of "Bird/Jones" design. 




The optical configuration is very poor and you will be lucky to get good views at magnifications over 50x. 





The idea is that you can avoid spherical abberations (non-focusing of some of the light-rays) by inserting a 'corrector lens' into the system. But, this 'corrector lens' doesn't do the job, and it also makes collimating a nightmare too!


My serious, though admittedly simple and honest advice is:



As a rule of thumb:

Always, I repeat: ALWAYS make sure your prospective Newtonian telescope has a parabolic mirror!


There are some companies producing f5 and faster optical systems with spherical mirrors - these give an attrocious image quality - Check out the spec of your prospective telescope before wasting your money!



You may be interested in other types of telescope. I will indulge your interest... a little.

The Dobsonian Mounted Telescope

(For your information)


This type of telescope uses precisely the Newtonian optical layout.


This type of telescope comes on an Alt-Az mounting which does not follow the stars. It is easy to use with low to medium magnifications, however, tracking an object at high magnifications is difficult for the beginner.  I would only recommend a Dobson mounted telescope for a beginner who was despirate for light-grasp and Deep Sky Object observation as the 'Dobsonian' can be cheaper than smaller equatorially mounted scopes.


Picture: John Dobson with a 10" Dobsonian Telescope.


The Dobson mounted telescope is intuitive and it's easy to line the telescope up on an object. Following it can be simple enough at low to medium powers.  The telescope comes off the mounting easily for transportation and storage. Setting up doesn't take long and the experience can be less challenging.


Recommended for the beginner if your interest is soley in Deep Sky. The easy use mounting and easy-peasy set-up is intuitive and natural.  At low to medium magnifications these telescopes give great images and can be recommended for Deep Sky fanatic beginners.





This type is a catadioptric system: A catadioptric optical system is one in which refraction and reflection are both employed in an optical system via lenses (dioptrics) and mirrors (catoptrics).


This type of telescope has spherically figured optics and consists of a spherically figured meniscus lens at the front, the spherically figured main mirror and a secondary mirror spot (aluminised onto the inner surface of the meniscus lens).  These telescopes are sealed tube instruments, typically f11 to f15 in focal ratio. Consequently, they have quite narrow fields of view, ideal for planetary, lunar and double star observations. Unfortunately, because of the high focal ratio, Maksutovs don't do very well with Deep Sky Objects.


I don't recommend them to beginners only because of the relative expense. If space is a premium, or weight is a consideration, please see my MAK page on the website for full appraisal of this capable telescope type.  They are excellent telescopes and can be treated as very similar in performance to a refractor of the same diameter, however, because they have a folded light-path, physically, they are very much shorter than an f15 refractor of the same diameter.


The f12 SkyMax127 in this picture has a focal length of 1,500mm in a tube less than 400mm long.  It gives planetary images equivalent to a 150mm Newtonian on the best nights, it will rival a 130mm Newtonian on any night. 


Maksutovs, like refractors, take longer to cool down to become optically excellent, because they are sealed optical systems.


The Maksutov-Cassegrain is a sealed unit and consequently the only parts, like the refractor, that we come into contact with are external.


The light passes first through a spherically figured Meniscus Lens (Thicker at the edge) and falls on the spherically figured Primary Mirror. This reflects the light back up the tube toward an aluminised spot on the back of the Meniscus Lens which reflects the light back down the tube to the Eyepiece.




Focusing in the Maksutov is done by moving the primary mirror. The focusing knob is found on the back plate and not the draw tube in this design.


NOTE: The light rays are slightly modified by the meniscus, converging by a few degrees. Consequently, the primary mirror doesn't need to be 'full diameter' and is usually about 94% of the meniscus diameter.  The light gathered therefore is the full meniscus diameter and not merely the primary diameter.  EG: All the light that falls on the meniscus of a SkyWatcher SkyMak127 is reflected from the 119mm primary to the secondary and out of the draw-tube apperture  to the eyepiece. So, the light collecting diameter is 127mm, not just the 119mm of the primary mirror as some people mistakenly believe.


The telescope Tube holds everything in place.  This must be a solid light-tight tube, it can't be a lattice or framework.

The mirror support holds the spherical Primary Mirror and has adjustments that direct the light precisely down the center of the tube toward the secondary mirror (This is set at the factory).

The Secondary Mirror  or 'Spot' is a small aluminised circular zone (usually between 16% and 20% of the diameter) on the back of the meniscus lens and directs the light back down the tube to the correct point near the eyepiece. The light ray angle is much reduced so optically increasing the focal ratio.

The tubular Baffles stop stray light from the meniscus lens, other than that from the 'spot' entering the eyepiece.

The Draw Tube holds the Eyepiece in the correct position to view the image.

The Focuser knob on the back-plate allows precise movement of the Primary Mirror to affect focusing.


Maksutov-Cassegrain: Not recommended for the beginner on price! - If you're not sure about astronomy, spend little first! (See above types)

Pricewatch: New equatorially mounted SkyWatcher Skymax 127, Maksutov-Cassegrain f12 = £530 (Oct 2022)



(CLICK HERE! There is another page all about the Maksutov type, if you require a little more information.)

SCHMIDT-CASSEGRAIN (For your information)


This type is a catadioptric system: A catadioptric optical system is one in which refraction and reflection are both employed in an optical system, via lenses (dioptrics) and mirrors (catoptrics).


These telescopes are externally similar looking to Maksutovs. They too have a corrector plate but have a different optical configuration. Optically they have some inherant field curvature problems and are even more expensive.


This type of telescope has spherically figured optics and consists of a corrector plate at the front, the main mirror and a secondary mirror.  They are sealed tube telescopes, typically f10 to f12 in focal ratio and have quite narrow fields of view, ideal for planetary observations. Consequently, Schmidt-Cassegrains don't do very well with Deep Sky objects.


I avoid recommending them to beginners because of the relative expense.  They are good telescopes and can be treated as very similar in performance to a Maksutov of the same diameter, however, because they have field curvature problems they don't quite stack up to the refractor of the same size.


The Celestron C5 Schmidt-Cassegrain f10 127mm in this picture (Image Courtesy: 'Celestron') has a focal length of 1,250mm in a tube less than 350mm long.  It gives planetary images equivalent to a 130mm Newtonian on the best nights, it will equal a 120mm Newtonian on any night.  Schmidt-Cassegrains, like refractors and Maksutovs, take longer to cool down to become optically excellent, because they are sealed optical systems.


The Schmidt-Cassegrain is a sealed unit telescope and consequently the only parts, like the refractor, that we come into contact with are external.


The light passes first through a Corrector Plate and falls on the spherically figured Primary Mirror. This reflects the light back up the tube toward a Secondary Mirror housed in its own cell set into the corrector plate. The Schmidt-Cassegrain has the largest obstruction to the lightpath of all reflecting types.  The Secondary reflects the light back down the tube to the Eyepiece.






Focusing in the Schmidt-Cassegrain is done by moving the primary mirror. The focusing knob is found on the back plate and not the draw tube in this design.


The telescope Tube holds everything in place.  This must be a solid light-tight tube, it can't be a lattice or framework.

The Baffle is tubular and stops stray light from the corrector plate being seen by the eyepiece. Only light from the secondary can enter the Draw Tube.

The mirror support holds the spherical Primary Mirror and has adjustments that direct the light precisely down the center of the telescope Tube toward the secondary mirror (This is set at the factory).

The Secondary Mirror, supported in its cell (Usually between 27% and 35% of the diameter) directs the light back down the Tube to the correct point near the Eyepiece. The light ray angle is much reduced so optically increasing the focal ratio.

Focusing is achieved by a knob on the back-plate that moves the Primary Mirror inside the telescope.


Schmidt-Cassegrain: Not recommended for the beginner on price! - If you're not sure about astronomy, spend little first! (See above types)


Pricewatch: New equatorially mounted Celestron VX5, 127mm Schmidt-Cassegrain f12 = £1,344 (April 2020)



So now we move onto the detail behind my size and type recommendations.

Buckle up - There's a lot to consider!


Why Does Aperture Matter So Much?


Resolution in the image and 'light gathering power' is dependent on the diameter of the mirror or lens. If you can't resolve the detail then NO AMOUNT of magnification will show it!  You can only see stars of a given minimum brightness with a telescope of a given aperture. Bigger telescope = More detail and dimmer star limit. Some telescope companies state things like, 'Collects 250 times as much light as the human eye,' in their adverts. This sounds like a gimmick, but actually it's a very good point. Light gathering power is dependent on aperture minus any obstruction.

If you like to think in terms of multiples of the human eye that's fine. (which is pretty standard at 7mm diameter = 38.5mm squared, fully dilated).

Just be aware that the eye is a funny thing and twice the light doesn't look twice as bright!


A Quick Word About Star Magnitudes:

The brightest stars are known as 'first magnitude'.  The unaided eye can see stars as faint as magnitude 6, without optical aid.  The magnitude scale is logarithmic. Each successive magnitude is dimmer by a factor of 2.512 times. 

Mag 0.0 (Vega) is about 2.5 times as bright as Betelgeuse (Mag 1.0) which in turn is 2.5 times as bright as Dubhe (2.0) Dubhe is, therefore, only about 1/16 th as bright as Vega even though it's a magnitude 2 star and Vega is magnitude 0.  Which means that a 6th magnitude star is about 100th of the brightness of a 1st magnitude star...

The bigger the diameter of your objective (lens or mirror) the more light (fainter stars)

and more detail will be available for observation.


The eye has a pretty standard performance...


Eyeball Performance:  Resolution 'R' = 3.0'  Magnitude limit = 6.0

This is enhanced by the use of optical instrumentation.


ROUGHLY SPEAKING:  Telescopes perform on stars in the following manner:-

Diameter of Primary      Resolution   Magnitude Limit

  30mm   1" telescope:  R = 4.5"       M=9.0

  50mm   2" telescope:  R = 2.3"       M=10.5

  80mm   3" telescope:  R = 1.5"       M=11.4

100mm   4" telescope:  R = 1.1"       M=12.0

130mm   5" telescope:  R = 0.9"       M=12.4

150mm   6" telescope:  R = 0.7"       M=12.9

200mm   8" telescope:  R = 0.6"       M=13.5

250mm 10" telescope:  R = 0.5"       M=13.9

300mm 12" telescope:  R = 0.4"       M=14.4


NOTE: The magnitude limit for nebulae / Galaxies is about two magnitudes less.


Star limit = Mag. 10.0

DSO limit = Mag.  8.0


You NEED the BIGGEST objective you can manage (either financially or physically - telescopes get heavy!) to allow detail to be shown and faint stars and galaxies to be glimpsed, no matter the magnification used!


Here's an analogy:

If you 'zoom' in to a high resolution picture on your computer you can find more detail than if you view the picture small on screen.

But if you print the small picture and use a photocopier to enlarge it there is no more detail to be had.

This is analogous to a larger telescope where higher magnifications can be used as they bring out details that cannot be seen at lower magnifications. (The eye can only see detail that has been presented to it above a certain size hence the need to magnify at all!) The tiny telescope presents all its information at a smaller size - Further magnification is wasted, it just enlarges the low-resolution image without bringing out any new detail.

If you use more magnification than the telescope can show, this is called 'empty magnification'. See magnification section later for more.



Go Compare: Comparing the refractor views close up! (View in 50mm on the left, 90mm on the right in this comparison). (Actually Left side Saturn image is a little bit too detailed!)


Left of image: Approx view of Saturn with 50mm (2.0") refractor or a 80mm (3.1") Newtonian.

Right of image: Approx view of Saturn with 90mm (3.5") refractor or a 130mm (5.1") Newtonian. (I'm happy with this image representing a really still night!)






As far as star images go:


There are two main considerations: Splitting binaries and the visible star magnitude limit. A larger telescope will always show closer pairs as individual stars and show fainter stars than a smaller telescope will. (Close pairs of stars cannot be split with tiny telescopes or higher magnifications)



Left: See the Difference:


On the upper frame is the Pleiades Cluster as seen with a 50mm telescope.

On the lower part there is another 20x view of the Pleiades (M45) as seen with a 6" 150mm telescope.



Note: not only the many more stars in the larger telescope, but also notice the splitting of the double star, circled, just below centre.  Not just two stars but four are visible in the larger telescope.

Resolution:  The triple star shown near the centre bright star (Alcyone) resolves itself through use of larger telescopes - Not higher magnifications.


< Have a Look!:

This video demonstrates the effect of widening the aperture from 50mm to 150mm and back.


Deep Sky Objects


Left: Orion Nebula: The nebula M42 in Orion. Once again the central star is not split with a tiny telescope but shows itself to be a 'Trapezium' of four stars in the bigger 'scope.



Also note the amount of nebulosity that can be glimpsed in the bigger telescope.  Both these scopes are f6 optical systems.



The reason bigger telescopes have better definition and resolution of detail is that the bigger the telescope primary, the smaller the point image it can produce. So, small telescopes produce big 'points' of light (Low Res. Images) but big telescopes produce smaller 'points' of light (High Res. Images).

So it follows that if the two stars are closer than the minimum 'point' that your telescope can produce, you will only see an elongated blob where the two circles overlap, instead of two distinct points.  As you use larger and larger telescopes the star 'points' become smaller until you can see the two distinct stars clearly.


See diagram, Left.  The double-star shown is just a connected blob, at any magnification, for any telescope under 120mm (Parabolic Newt.) or 90mm (Refractor) - Where they are just about separate points.

Also: Note that as the telescope gets bigger it collects more light and the stars appear brighter in the eyepiece.
  More light concentrated into a smaller point!



The reason that refractors show more detail than the equivalent reflector is that there are no obstructions in their closed system of optics. The Newtonian has a spider to hold the secondary mirror, the secondary mirror itself and tube currents that degrade the image. The mirrors transmit only about 80% of the light that falls on them and there are two in most telescope designs except the refractor which transmits about 98%! Although a Newtonian's optics may be 'diffraction limited' (See later section) the optical system certainly is not!  A refractor on the other hand can be pretty much perfect for its aperture. 


For simple reckoning of equivalent sizes, multiply the refractor object glass diameter by 1.3 and you will get a good approximation of the equivalent Newtonian primary mirror diameter.  Or divide the Newtonian by 1.3 to get the equivalent refractor size.  Be aware it is only an approximation - Different manufacturers and different observing conditions can alter the perameters somewhat - But as a general rule it's accurate enough.


Proof of the pudding: I have owned a 130mm f5 Newtonian and a 100mm f9 refractor and a 127mm f11.8 Maksutov-Cassegrain all at the same time.  I can tell you that in extensive test comparisons, all three are equal in resolution and light gathering performance.   This is a debate that has raged since the seventeenth century.


However, the swings and roundabouts of astronomical isnstrumentation come out to play:


The Newtonian 130mm f5 is better at Deep Sky than the 100mm f9 Refractor or the 127mm f11.8 Mak. 

But, the closed system of the refractor and Maksutov perform to their best on more nights than the open tube Newtonian! 

The 100mm ED refractor gives a generally better image quality than the 127Mak, because the Mak has a secondary mirror obstruction and the meniscus lens adds a bit of chromatic abberation.

But the Mak has slightly better detail when the atmosphere is perfect!

All three have the same light gathering because although the 130 and 127 have a larger overall light gathering area (about 1.8x the 100mm refractor's) the reflections (2x 20% light loss) and obstruction of light from the secondary (8% light loss) leaves aproximately the same amount of light (59% of 1.8) as the 100mm refractor with its 98% transmission.


I could not choose a favourite between the three!  In many aspects of astronomical instrumentation it's 'swings and roundabouts'.  The decision of "Newtonian or Refractor?" is something that you will spend hours thinking about. There's no easy winner!


Planetary Images.


You get to choose which view you would prefer by picking a telescope of the size I advocate in this guide!

Authentic View Left: Mars at a favourable opposition (Close to Earth). You'll need to look closely - But observe the differences in detail between the three telescopes. Once again you can choose which view you would like by buying the size that gives the view you want!


A reflector transmits only about 85% of the light that enters the system, a refractor something like 98%.


Observe the detail and the brightness differences in the pictures above - These views are representative of the real difference between the different telescope sizes as they appear to the eye when observing on a still night.



Chromatic Abberation 'False Colour'

When light passes through a lens the light is split into its component colours somewhat.


Depending on the type of telescope you choose or the lens it has, you will get more or less chromatic abberation.  This 'false colour' shows itself around bright objects as red and blue fringes like a rainbow effect seen in very cheap, kid's toy binoculars.


Generally speaking, the better the telescope the less chromatic abberation will affect your view.


Newtonian reflecting telescopes do not suffer from chromatic abberation as the light does not pass through the glass of the mirror but is reflected off. All light is reflected the same.


LEFT: You will see that the different lens types have different levels of success at combatting chromatic abberation. These lenses have a medium f8 focal ratio... When selecting a refractor it is important to know what type of object glass you are going to get and therefore you can be prepared for the level of chromatic abberation to expect.  Newtonian buyers, you can ignore this worry (You have enough on your plates!)


These lenses have a medium f8 focal ratio... The focal ratio has a significant effect on chromatic abberation.  An f6 refractor will have significantly more chromatic abberation than an f15 refractor with the same lens type.







ABOVE: In this diagram the percentage figures are not strictly accurately represented, but it does give a good idea of the relative differences between the different lens types in medium focal ratios.

The longer the focal length of the lens, the less the effects of chromatic abberation are seen.  The reason for this is that if the light is refracted less, then the difference between the red and blue focus is also less.  The shorter the focal ratio, the bigger the difference.  Refractors in the early days of telescopic observation grew to huge lengths to try to get rid of the 'false colour'.  These cumbersome instruments were known as aerial telescopes and some were as much as 150 feet in length!


Luckily, a British inventor called Chester Moore Hall created the achromatic lens in the 1730s, using a combination of a positive focus Crown Glass lens and a negative focus Flint Glass lens, which have different dispersion properties, to bring the red and blue light to a much closer focus.

 In modern times, manufacturers have used exotic types of glass to lessen these effects yet further (But at a price! These exotic glasses aren't cheap.)  I am lucky enough to own an ED telescope and I must say that the colour is very good.



Above: Pedro Ré, and his "150 foot" Aerial Telescope.  Johannes Hevelius had a similar instrument in Danzig, Poland.




Example1: Chromatic Abberation is very apparent in this poor quality medium focus achromatic f7 60mm Astral refractor. (Observe the red and blue fringes at either side of Jupiter's disc).










Example2: Chromatic Abberation almost nonexistent in this excellent long focus achromatic f15 60mm Prinz 550 refractor. (Twice the focal length, twice the focal ratio)


< You can click this illustration to enlarge if it helps see the chromatic abberation effect.


The brightness of the image in a telescope and the field of view (The size of the circle when you look into the eyepiece) is determined by the diameter of the telescope objective (The main lens or mirror) divided by the focal length. It is also determined by magnification (Higher magnifications narrow the field of view). If the f number is low (4 - 6) the field will be wide and if high (8 - 15) the field of view will be narrower for any given magnification.

Here's another analogy!
Think of looking through a toilet roll tube. They're fairly short in comparison to their width (about f6). Now look through a kitchen foil tube (about f10). The field of view is much smaller in the long tube. This is exactly how a long focal length gives a restricted view and can be used to demonstrate well the f number effect on field of view.


Eg: 900mm focal length divided by 90mm objective lens gives f10 (900/90=10) (Which is fine for a refractor but a bit unwieldy for a Newtonian.)


It is usual for Newtonians to be shorter focal length than the equivalent refractor - So f4 - f8 is usual in Newtonians, whereas, f6 - f15 is more usual in refractors. A refractor with a shorter focal ratio (f number) than f5 starts to show some focussing problems and the false colour around bright objects becomes much more pronounced (Red and Blue fringes) so try to avoid refractors shorter than f5. (A focal ratio of around f10 is a very common size for good astronomical refractors but as a guide f5 should be the minimum you look for. Parabolic Newtonians of f4 – f8 are common and give good images.)  There are many excellent f5 achromatic refractors out there, just avoid the bargain basement models!

Visually, the stars are not affected by f number but extended objects, like galaxies and nebulae, seem brighter in low f number systems. If you're interested in 'deep sky objects' then a reflector with a focal ratio of between f4 and f6, or a refractor with a ratio of f5 - f7 would be your best choice.



This applies ONLY to Newtonian Telescopes. It does not apply to Refractors, Maksutov-Cassegrain, or Schmidt-Cassegrain telescopes.

Small Newtonian reflectors often have spherical figured mirrors. The mirror's shape is incredibly accurate and MUST be parabolic to bring all the rays of light to the same focus.


Manufacturers of small telescopes don't bother with this final 'figuring' of the mirror. It's time consuming and needs special testing of the optics.


Picture: See the sharp stars on the Parabolic image. See the fuzzy stars on the spherical image.



You might think, "Ah, but I can still see the stars!"  Yes, you can. But if they are fuzzy, what about detail on the planets or the Moon? Smudging of a tiny point is not as noticeable as smudging on a surface. You will also lose maximum magnitude because the fainter stars will just be spread out smudges instead of a detectable faint point.  So your parabolic Newtonian might reveal magnitude 13 stars but the spherical one of the same size, only magnitude 11.


Another annalogy Regarding Resolution!  If you had a piece of paper with a smudged point on it, you'd know where it was. It would be obvious and the smudging wouldn't matter too much.  What if it was a page of very small writing? What if that were smudged?  Similar to looking for detail on a planet. Smudging could leave you seeing just a mess!



So: Why aren't all Newtonians parabolised?  The manufacturers leave out the parabolising process because it's an expense they can avoid - Their telescopes, however, are sub-standard as a result and give awful image quality. They try to get around this fault by supplying long focal lengths (f9 - f12) because the difference between the spherical and parabolic mirror is smaller (But still significant!).


If you want to get the best out of your telescope, and use the optimum magnification and have the best images you need to have a telescope with a parabolic mirror. That usually means at least a 130mm Newtonian.




 Ray Diagram: The parabolic mirror gives much sharper point images and the detail on the planets will also be smudged by a spherically figured mirror. The spherical mirror produces out of focus blurry stars and smudged planetary images.

The parabolic mirror brings all the rays of light to a focus at A.  Whereas, the spherical mirror brings light from the centre of the mirror to a focus at A, whilst it brings the light from the edge to a focus at B.  Providing some focussed light and some slightly out of focus light at all points between A and B.  There is no clear focus with a spherical mirror.


Yet another reason to get a larger Newtonian than these toys of 100 - 115 - 120mm diameter!




NOTE: This is not different colours being reflected to different points like chromatic aberration - It is all light - The colours in the diagram are to show different light rays only.  Mirrors reflect all colours the same (unlike lenses which refract different colours different amounts).



"Diffraction Limited Optics" – A good thing or a Bad?


You may wonder why your telescope has 'Diffraction Limited Optics'.


It sounds as though that's a bad thing, doesn't it?


Why should you accept 'limits'?


Well, the truth is that all optical systems have a theoretical limit to the detail thay can show. A small telescope can only possibly show so much detail - A larger one will show more detail.  But a badly made lens or mirror, of any size, won't show all the detail a lens or mirror of its size should.


So, the limit for seeing fine detail in an image is dependent on the accuracy of the telescope mirror or lens.  A lens or mirror that is perfect, optically, is said to be 'Diffraction Limited'. Because the only thing that is limiting its abiltiy to show detail is optical physics. That is, it's as good as it can be, given the size of the optics.


So: Diffraction limited optics are the best.



Always look for "diffraction limited optics" when buying your telescope.  If it doesn't say in the blurb or specification page - ASK!


In a Newtonian Reflector, for example, the mirror MUST be parabolic to achieve diffraction limited images.  A spherically figured mirror is always out of focus at some point!


The Dawes Limit


This is the formula for finding the resolution limit of your telescope (If it has diffraction limited optics).

It is also known as The Rayleigh Limit.


R = 4.56/D D in inches, R in arc-seconds


R = 11.6/D D in centimeters, R in arc-seconds


where :


D is the diameter of the main lens (aperture)


R = Resolution: The resolving power of the instrument


You can use this information to work out if you can see the moons of Jupiter as discs, or whether you should be able to split the double double in Lyra or not, for example - A very useful formula.


Modern manufacturing techniques mean that it is easier than ever for manufacturers to produce very good optics. A company like SkyWatcher, for example, produce excellent value, diffraction limited refractors, Maksutov-Cassegrain and Parabolic Newtonian telescopes. They include this information on all their specifications. If you're looking at a telescope and it doesn't say 'Diffraction Limited', you can bet that it isn't and you'll have inferior views.


The biggest obstacle to your viewing clarity, however, is the atmosphere. Whilst your telescope may be diffraction limited, it will also, always be 'atmospheric seeing limited' too. There are bad nights, good nights and very good nights. It's all part of the game!


An oft neglected part of telescope use: Focusing.
I get letters from beginners who are having a bit of trouble finding or seeing details on objects in their telescopes. Rarely it is because they are trying to use the Barlow without an eyepiece, or no eyepiece is in the drawtube, or they have the telescope on the mount upside down and they're looking at the ground!  But, much more often, it's because they haven't focussed properly.
The first thing to note, when focussing is that the image has to be as small as possible!  This sounds crazy, right?  But if the image is out of focus it becomes larger.  If it's really out of focus, it can begin to look like the picture at the top!   The image on the left is in focus, the image on the right is way off!  You can, in extreme cases, even begin to see bits of the telescope - Now, this is clearly wrong! 
As you focus, by turning the focussing knobs on the telescope, the image becomes clearer and smaller at the same time. If it becomes larger and more fuzzy, turn the knobs the other way. When focussing the correct direction, details begin to appear to your eye and at some critical point they begin to unfocus again.  At that point you stop turning the knob and very slowly turn it the other way until the image becomes sharp and clear again.
I was at an astro-talk by an eminent astrophotographer who said that he attributes the excellence of his photographs to the amount of time he takes to get the focus perfect.  He told us that he regularly spends an hour on focussing (That got a gasp from a room full of astronomers, I can tell you!) Now, I'm not going to suggest that you take that long, but you do have to spend enough time to make sure you have the best focus you can achieve. On a night when the seeing is troubled and unsteady it can be quite a task!  You need to be ready for the steady moment, when the detail really comes out...
Those of you who have a telescope already:
So, let's do a bit of focussing practice, eh?  (This can be done in the daytime).
Try putting a reasonably long focal length eyepiece, something like a 20mm eyepiece (Eyepiece sizes are written on the side or the top). A 20mm eyepiece gives a nice wide angle view and will be best for first steps in focussing practice.  Carefully aim at something in the distance (at least 500m away), a tree or lamppost or something with an obvious shape.  The image should be clear and sharp with no blurring or fuzziness. If it is not clear and sharp you need to turn the focusing knob so the eyepiece tube comes out / in until the image is sharp. 
When the image is really unfocussed (or there's no eyepiece in) you may even see the image of the 'spider' coming into focus in a Newtonian (see top pic).  When you're happy with the focussed image turn the knob a few times one way or another, then look through the scope and focus again. Do this until you're confident that you can focus on images in the telescope.  Change object if you wish and focus on that. 
Let's try a shorter focal length eyepiece (if you have one). Look through the telescope at the object in the distance and use the focusing knobs to bring the image into sharp focus as you have done before - In a shorter focal length eyepiece it's harder to decide when it's at best focus. This is why practice is important.  You will notice that when you change to a different focal length eyepiece the image may become a bit blurry, and you might need to refocus to get the best image. This is usual.  When you are looking at the planets and stars you will still need to refocus to get the best image.
You can also take the opportunity, while the object in your telescope is staying put and not sailing out of the view as astronomical objects do,  to check the alignment of the finder.  Look through the finder and make sure that what you can see exactly in the centre of the finder is what you see in the centre of the telescope when you look through the eyepiece.  If not you'll have to make adjustments - They usually have screws for adjusting the direction.
I have a guide about setting up your finder, here:> FINDER TYPE GUIDE
There is a separate guide about focusing including more information, here: > FOCUSSING GUIDE
Once you're reasonably happy that you can focus correctly on images in the scope, try the moon, using the same technique - You'll be amazed. 
Another point about focusing is that if your telescope is a spherically figured Newtonian it will not put all the light into one point and focusing will be a lot more hit and miss - There will be no single best point of focus. All the more reason to get a parabolic Newtonian. 
Refractors are slightly different, even long focus single lens refractors have a single point of focus that the eye will use, in the Yellow/Green part of the spectrum. You will therefore see more or less of the red and blue colour fringing, 'Chromatic Abberation', depending on the type of telescope object glass and the focal ratio.  Generally though it is easier to achieve a good focus in a refractor.
Top Tip: When focusing on the planets or the moon, it's best to use a nearby star, or the planet's moons to get the best focus, then, switch back to the planet or the Moon.
BTW: Both spellings of focussing and focusing are correct - I try to use justone s!



Something I must tell you: I was asked this question by someone who had read this guide.

My answer is simple: There are no such things as "novice needs!”

There are objects that are visible in 'small' telescopes and these same things are beyond a 'tiny' telescope - I'm here, writing this guide, telling you that those things that aren't visible in a 'tiny telescope' make up 95%+ of what you will read about and want to see.


We buy ourselves some optical equipment because we have read that we can see Saturn's rings, but once we've seen that things like that can be seen we want to see more detail, the 'Cassini Division', or the polar caps on Mars, or the belts and bands and the 'Great Red Spot' of Jupiter's atmosphere, or the Ring Nebula, M57, in Lyra - To view these wonderful things requires at least a 70mm refractor or a 130mm reflector! Buy anything smaller and you will not be able to see these wonderful details.

You may think, "I'll just be happy to see Saturn's rings, Jupiter's moons, and the disc of Mars". But I'm telling you, once you've seen them you'll immediately want more detail! So why start off on a minimum? Why not give yourself a good start and at least have a detail level that allows you to see even more with experience?

Do yourself a favour, read and digest this guide and buy at least a 70mm refractor or a 130mm Newtonian and set off on your stargazing journey on the right foot.


See for Yourself:

Left Pic. a 76mm Newtonian at 200x Magnification does not reveal detail.


Right Pic. a 150mm Newtonian at 100x Magnification but with a larger aperture does,



You can see more detail at lower magnifications with larger telescopes!



The benefit of an equatorial mounting is that you only need to move the telescope about the polar axis to follow a star as it moves across the night sky. The polar axis points to the pole star, approximately, and therefore the polar axis needs to be set to the observer's latitude.

It's easy to find your latitude using Google! For the English Midlands, for instance, that is about 52.5 degrees. Once an object has been centred in the view, lock off the declination axis (The one with the counterweight) and as the telescope is moved (With motor or slow-motion knobs) about the polar axis the planet or star will stay in the field of view without further adjustment. An Alt-Az stand, such as is found on the Dobsonian Telescope needs constant adjustment of both axes at the same time and can be frustrating to the beginner.


Above:This diagram looks a bit confusing I know.
This is an attempt to demonstrate the difference between the movement of the stars and an Alt-Az telescope. You can see that the two move in different ways. Necessitating constant adjustment of the mounting in two directions (up and right).
An equatorial mounting automatically follows the white lines.


LATER: For those who need to know just how easy it is to set up an EQ equatorial mounting, please see my other guide "How do I set up an Equatorial Telescope Mount?"


They can be a bit of a mystery for the uninitiated, but basically you point the polar axis at Polaris then move the other two axes to find and follow your object.  (The other guide explains in detail)





Should You consider a Go-To mounting?

As with many aspects of amateur astronomy, there are swings and roundabouts in the choice of mounting.  You need to take some time to consider which type of mounting will help you achieve your goal.


If you want to learn your way around the sky and be able to use any telescope or binoculars to locate objects at any time, you might opt to learn 'the manual way', with an equatorial mounting that you have to operate yourself and point in the right direction. Use your skill to find convenient guide stars and 'star hop' your way to the object you want to observe. There is a lot of fun in this and it is very rewarding.


Alternately, you may want to be sure that you're looking at the right object and forego the learning of the stellar signposts, in favour of a robot telescope that will point to the object you want to see off a list. For you, the 'GoTo' mount is a good option.  Beware, though, you will take much longer to learn where astronomical things are! 

Go-To Mountings have motors and computers that find objects for you from a list. Some of them have a hand-set attached to the mounting to select your objects, while others use an app on a smartphone to control the telescope.

Granted, they take some of the fun and all the skill out of amateur astronomy and will not allow you to advance much beyond 'telescope user'.  However, you will have seen lots of things within the range of your telescope without the palava of tracking them down (sometimes frustration builds beyond belief trying to locate supposedly easy objects - In my case Messier objects M1, M97 and M78...  I've still not seen any of those after all these years, and it's not for the want of looking, believe me!)  Astreoids are another set of objects within the range of small telescopes which are feindishly difficult to observe with certainty without a GoTo...

There is a Go-To version of the Equatorial, which is useful once you have learnt your way about the sky.
The Alt-Az Go-To is also not suitable for long exposure astro-photography as the field rotates as you track an object smudging the detail.

However, short exposures of ten seconds or so can be stacked to produce excellent pictures with experience.


As you have seen. There are a lot of "howevers" and "alternativelys" in this section.  You have to decide which way is best for you.




You may notice the obvious difference between the two telescopes I'm advocating. Their size!


The Newtonian is one and a half times the diameter of the refractor (3 x the light gathering area!). This means that the 130mm Newtonian transmits about two and a half times the light of the 90mm refractor. (It's only two and a half times, not 3x, because of light losses due to reflections and the shadow of the secondary, cell and spider).


You may wonder what effect this will have on your observing. The difference in light gathering area means that you will gain just about one magnitude (ie. You can see about twice as many stars) with the Newtonian. Remember the logarithmic magnitude scale, each magnitude is 2.512 times fainter than the previous one?  If you had a 130mm Newtonian you would get just over one magnitude fainter stars than a 90mm refractor.


However, the surprising thing is that the refractor will under most conditions show just as much, or even more detail as the bigger Newtonian! This is because the obstructions in the light path and the open tube arrangement of the Newtonian means that the resolution suffers somewhat. The refractor, whilst not gathering as much light, will show planetary detail at least as well as the bigger reflector and sometimes better!  With planetary observations high light gathering is secondary to detail in the image and a refractor is the instrument of choice (Or a Maksutov which, you will remember, operates very much like a refractor).


Consequently, I can recommend a good 90mm refractor <(Click this link if you like), and tell you that, on the best nights, it will be just as good at showing detail, as a 130mm Newtonian <(Click this link if you like).

If you're really interested in spotting very faint objects, (comets, nebulae, galaxies, globular clusters, variable stars and star clouds) then the
Newtonian would be the one for you. If you're interested in planetary detail, the moon or double-stars, then the refractor would be a better bet.


However, if you're interested in having a good stooge around and having a look at everything within range then either type would be fine! (Just buy one big enough to show you the detail you want to see).



Table: A rough guide to the differences - By no means exhaustive, but I think there are several things there that show the essential differences between the different sizes.


The objects seen in the larger telescope are visible because of the larger light gathering capability and greater resolution of the larger optics.


For the sake of getting a telescope of entry level (130mm) rather that a tiny 76mm telescope, you can see all these extra things!


A similar set of differences are seen between a 50mm refractor and an 80mm refractor - The smaller isn't much better than a toy, whereas, the larger will satisfy for many years!




This is a basic overview: For full information, including: Barlow lenses, Exit Pupils, and other considerations, please access my Complete Magnification Guide HERE.


Magnification is not the be-all-and-end-all of telescopes! You can see a surprising amount with relatively modest magnifications - If you were to pin me down and say I could only have ONE eyepiece (therefore one magnification) as a planetary observer I would choose one which gave about 150x magnification. Not 200x or 300x or even the fantastic 525x magnification - Just 150x*.

*If I was only interested in galaxies, nebulae and comets, I would reduce this to 80x because then I would need a wider field of view for Deep Sky Objects :o)

Useful magnifications vary with the size of the telescope.


The larger the telescope, the higher the maximum magnification, but this is always dependent on the 'seeing' or atmospheric turbulence.

Examples: You would be forgiven for thinking that really big telescopes use magnifications of 500x and maybe 1000x, but the truth is...

l have used the 9" (225mm) Refractor at the Godlee Observatory, Manchester University. We had excellent views of Jupiter at 175x magnification.

The best view I ever had of the globular cluster M13 was in a 22" (550mm) amateur owned Newtonian, at just 200x magnification.

I used the 30" (750mm) Newtonian at the Amateur Astronomy Centre Nr. Bacup. And, once again, we had some lovely views of Jupiter, at 200x magnification. (See Pic. below.)


< Have a look at this telescope 


Even 'huge' telescopes utilise 'reasonable and usable' magnifications.














Above: The 30" telescope at the AAC was used to view Jupiter at 200x.

(That's me when I wasn't grey! Approx 1985)


Why do we need different magnifications?


The objects we look at are of different sizes and sometimes we want to see the whole, and sometimes the detail. Sometimes, you just think that a smaller or larger view would help see the detail of what you want to view.  Also there are exit pupil considerations - See magnification guide.


This means that we need to be able to 'close in' on something. For instance, if we want to look at the whole of the star cluster "The Pleiades" (Seven Sisters) a 6"/150mm f6 reflector would need a magnification of only 35x to fit them all in to the view - However, if you wanted to show the double star Alcor in Ursa Major (The Great Bear) then you would need a magnification of around 100x - 120x to show the double to best advantage.


Likewise, if you're interested in planetary detail, you'll want magnifications from 80x to the maximum your telescope and the atmosphere will allow. If you want to observe the moons of Jupiter and Saturn, you will only need to use 60x - 100x.


The main consideration is the atmospheric turbulence. If the atmosphere is unsettled, then you will need a lower magnification to get the best view. The higher magnifications can only be used effectively when the atmosphere is calm and 'quiet'.  It's not all about big magnifications, it's about getting the best view on that particular night.




How do we work out magnification?

Each eyepiece (The small lens you look through) has it's own focal length (Usually printed on the side or top) If you know the focal length of the telescope object glass or mirror, then all you need to do to find the magnification is divide the focal length of the telescope by the focal length of the eyepiece.






Eg1. 900mm / eyepiece 15mm = 60x magnification.

Eg2. Focal length 1250mm / Eyepiece 8mm = 156x magnification.


It can be seen then that the same eyepiece in different telescopes will give different magnifications!


Eg3: a Refractor, focal length 1200mm with eyepiece 10mm = 120x magnification. The same eyepiece in a 150mm Newtonian with a focal length of 750mm = 75x magnification.


Magnification is a big selling point for telescope sellers so you have to accept that they will throw in an eyepiece that is virtually useless just to give that 525x magnification claim. As long as they give other eyepieces that cover the 50 - 100 range and the 100 - 150 range you'll be OK.

But do you know what? I have noticed that once you get away from the ridiculously small telescopes manufacturers tend to give more reasonable magnifications! Quite often 76mm Newtonians come with 525x (useless), but a 150mm Newtonian will more likely have 150x as it's 'best' magnification (One that could
actually be used on most viewing nights).


 If you don't know your primary focal length you can measure the outside of your telescope to approximate (to about 1cm) the focal length.


Remove the eyepiece, wind the focuser right in, measure as shown in the diagram.
Once you know this length in millimetres, you can find magnifications easily.
Don't forget you can always buy more eyepieces to get the usable magnifications you require.



So... Following my advice further it stands to reason that to utilise the higher magnifications to the full larger telescopes are essential! A six inch 150mm Newtonian will stand 300x magnification on a really good night, but an eight inch 200mm would be able to deploy 400x on very still nights.



Logically, then: If you buy smaller than my recommended size you won't see the detail that's available. You can really only expect to use magnifications up to 50x per inch of aperture for Newtonians, and 100x per inch for refractors.

THE MAXIMUM USEABLE MAGNIFICATION on a very good seeing night is:

About 2x per millimetre diameter for any telescope type. 

(It's not a hard and fast rule, just a guideline. If the image quality starts to drop off, reduce magnification!)

I would recommend you read the complete Magnification Guide.


What About Light Pollution?

If you live in a large town or city, please don't think that you have 'no chance'.  There are many astronomy projects that are well within the scope of your instrument even in a city center.


The Moon and the planets are bright enough to be viewed from a very light poluted area.  These days street lighting is nowhere near as bad as it was last century. When you hear about the woes of light pollution it generally relates to 'deep sky observation' and is stressed by purists banging on about 'the ideal'. 


Observing in the city is rewarding and interesting with the right subject.  There may be a need to observe from a park or piece of common land, rather than your local built up area, depending on your horizon. But this is well worth it to get you into this brilliant hobby.



Country dwelling observers have their own problems, believe me!  For a while I lived in a house with huge trees growing all around and this limited my view to 45 degrees up all round and the altitude of the trees was worse still in the south (where we like to observe the planets) limiting me to objects over 70 degrees altitude!


I grew up in the seaside town of Blackpool, with its closely positioned rows of terraced houses and back alleyways, much like the picture above.  The view of the sky from my back yard was: North above 25 degrees, East above 60 degrees, South above 25 degrees and West above 20 degrees.  Far from ideal.  However, this is where I learned my way about the sky and made many many observations.   With my first telescope and subsequent instruments in this well illuminated town I observed the Moon, the planets, double stars, comets and many Messier objects.  Although it was far from ideal it was a fantastic time of my life learning what I could see.  I never once took my scope out into the countryside!  If your circumstances aren't brilliant, I urge you to give it a go anyway - You'll be surprised at what you can see.


The atmosphere is a major player in the 'What's the highest magnification I can use?' debate.  The rule is to use the highest magnification that still shows a steady clear image.
Sometimes the atmosphere is really steady and you can use the highest magnification your telescope will allow.  But, more often, there will be warm and cold currents churning up the view.  In that case you have to moderate your magnification so you can see the best view you can get.  This usually means that you have to use a maximum magnification of about 150x on a 150mm Newtonian telescope, one that boasts a maximum of 300x.

When I went to live in Lanzarote, I was looking forward to 300+ clear nights a year, but what I didn't realise was that the atmosphere would boil from sunset until four a.m.  The views before midnight were nearly always rubbish!  The best viewing was before dawn once the ground had cooled down.

In temperate latitudes we have it a bit better.  The atmosphere in the UK is usually good after the Sun's been down an hour, getting very good after midnight any time of year.  Once the atmosphere is calm you can regularly use half your telescope's maximum magnification.  On really clear and still nights you can observe at the max.
A slight mist can sometimes be a good thing - It can act like a filter when looking at the bright planets or the Moon and actually enhance the image! Clouds can be observed round by looking through the gaps.  But rain is a definite no-no.




Should You "Go Out Into the Country?"

Generally speaking, light pollution is not about having a streetlight close to your house. It is concerned with the whole area you observe from.  I get letters from people all the time telling me they're going to take their scope out into the country, or up a hill*, to get better views.  The fact is that unless you live in a major city, you won't get much better views at all.  If you observe from a town of less than about 15,000 inhabitants, going to the 'country' won't make any difference - You're already in the country!  Cities and large towns have a bit of a light problem as far as DSOs go, but there is no problem with the Moon or planets from city centres at all.


* Going up a hill will not improve your view. Your altitude makes no difference below about 14,000 feet.  The highest mountain in the UK is only 4,400 feet.  So, please, forget about going up a hill - It won't help.


My advice about your observing location is this: Make sure you're in a dark environment. If you're in the garden, make sure there are no house lights on. Make sure you have a reasonably clear view south.  If these requirements are not possible at your home then maybe a local park will get you into a better location. But there is certainly no need to go out of town into 'the countryside' seeking better viewing.


Never underestimate the proximity of a toilet and a kettle when you are enjoying looking at the stars!




The maximum magnification, dependent on atmospheric condition, is:

50x per inch of aperture = 2x per mm. (Half this for Newtonians)
ollowing this statement above it can be seen that the maximum magnification you should use on a 60mm refractor on a normal seeing night would be 120x and on a parabolic 114mm Newtonian, just 114X

We have already seen that the planets are best seen at around 140x up to 250x , on a still night.

This means we NEED a bigger telescope to handle these magnifications successfully.

ie: A Refractor of 70mm or Newtonian of 130mm - The magic sizes I recommend!
(You can therefore see: I haven't just plucked these sizes out of thin air :o)


You cannot change the laws of physics!


Obviously: Working at the instrument's limit isn't going to produce excellent images. So, I would recommend the larger 90mm refractor or the 150mm Newtonian. That way, you are well within its capabilities with your 150x magnification planetary views.


So, that's about it for the nitty gritty of what makes a telescope suitable or not. 


You can stop here if you wish, now you are armed with the information.


The following extra titbits can be considered 'further reading'.


I have included these interesting points for anyone who wants the fuller picture.  Please read on!

How Did I Learn the Inadequacies of a Tiny Telescope?

In 1977, my parents bought me a 60mm diameter refractor for Christmas - I thought I knew better than the books I'd read and I'd pestered for this particular telescope... It had an Alt-Azimuth mount, no finder and four set magnifications: 15x 30x 45x 60x... The Object Glass was so poor that there were no markings on Jupiter and although I could see the rings of saturn, I never saw Titan with this telescope (An easy object in almost any instrument over 25mm diameter) With that telescope I learnt, very quickly, that with a small, cheap telescope, you can see a few things, but not enough!


Left: The Astral Telescope: £55 A week's wages in 1977.

Since that enlightening Xmas, and through the last forty years of observational astronomy with some brilliant telescopes, I have been of the opinion that it is a good idea to start off in astronomy with a telescope that will show you more than the very basics. (Does it show?)

Yes, a tiny telescope will show you A FEW things, but why bother with this first step?


When I started off in this hobby you could only get these two types, Refractor & Newtonian Reflector (Nevertheless, they are still the best for starting out) - There were no Maksutovs, no Schmidt-Cassegrains, no Catadioptrics, and even a simple 6" Newtonian was £400 bought new! The only option was to buy a telescope we could afford - a 60mm Refractor on a useless AltAz tripod for £55.

Nowadays, there is a wealth of different kinds, and the prices have come down massively. So - why would you suffer the tiny telescopes when decent sized telescopes are available for you at very reasonable prices?

YOU CAN NOW GET YOURSELF A VERY GOOD TELESCOPE FOR A WEEKS WAGES OR LESS!  (A week's wages is now around £350  for most people)
You're looking for a Refractor 80mm and larger... Or a parabolic Newtonian 130mm or 150mm...




Please DO...



Look out for fantastic bargains on the second-hand market.


Second hand is as good as new with most astronomical instruments, as they very rarely sustain damage.


Grab a bargain...

Left: This 100mm SkyWatcher 100ED-PRO Refractor was found in a 'Cashconverters' store and they sold it to me for £89.99 - WOW!  (This telescope OTA is £720 new!)


The scope has ED optics (Almost no chromatic abberation), it has a 50mm finder (Brilliant) and comes with a very good quality 2" diagonal and focusser.  All-in-all, this is a superb telescope and I am very pleased - You could be the happy owner of such a scope for peanuts - Read my Second-Hand buying guide.



BUT, Please DO NOT...



Be tempted to buy any telescope SMALLER than the sizes I recommend in this guide or you will not see the detail that you deserve to see for the money.  If you have patience you can get the scope of your dreams for the same price as one of these binnable objects!  See my second hand telescopes guide!


Left: This tiny spherically figured telescope will not begin to satisfy even your most basic astronomical needs.



 PS: You wouldn't buy this one anyway, would you? The focal ratio is over f6.5... 







 I get quite a few messages asking what else you should buy to get started in astronomy when you get your new telescope.





A Good Star Atlas.



The most useful thing you can get is a good star atlas. I recommend two publications. Peter Lancaster Brown's excellent introduction to visual observing 'Star and Planet Spotting' (Available second hand on eBay from 60p) and for those observers who have some experience Wil Tirion's 'Sky-Atlas 2000.0' which shows stars down to magnitude 8 (A real must for those with telescopes above 130mm) Which you can pick up for rather a lot more (Mine was £30 when I got it in 1981).

Left: My two superb reference works.


Left: My SkyAtlas 2000.0 is an invaluable tool for finding anything you need to see.

A superbly detailed star atlas it shows stars down to magnitude 8.
It has 26 maps of the night sky with colour-coding.
The Orion area of the winter sky shown left. (Note: Orion Nebula Enlarged at left showing magnitude 10 stars).


Left: My copy of Star & Planet Spotting is invaluable as an observer's reference. It gives lots of useful information on the Constellations, Observing the Planets, Variables, Comets and Meteors. There are also basic and very usable star chart pages in the middle. They show stars down to magnitude 6.
The winter sky, including Orion, shown left.

I would recommend that every amateur astronomer should have a copy!



PC Based astronomy programs.


It is also very useful to use a computer based program such as 'Stellarium' (invaluable Free download). There are many different ones but for now why not try a free one?

This will, after a bit of playing about with the options, equate you with the way the sky moves. You can set specific times or view the sky as it presently is. Move forward or backwards, in time, at varying speeds. You can zoom in, use a telescope or have a whole sky view! You can get information on any object. You can look at how the planets move and work out where and when to see specific things. You can turn various useful things on and off: ecliptic, planet orbits, EQ grid, Alt-Az grid, labels, ground, atmosphere, daylight, clock, constellation lines and art!

< Laptop View: Screen-shot 'Stellarium'. Orion area again with Betelgeuse selected, view South with atmosphere, daylight and ground selected. Equatorial grid is on. Constellation lines are off.


It's easy to set your location so you can see the exact sky where you are, and easy to change it so you can see the sky for where you will be going on holiday for instance! Lets you see the Southern sky or work out which stars are visible from various latitudes. It's a lovely program visually, and is worth a look. It will never replace the star atlas for usefulness 'at the telescope' - But it is absolutely invaluable for seeing how things work and planning your observing sessions.


What Might I Need Equipment wise?:

It is always handy to have a pair of binoculars. They really are almost a must to accompany your telescope. Almost any size will do, except the tiny sports types. Binoculars are 'sized' by their magnification and their diameter. For example, a pair of 10 x 50 binoculars give a good view. They magnify 10x and have lenses 50mm across and are referred to as "Ten by Fifty" (not 'Ten times Fifty'). Personally, I have a pair of 8 x 40 binoculars and a pair of 10x50s too! You can never have too much optical equipment, ask Mrs SuperCooper.


Binoculars will help you find the exact position of the bright galaxies, check quickly on the position of Jupiter's moons (Oh, yes, don't let the claim that a telescope will show Jupiter's moons be a selling point – If it wasn't for the proximity of the very bright Jupiter you could see them without a telescope!)


With your binoculars you can see and the planets out as far as Neptune, and they give many wide views of the milky way. They are useful tracking down the Messier objects too. I would recommend you get a good pair of 8 x 40, 7 x 50, or 10 x 50 binoculars.



Above: Bino-Bargains: My excellent Swift 10x50s are superb and were got second hand for £19


My I.R.Vision 8 x 40 wide angle view binoculars are not ideal for astronomy but were better than nothing for a tenner whilst on holiday!

Try to keep things simple - No huge magnifications, No Zoom Eyepieces and make sure you check the size - Lots of misleading sizes advertised on the interweb!  200x150 being one set of 10x50's... I don't know what the seller thinks the figures mean!  Overall size in millimetres?  Anyone's guess!

Whilst the 60mm and 70mm and even 80mm binoculars are very nice for astronomy, you do need a tripod and mounting to hold them steady. The main object of using them in conjunction with your telescope is for conveniently looking at something before observing. It's not very convenient if you have to use a tripod! By all means get a large pair for use with a tripod if you like the views - But this would be an instrument in its own right and not really used for a quick check prior to observation with the telescope.


For more information on binoculars click the link below:

Jump to my Binoculars in Astronomy Page.



For your telescope:


Buy nothing extra for your telescope until you are familiar with using the telescope you bought. It's very easy to spend lots of money on things that you don't or won't need.  Take your time to get used to your instrument and your view through it.  After a while you will realise what you need.

The next thing you'll probably want is another eyepiece to extend the range, sometimes higher magnification and sometimes lower.

It can be beneficial to get a 2x Barlow lens. This device effectively (optically) doubles the focal length of your telescope and therefore all your eyepieces can be used as they are or in unison with the Barlow to give twice the magnification.  Sometimes useful but not always - Remember the magnification limit of your telescope and try not to exceed it. Another consideration, if you are looking for Deep Sky objects, the Barlow will also double the focal ratio - f5 (fine for Deep Sky) becomes f10 - Not too good for Deep Sky! 


There are many cheap plastic Barlow lenses available (£12-£19), but I would recommend spending about twice as much and getting a metal, achromatic Barlow (£22-£35) - The quality of the image is well worth the extra outlay!


Try not to duplicate magnifications (Waste of money) if you buy a 10mm it will give you a magnification. Adding your Barlow will give you another magnification twice that of the first.  If you then bought a 20mm eyepiece adding the Barlow would duplicate the magnification you get without it using the 10mm eyepiece.  Always take care you won't duplicate too much (Some duplication is almost inevitable).



Using a Barlow also means that if you have a refractor of f10 you will be using it as though it is an f20 with the resultant shrinking of the apparent field of view and 'photographic slowing of the system'... Care is needed and I wouldn't spend a lot on a Barlow unless you can try it first!

Another important part of the telescope is the 'Finder Scope'. This is the smaller telescope attached to the side of the main scope that allows you to locate things in the eyepiece. You can adjust the finder's cross-hairs to precisely line up with the centre of the field of view of the main scope, even at high magnifications. It can be tricky, but you can't really do any harm. Lining up the finder should be done with a distant earthbound object in the daytime so time can be taken to get it spot on. The more accurate the finder is set up the easier observations will be in the dark.




Some finders that are supplied with telescopes are woefully small for the telescope they serve. A finder of at least 25mm is essential and 30mm or 50mm is much more desirable for 150mm reflectors.


Eg: If you're hoping to line your telescope up on Neptune, you have to bear in mind that the planet is magnitude seven or eight and to find it easily in the finder, the finder has to gather a good deal of light too! (The 70mm Refractor I review later in this guide had a finder of just 15mm diameter - Couldn't see much of anything fainter than Jupiter!)


If you can replace this small finder with a big, 'straight through' finder (The right-angle finders are difficult to use for the beginner) your observations will become much easier.


The finder needs to gather light and show stars that cannot be seen by the unaided eye. Some scopes come with something called a red-dot finder. This should be replaced with a light gathering magnifying finder to enable you to see things that your eye can't. What use would a red-dot finder be in locating Neptune when it is Magnitude 8 and your eye can only possibly see to magnitude 6? You need a proper finder...



Left: Red Dot finder types in common supply.
These won't show you anything you can't see with your eye!







Left: A Proper Finder scope - 6 x 30mm

This will show objects down to magnitude 9. You can line up better because of the magnification and cross-hairs.





If you would like more information on this subject, I have a separate guide that deals with this subject in greater detail.





Moon Filters




Do not pay any extra money out for a 'Moon Filter' - No self respecting amateur astronomer would use such a gimmick

(You cannot harm your eye with the moon's light!).



REASON 1:  It can be bright but if you're looking at the Moon then there's no need for dark adaptation.


REASON 2: Astronomers spend most of their time trying to maximise the amount of light available to the eye - Why would you darken the image?


REASON 3: Each time light passes through a lens or filter there is some loss of light and there is some degradation of the image.



How many reasons do you need?  Please don't use a Moon filter... Leave it out and see more of what's there!


Travel Scopes:

You may find that your telescope is a bit big for just grabbing quickly to have a look at something, or it won't fit in your car with all the family when you're off to the seaside but you'd like a telescope in case you need to observe something. You could do worse than get yourself a 'Travel Scope'. This is an easily portable telescope designed for use on the go.

A travel scope should not be considered for your Main telescope, but will complement your Main telescope. There are plenty of short focus refractors in decent sizes that will still show you plenty when you're out an about. For instance I have an astronomy acquaintance who takes his in a back-pack when walking his dogs and sets up while they have a run about on the beach.

You can get a very good 70mm Travel Scope for around £50 new (Including the carry-bag).

The 70mm objective lens gives enough detail for casual observing and the f6 ratio gives a wide field and bright deep sky images too. A good all rounder of a decent size.


My own Travel Scope is a Helios 80mm f5 refractor on travel alt-az with slo-motion control, that I picked up for £22.00


You may have to buy a short focal length eyepiece (4mm) or 2x Barlow or even a 3x Barlow to get decent planetary image size (100x Magnification and above) as some come only with 20x and 40x magnifications which, while refreshingly conservative, are nowhere near enough to show detail on the planets.


70mm Celestron Travel Scope on extending tripod with carry-bag. Ideal for 'Astronomy On-The-Go' but should not be your main instrument, unless as a 'stepping stone'. These can be had for around £50 new! 

The travel scope should not be considered as a starter scope if you are interested only in the planets as the image is too small. These scopes are excellent for having a good look round the wonders of the universe and getting a feel for what can be seen with a 'proper' instrument (But will let you down on planetary views).

Resist buying the Newtonian layout reflecting types of travel scope at all costs! They are far too delicate for true travel scoping! Refractors and Maksutov optical layout scopes are much sturdier.

If you have limited funds and want to get into astronomy with a useful instrument, this is a great way. By getting started with a 65mm to 75mm Travel Scope you are able to spend very little and have a scope that will be of use, even if you do 'take the plunge' and buy a larger telescope in the future - It's always a good idea to have a small, easily used telescope handy!

This telescope is great for most aspects of astronomy 'on the go' and having a quick look around the night sky, but, once again, I wouldn't recommend it for your main telescope, although I must admit that it would have been much better than my first 60mm instrument! (You know why... Think about it... That's right, it's a BIGGER diameter lens!)


The most important thing you need... Patience!

Being able to 'see' with your telescope is something that is helped by the size of your telescope, but also needs a lot of experience. Start off with the magnifications I have suggested and keep observing. As you get used to seeing objects in a telescope you'll learn to really 'see' the detail that's there.


It might sound like I'm trying to put you off but actually I'm hoping that you'll realise that what I'm saying is, if you're initially a little disappointed with your telescopic view of the heavens, stick with it and you will gradually be able to see more and more.

Someone like me, who has spent years looking through telescopic equipment, can see a whole lot more than a raw beginner and somewhat more than someone with a year's experience. But this is a fantastic hobby and there wouldn't be much fun if everything was there on a plate for you straight off!


Most of the fun of this hobby is gradually learning more and more about the universe and learning to see it for yourself.




The Dobsonian Telescope: Facts!

I think a quick word about Dobsonian telescopes is in order. Having had John Dobson as a house guest when I lived in Horncastle, when he gave a lecture at the Horncastle Astronomy Weekend, I think I am able to provide you with informed information on this variation on the Newtonian telescope. The original design was for a large aperture telescope, using the Newtonian layout.  The mounting is a simple altitude and azimuth non-equatorial design for cheapness and simplicity (ie. up and down left and right).


The beauty of these telescopes was that the mirror could be made from much thinner, cheaper glass than the standard Newtonian because it was on an Alt-Az mounting and not an equatorial. The mirror was supported on bubble-wrap, cardboard float points and a 'sling' type support strap! The complicated mirror cell and thick glass mirror of the equatorially mounted Newtonian was avoided and much bigger telescopes became affordable to amateurs.


For the purists:


The term 'Dobsonian' has come to mean any telescope on the 'Dobson Style Mount'. This has become so prevalent that it is even in several astronomical dictionaries as the definition!


However, the true definition of a 'Dobsonian Telescope' is slightly different and is worth knowing.


Many manufacturers, and even experienced astronomers mistakently think that the term 'Dobsonian' refers to the type of mounting it is on. An Alt-Az of particular construction.


This is not the case.  Having spoken to John Dobson personally on this subject, these are the facts.


When John was creating the telescope design that became known as 'The Dobsonian' there were a number of considerations that influenced the design (ALL of which have to be present for a telescope to be a TRUE Dobsonian). 

John was not bothered about patenting his ideas, he just wanted to get as many people looking through big telescopes as possible - Consequntly, there is no rule about what you can call 'a Dobsonian'.


To that end:


1: He ground large mirrors (12" and larger) out of thin glass blanks, usually 25 - 35mm thick.  These were extremely thin by contemporary standards - A 12" 'ordinary Newtonian' would typically have a 65-75mm thick mirror and a 16" would have maybe 85-95mm thick glass.  John Dobson made telescopes of 20" and 24" aperture with 60mm thick glass. 


2: He avoided the complicated and expensive mirror cell by laying the mirror on a backboard with a simple plywood cell of three triangles and rubber points with a 'bib' over the top, sometimes with a layer of bubblewrap to support the thin mirror.  The weight of the mirror was taken by a strap underneath the circumference.  Because of this the telescope could ONLY be used on an Alt-Az mounting to keep the sling supporting the mirror.


3:This was OK in this design, because keeping costs low precluded the equatorial mounting.  An Alt-Az it was.  John made the mounting from plywood and a large bolt for the verticle axis and teflon pads to minimise friction.  The altitude bearings were made of tubing and rested on teflon pads in the mounting cutouts.


John was pleased to be able to make a workable 20" or 24" reflector for the same cost as a 6" commercially bought telescope.


There's a video by John himself: Here>

Watch video from about 55mins, for the mirror support part.



The above facts are largely unknown and figured only in interviews with Mr. Dobson in the astronomy magazines of the seventies and early eighties.  The majority of people, following the manufacturer's lead, believe that the type of mounting is all that denotes a Dobsonian Telescope.  It is a matter of fact, that this misconception will continue to prevail, and we will have to accept that these Newtonian telescopes of classic design, supplied on a 'Dobson Style Alt-Az mounting', will be referred to as Dobsonian Telescopes! (It's convenient and much less of a mouthful! :o)


Recommended for the beginner if your interest is soley in Deep Sky. The easy use mounting and easy-peasy set-up is intuitive and natural.  At low to medium magnifications these telescopes give great images and can be recommended for beginners.





Left: John Dobson at the HAW: A 14" Newtonian on the 'Dobsonian Mounting' can be seen behind them.


Although these telescopes are fantastic to use, in the hands of someone with little experience tracking the object is tricky and high magnifications are impractical. Consequently, planetary observation is difficult for the beginner. I would respectfully suggest that the Dobson / Dobsonian telescope should be avoided until you get some experience with the easier to use 'equatorial mounting'.


If you see a 'Dobsonian' telescope advertised make sure it's not just a Newtonian on a Dobson mounting, or worse, they're calling it a Dobsonian because it's on an Alt-Az mounting of simple design! ... To be a true Dobsonian telescope it has to have the thin main mirror supported on the sling type support and it should be BIG!

Call me pedantic, but they're selling you an Alt-Az Newtonian and telling you it's something special... NO.

The whole point John was trying to get across wasn't 'ease of use', if that was the point they would be mounted on equatorials! They were designed to be telescopes of huge diameter built at home on a shoestring! Mass producing Dobsonians seems to be missing the point entirely.

Anything that isn't huge*, and relatively cheap to boot, isn't really a Dobsonian at all, but a Newtonian 'dressing down' to snare the unwary!

Don't pay megabux for a Dob! (*By huge I mean 12" 300mm and over.)




What about Cat/Newt and Mak/Newt Reflectors?

There are a number of companies now selling so called 'Catadioptric Newtonians'. I would suggest that these are to be avoided.  The cheaper ones use a spherical mirror and a negative lens to add focal langth.
They have optically extended focal lengths - So you'll find a short tube but with a focal length of typically 1400mm. (The telescope will look on the outside like a typical Newtonian f5 telescope, but may state f10). The view in these 'Bird - Jones'  telescopes is narrow and unless the mirror is very good the image quality will suffer.  


PIC: Maksutov/Newtonian telescopes, on the other hand, are very nice instruments, though costly. I have yet to come across one on the second hand market place! (They must be very good!)

There are two types of hybrid: Mak/Newt and Cat/Newt ... The better designed ones (Mak/Newt), which cost £1,000 plus for the smallest and produce very good image quality, use an optical 'corrector plate' which is a thin optically precise piece of glass at the tube aperture and are consequently 'sealed systems' (See picture) - The Cat/Newt versions use a Barlow lens in the focuser to extend the primary focal length in a short tube and are not closed optical systems.

The Cat/Newt design is not particularly good, however.  This design is also called a "Bird/Jones" optical system and l can't honestly see why anyone would have designed it that way!  Look it up when you have a minute (and prepare for abusive talk!)

What the design sets out to do is utilise cheap spherical mirrors and rectify their faults by adding in a corrective lens. The corrective lens, however,  doesn't actually correct anything, rather it compounds the problem and actually makes it worse if anything.

It's like you've got a pair of glasses that aren't right for your sight, but instead of getting some that are, what your optician does is adds another pair of glasses to counteract the first pair. The light is working twice as hard and has twice as many lenses to navigate. Now, obviously, this might be ok if the lenses were very good, but we're dealing with a means of avoiding expense... so, you can imagine how good the lenses are.

This is a reasonable analogy.  They have a cheap and incorrectly figured mirror and add a small and equally inaccurately figured lens to try to make it better!

Far and away the best solution is to simply use a proper parabolic mirror in the first place.  Why manufacturers insist on creating these complications when the solution is so simple is beyond me!

These telescopes are difficult to collimate, even with a laser collimator, as the extra lens ruins the return light path. The best you can get is an aproximation of collimation with a catadioptric-Newtonian (Not good enough for optics!).

The best image quality you'll be able to get is by using low magnifications.  I used one recently which boasted 1006x magnification as supplied by Bresser (usually a very good company), but, to be honest, it was all out of useful magnification by 50x.  I'm sorry to say that it's a very poor design and it will be useful for only the very basics of astronomy.


To be fair to the closed tube catadioptric Maksutov/Newtonian telescopes: There are three advantages over the simple Newtonian:

Firstly: The tube is closed by the corrector plate and this eliminates tube currents.

Secondly: A closed system also lessens the tarnishing of the mirror surfaces.

Finally: The secondary cell doesn't need a spider to hold it in place, so there is not as much diffraction of the image (But it's still there because of the secondary).



Note: Both these systems are catadioptric telescopes. They make the distinction between the two by using the word Mak to show the different layout only.  Any telescope system using mirrors and lenses is by definition 'catadioptric'.


Buyers make sure: The main mirror and the corrector plate must be very good quality as the optical lengthening of the focal length means that any errors in the mirror or plate are magnified!

A well made quality Catadioptric-Newtonian of 150mm could be an excellent instrument for observing the planets. If you're buying new, they will cost over £1,000 for the OTA. If the price is in the low hundreds, walk away!  If your interest lies in this direction you could consider a Mak/Newt as an option but the accuracy of the parabolic main mirror and the corrector plate is paramount!  If the mirror isn't parabolic, run away!

The idea is that you get a nice long focus instrument in a much shorter tube. (It certainly isn't to make it simpler to manufacture, or to give better image quality, or to be simpler to use and maintain!)




A Word About Telescope Merchandising Codes or Naming Systems.

 The codes used by manufacturers to describe telescopes usually relate to the various sizes and focal lengths mentioned above: There is a sort of de-facto system that a bit of logic will unravel.

A telescope called 'StarPlod
70070' will be 700mm focal length and 70mm Diameter.


One coded 1200/150 will be 1200mm focal length and 150mm Diameter.

60/900, you guessed it, 900mm focal length and 60mm D.


What about a '3003' ?
This company is selling a
30" focal length with diameter 3" lens (f10).

Trying to catch us out!
Make sure you check any code against the description.  Sometimes it doesn't relate to anything! (See left).


Left: This Bresser has a code that doen't relate to the optical perameters, but they are detailed on the label.


Some telescope sellers / companies:


Obsessed as their marketing departments are with magnification - They will advertise a magnification as an AREA compared to normal view. This means where we astronomers would say a telescope magnifies 50x they may claim 2500x Which is 50 x 50 = the difference in the area. This doesn't happen often nowadays, but watch out.

You won't be fooled by this because you know that magnification from 25x to around 150x is all you need for most objects.

Sometimes they are selling just the telescope without a mounting. You WILL need a mounting, and an equatorial mounting at that! Make sure it's the whole telescope and the mounting and not just the optical tube assembly ('OTA')... watch out.


So... the telescope that I found called a PlanetViewer F90076 is a Newtonian reflector, with focal length 900mm, and diameter 76mm (2.99") which, as you can now work out is f11.8 (Newtonian f11.8... Eek! Wobble wobble, very dim image, very narrow field of view like looking up a straw!)


Which is a little less than half the recommended MINIMUM size - Which will perform, I'm sorry to say, approximately a QUARTER as well!


Don't forget that a Newtonian reflector operates, detail wise, as well as a refractor of half the size - Would you really buy a 38mm refractor to do amateur astronomy? Of course not!



< Smudge Saturn: Close up!


This image is representative of a 76mm (3") Newtonian reflector or a 40mm (1.5") refractor image at 100x.

I JEST NOT dear reader... Please be guided by the voice of experience :O)










IF some well-meaning soul has bought you a 'tiny telescope' for a present, and having read this guide you know they have bought you something that will fall short of the ideal.




Message me and I'll give you some advice.


Owning a tiny telescope is not the end of astronomy but you have to accept the limitations.


It's not ideal to own a tiny telescope, but there are some projects you can embark upon that will help you appreciate your next telescope and help you to 'see'.



In the interests of checking the accuracy of my claims I bought myself a 70mm f10 Bresser Refractor (off eBay, naturally!). As you know by now this is my minimum recommended size. I did extensive observations of the planets, stars and Messier objects with an aim to either corroborating or re-calibrating my 'minimum' telescope size for you. I feel that I must tell you that this telescope performed well enough for a beginner to get a real feel for astronomical observation. Falling only slightly short on the detail on the planets that I was hoping for.

The Findings:
The 6x20mm finder was adequate for the objects this telescope could show.  Really a 25mm finder is the minimum you need (50mm diameter on telescopes over 150mm). See finder scopes section under 'What else will I need?' section above.

The eyepieces supplied were not the best qualtiy (or optical layout) and had many internal reflections.  Used with good quality Pl
össl eyepieces (recommended) the telescope was markedly better.


The original eyepieces: Planets were almost featureless (Some observable detail on Jupiter, but not as much as I had hoped on Mars or Saturn.). Some improvement was possible with a much better quality Plossl eyepiece.

Plössl eyepieces: Planets showed some detail - About what I would expect.


If you can't afford anything in the 90mm range, then the Bresser 70mm f10 is just about adequate and will give you detail enough to keep you interested - Don't pay over £80 for a second hand one!  At least we know where we stand!




I recently had use of a Meade 70DS telescope.  This too was f10, and gave excellent images of stars and planets.  The Moon was really nice though this electrically operated non-goto refractor.


The image at left is a single shot (not stacked and hense not as crisp as you might imagine) image taken by me at the controls of a Meade 70DS.

"The Watch-It Gallery" - Who would buy these?

Get to know what a telescope looks like when it is set up. If you come across a telescope that is being advertised (EG on eBay) and the 'scope is obviously upside down, or assembled in a totally whacky manner - Please leave it alone!






You may laugh, and if you don't, you need to learn a bit more about how to set up a telescope.


Seriously, these telescopes could have sustained serious damage being upside-down!




Check and double check - Before buying.

 No answer? Move on, buddy!


I came across this telescope on eBay whilst looking for a SkyWatcher 150mm Newtonian telescope for someone... This telescope was advertised as a 150mm Telescope.

This is NOT a 150mm SkyWatcher!

It's a 130mm SkyWatcher - To make matters worse it's not parabolic either! To the expert the difference is obvious. To the beginner, you could end up paying rather a lot of money and ending up with something substandard.

I tried contacting the seller, but no joy. The answer there is to walk away quickly and look elsewhere.










SkyWatcher make the only 130mm Newtonian I would recommend.  SkyWatcher make excellent telescopes and their 130P EQ2 model is smaller than I usually suggest for the beginner, but if you really can't afford a 150P EQ2-3 then this might have to serve!  The optics are parabolic, which means that you'll get the best view you can from such a small telescope. (Make sure it's a 130P if you have to get one of these. If it's not specifically stated look for the focal length. The 130P has a focal length of 650mm.)  The mounting, just as the scope itself, is well designed and constructed. 




There is a very good reason that so many people are selling their tiny telescopes.

It's because tiny telescopes are no good for astronomy!







I will answer as soon as I can and it's no bother – Honestly!
If you buy smaller than this you will sell yourself short of the experience you deserve!



Clear skies and good seeing... I hope this guide has been useful.