Quick Reference and Information
This section contains lots of information contained in the other guides,
but all in an easy to find page.
It also includes a glossary and useful websites sections as well as all the information about telescope perameters and copabilities.
Hopefully, it will save you time scouring through the other guides for the information you need.
The recommendations here relate to small to medium sized equipment. The maths is rigid for all equipment!
Each heading is a link to the relevant section in the website. If you have further questions the relevant guide will answer them.
To find the magnification:
Divide the telescope primary focal length by the eyepiece focal length. Eg: 1,000mm FL / 8mm eyepiece = 125x
Remember, if you don't know your telescope's focal length, you can only measure the tube of a refractor or a Newtonian to calculate the focal length.
Catadiapotric telescopes have incalculable (by tube measurement) focal lengths. Measure the exit pupil for catadioptric scopes. (See magnification section)
Maximum Recommended Magnification:
Don't exceed 2.5x per millimetre of aperture in any telescope type (50x per inch) Eg: 80mm refractor 80 x 2.5 = 200x
Barlow Lens:
My recommendation is to use a 2x Barlow if you need one to reach magnifications without resorting to eyepieces under 6mm. Make sure it is a good quality
Achromatic Barlow.
The Barlow used with an eyepiece doubles the magnification by doubling the focal length of the primary, but also the focal ratio. It preserves the eye relief (The
distance your eye is from the lens when observing). For this reason it is better to use an 8mm eyepiece with a 2xBarlow than a 4mm eyepiece on the
planets, but the Barlow has to be very good quality!
My magnification recommendation for Planetary views: is around 150x to 175x in general atmospheric viewing.
Very calm nights upto 250x may be used in larger sized equipment (90mm refractor or 150mm Newtonian)
My magnification recommendation for Deep Sky views: is up to 100x in general atmospheric viewing.
Very calm nights upto 150x may be used in larger sized equipment (90mm refractor or 150mm Newtonian)
Divide the Focal Length of the primary by the Diameter of the primary. Eg: 750mm FL / 150mm D = f5
Focal length of the eyepiece divided by the telescope focal ratio. Eg: 30mm / f5 = 6mm exit pupil
As long as the exit pupil is 7mm or under, you will be ok.
Longest focal length eyepiece usable in a scope (ie: lowest sensible magnification for astronomy)
Focal ratio multiplied by 7. Eg: f6 telescope would be 6 x 7 = 42mm
This is the width of the view when we look into the
eyepiece.
Eg: AFOV is stated on new eyepiece blurb
This can be worked out by dividing the Apparent field of view of the eyepiece by the magnification of the optical system.
Eg: AFOV is 35 degrees and scope 120x
So: 35/120 = 0.29 degrees of sky observed
Magnitude works on a logarithmic scale:
A 2nd mag star is 2.512 times fainter than a 1st Mag star, and on it
goes.
To see one magnitude fainter, your larger telescope has to present 2.5+ times the light to the eye than the original scope. Remember the secondary obscures some light
if you have one, and reflection light reducytions need to be taken into account if you are comparing refractors and Newtonians. Generally, if you double the diameter comparing refractors
then you have four times the light collection. If you're comparing refractor with Newtonian then double the diameter is only about three times the light
gathering!
1st Mag = Star Magnitude
2nd Mag = 2.512 x dimmer
3rd Mag = 6.310 x
4th Mag = 15.85 x
5th Mag = 39.82 x
6th Mag = 100.0 x
Note: You can shift these values up or down to find relative values:
A 2nd Mag star is 100 times brighter than a 7th mag star or a 3rd Magnitude star is 15 times dimmer than a 0 mag star.
Magnitude scale is bipolar! There are positive (dimmer than 0 magnitude) and negative (brighter than 0 magnitude) objects + & - values.
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Pedantic: Purists might argue that a long focal ratio Newtonian and a short one will have different light gathering areas due to the different size of the secondary, but visually the faster ratio of the shart focal ratio scope will generally outweigh the small magnitude gain given by the smaller secondary shadow.
Eg: The difference between a 150mm / f4 and a 150mm / f8 is about 9% more light in the f8, but that equates to only a difference between a magnitude limit of 13.55 and 13.45 for stars! Meanwhile, DSOs will 'look brighter' in the f4. Not worth calculating. Magnitude limit calculations really only relate to stars. Atmospherics and eyepiece choice will negate the difference between focal ratios in same diameter telescopes regarding stars!
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This is the formula for finding the resolution limit of your telescope (If it has diffraction limited optics).
R = 4.56/D (D in inches, R in arc-seconds)
R = 11.6/D (D in centimeters, R in arc-seconds)
where :
D = Diameter of the main lens (aperture)
R = Resolution: The resolving power of the instrument
Telescope Type and MINIMUM Size Recommendations. (Why would you settle for the minimum? NO
smaller)
This is a good "Children's" sharer size:
Short focus (f5 - f10)
Minimum size: 60mm on AZ mounting - Ideally 70mm - 80mm
Best for: General Observation / Planets / Deep Sky / Moon.
Recommended Example: Bresser SkyLux AZEQ 70mm f10
Refractor for Children's Shared Telescope Observation (See specific 'Supporting Parents' section): Click this link.
"Teen / Adult" Telescope Minimum Sizes:
Refractor: Short focus (f5 - f9) EQ Mounted
Minimum size: 70mm - Ideally 90mm - 120mm
Best for: General observation / Deep Sky / Moon.
Recommended Example:
SkyWatcher StarTravel 102 on EQ2 or EQ3
Refractor: Long focus (f10 - f16) EQ Mounted
Minimum size: 70mm - Ideally 90mm - 102mm
Best for: Planetary observation / double stars / Moon.
Recommended Example:
SkyWatcher EvoStar 90 EQ2 or EQ3
Newtonian (f5 - f8) EQ Mounted or AZ Mounted
Minimum size: 130mm Parabolic - Ideally 150mm parabolic.
Best for: General Observation / Deep Sky / Moon
Recommended Example:
EQ = SkyWatcher Explorer 130P on EQ3
AZ = SkyWatcher Heritage Flextube 130P Dobson Mount
'Dobson Mounted' Newtonian (f6 - f8)
Minimum size: 150mm Parabolic - Ideally 200mm parabolic.
Best for: General Observation / Deep Sky / Moon
Recommended Example:
SkyWatcher Classic 150P Dobsonian
Maksutov-Cassegrain EQ Mounted
Minimum size: 100mm - Ideally 127mm -
180mm
Best for: Planetary observation / double stars / Moon.
Recommended Example:
SkyWatcher SkyMax127 on EQ2 or EQ3
Schmidt-Cassegrain EQ Mounted
Minimum size: 127mm - Ideally 127mm - 252mm
Best for: Planetary observation / Moon.
Example:
Celestron C5
Binoculars (Hand held)
Min 8x40 or 7x50 - Ideally 10x50 with porro
prisms for your first pair.
There are many larger sizes available, and roof prism types, but they would not be a good idea for your first pair!
Example:
Swift Ranger II, 10x50s
Travel Scope AZ mounted.
The Celestron Travel 70 is ideal in many ways, it's the obvious choice. However, it is not
recommended that you try using a travel scope as your main telescope. They don't have the image scale to give enough detail, but are good for wide field views.
My Recommended Telescope Manufacturers
This is a list of 'trusted' makers who usually delight. As with any telescope, have a look at the manufacturer's specifications.
Make sure your Newtonian is a parabolic one! Refractors must be over 400mm focal length.
I would recommend any of the following makes as having a very good chance of getting good optics, good views and excellent build quality:
SkyWatcher Their EvosStar and StarTravel refractors are very good. You want a "P" in their Newtonian models (eg: 130P / 150PL / BKP1145)
Celestron Their AstroMaster refractors are nice. LT model refractors are great for wider angle views. Powerseeker refractors are good.
Watch the Powerseeker Newtonian telescopes, they are spherical or Bird-Jones rubbish! AstroMaster Newtonians are all spherical tat! To get a parabolic Celestron you need to look at C6-N at least! Celestron are not nice to their customers!
Orion Nice instruments. Check for parabolic Newts (They do mostly parabolic scopes, but make sure!) Nice build quality.
Bresser Refractors are all excellent. Newtonians: Watch for Bird-Jones and non-parabolic versions.
Meade Usually good, but their small refractors (60mm to 70mm) can have poor images. Always check Newtonians for parabolics.
Konus Good refractors of 60mm - 90mm. Nicely made. Don't let the tube colour put you off, these are good scopes!
Obviously there are lots of other manufacturers, but these are the ones I would suggest are a safe bet of getting a good telescope.
Do I have a list of telescopes I would definately NOT recommend? I certainly do, my friend:
In all honesty, I have never come across a 'scope by any of these manufacturers that I would be happy to ask someone to use!
As a rule of thumb, PLEASE AVOID THESE:
They tend to have poor quality optics, poor design, poor manufacture or all of these failings!
Astral
Bushnell
Edu Science
Jessops
National Geographic (Own Make Bargain Basement models)
Opticraft
Optus
Science Tec
Seben
Simmons
Tasco
Vision King
Vivitar
Zennox
"Types" of telescope to AVOID:
ANY "Bird-Jones" / "Catadioptric Newtonian" design with no full diameter corrector
plate.
They all give awful image quality and include: Celestron PowerSeeker127 & PS1000, SkyWatcher 1141.
ANY Spherically figured Newtonian:
These include popular scopes - Celestron AstroMaster 114 and 130, SkyWatcher 1309 and BK1145,
(BKP1145 is OK= Parabolic)
ANY 76mm Newtonian:
They are just too small (SkyWatcher do make a 76P, but it's just too small to be useful in astronomy!)
ANY refractor telescope under 60mm diameter:
They are just too small to be useful in astronomy.
Long focus (f13 - f16) Japanese 60mm refractors represent the best of the 60mm brigade but, are the absolute minimum size.
Optical finders are much better than Red Dot finders in every respect, even for GoTo set up!
Optical finders of at least 30mm diameter are desirable on scopes up to 150mm diameter. A 22-25mm finder is OK upto 70mm as long as the clear aperture is 22mm+
(Some finders have single OG lenses and stop them down by half - No need!) But, you really want achromatic optics.
Larger scopes, especially used for observing Deep Sky Objects, benefit from a 50mm finder, as you will perhaps be searching for much fainter objects to view.
A 50mm finder is a useful piece of equipment and can be used on any scope. I would always have one on a DSO telescope of 150mm diameter and larger, for instance. But, if your interest is primarily the Planets or The Moon, a 30mm is big enough.
Recommended Physical Star Atlas:
Wil Tirion's SkyAtlas 2000.0
Absolutely fabulous atlas with 26 large-scale fold out pages showing stars to magnitude 8.0
Beautifully printed and amazing just to look at. It's not cheap, but well worth the outlay if you're serious.
Peter Lancaster-Brown's 'Star and Planet Spotting'
Only available second hand, but well worth tracking down for some usable field charts and lots of guidance and information.
Recommended "FREE Download" PC Based Star Atlas/Planetarium.
Stellarium < Click
the link to go to download page.
Change the time and date. Search for the object you want. Zoom in to have a close look. Plan your observations. Check the exact positions of asteroids. It's a piece of cake with this easy-to-use downloadable star atlas/planetarium. It even has a night time mode for use at the telescope!
There is a Stellarium App for your mobile phone, of course. It has a 'lite' version which is free, or you can pay for the full version (Which, to my mind, isn't as good a deal as the totally free computer version). Useful when out at the telescope.
You can pic objects and zoom in, just like the PC version. I zoomed in on orion, M42 and even right in to the stars of Theta Orionis, 'the Trapezium' with this app - I was quite impressed.
Useful Websites:
Information Websites:
An Excellent Astronomy Astrospace website from the BBC.
With monthly charts of what to look out for and an amazing amout of information on everything you could want to observe:
Another Excellent Astronomy website. With up to date
observation suggestions and finder charts. This is
a great site to get ideas for your observation projects.
Also includes live video sessions. A great resource!
Pic: If you think this is a picture of the Moon, look again!
Starting with binoculars? Here's a brilliant site.
With ideas about what to look out for and great
advice regarding binocular observation:
Stephen Tonkin's BINOCULAR SKY.
If you're wondring where to go to observe the night sky.
Where you can meet other, like minded, people.
Or where to try out telescopes before you buy, you could
visit this non-profit organisation, which helps beginners and
experts alike, to find meetings and locations to observe the sky:
Telescope Supply Website:
Where would I recommend you buy your new
telescope and equipment from in the UK?
Excellent products and great advice from Ed Harrison,
personal attention and quick affordable scope delivery in 48hrs!
I have excellent service from:
Telescope Perameter checker.
If you want to check the physical perameters of any optical system, visit:
International Space Station Passes for the UK.
If you're interested in observing the ISS,
which can be seen without optical aid,
this website has details of where and when to look.
Customisable to your location:
Starlink Satellite passes page:
Just enter your location and see when the spectacle of the Starlink passes can be seen.
A very useful planetarium program.
It's easy to set your location so you can see the exact sky where you are. This is a very useful tool. You can use it to plan your observing sessions, or to confirm what you have seen. there is a dim setting so you can use it at the telescope too. Also it is very 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. Download Here