Newtonian Mirrors.
A VERY IMPORTANT CONSIDERATION: PARABOLISATION.
IMPORTANT:
This section applies to NEWTONIAN reflecting telescopes ONLY.
It DOES NOT apply to Refractors, Maksutov-Cassegrain or Schmidt-Cassegrain telescopes (Which are good optical designs using spherical mirrors and an optically excellent meniscus lens or corrector plate!
Small Newtonian reflectors (under 150mm) often have spherically figured mirrors. The mirror's shape is incredibly accurate and MUST
be parabolic to bring all the rays of light to the same focus, especially in short focus instruments (f4 - f6).
NOTE: Parabolic mirrors are sometimes, rather confusingly, called 'Aspherical' mirrors.
Some manufacturers of small telescopes sometimes don't bother with this final 'figuring' of the mirror. It's time consuming and needs special testing of the
optics. They leave it out because it's an expense they can avoid. Their telescopes, however, because of this short-cutting, 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!) and therefore the error will be minimised. Some companies seem to have forgotten even this fact of physics and are producing f6, f5 and even f4 spherically figured
telescopes! Now, f4 parabolic mirrors (the focal length is four times the diameter) produce excellent bright images and can stand high magnifications on the best nights. But,
spherical mirrors of less than f8 can only produce 'so-so', at best, to 'truly awful' images!
Some well-known and respectible manufacturers produce non-parabolised Newtonian telescopes. Watch out and only buy the parabloised models. One
day it will become pointless to make spherically figured telescopes... 'Til then make sure you only buy the Parabolic Newtonians.
Spherically figured mirror focus:
Some light focused at A, some at B, and some at all points in between!
In the diagram: The spherically figured mirror gives smudged images because the light reflected from different parts of the mirror is focused at different focal length points (Represented by A & B).
The spherical mirror always produces out of focus blurry images. You also lose some of the faintest stars as their light is
spread out. So, your telescope will not show stars to its theoretical magnitude limit.
Parabolic focus:
All light to the same focus - A.
In this diagram: The parabolic mirror brings all rays of light to the same focus and all image detail is pin sharp.
A Newtonian telescope with a parabolic mirror produces the best images a telescope of its type and size can produce. All focal ratios are equally crisp and sharp with the best magnitude limit
achieved.
This holds true for all focal ratios.
If you want to get the best out of your Newtonian telescope, and to be able to use the optimum magnification and have the best images, you MUST have a
telescope with a parabolic mirror.
"Hang on, SuperCooper", you say, "Surely we can get around this with a longer spherical focus?"
The spherical mirror always produces out of focus blurry stars and smudged planetary images. Longer focal ratios lessen the degradation somewhat, but why would you want any at all? See the smaller distance between A and B on this diagram. Nevertheless, the image is still not completely focused. The parabolic mirror avoids this blurriness of the image by focusing all the light into one point.
Optical specialist testing may certify an f10 spherical mirror as 'Diffraction Limited', because the margin of 'error' is minimal. The difference between spherical and parabolic can be lessened to negligable, but at the back of your mind is the fact that the image could be better! Why would you accept margins of error? Why not get a perfect, parabolic telescope?
Diagram: Once again, in the diagram: The spherical mirror gives smudged images because the focus of light reflected from different parts of the mirror is at different points (Represented by A & B).
Some light focused at A, some at B, and some at all points in between!
Longer focal lengths can be accomplished two ways. An actual longer focal length, or inserting a Barlow lens into the system to optically lengthen the focal length. Neither way is satisfactory.
Long focal lengths give dimmer images and you lose the best part of Newtonian telescopes and that is to observe DSOs. With DSO observation the shorter the focal ratio, the better. So, why would
you want a telerscope of f12 to observe DSOs? The answer is you don't. The best solution is an f5 parabolic telescope that is excellent for DSOs and planets and stars and everything! Don't
accept compromise - Get a parabolic Newtonian.
How Much More Will You Have to Spend for a Parabolic Mirror?
The difference in monetary outlay for a NEW parabolic telescope can be as little as £30 ($40) for a telescope of the same
diameter. If that means that you have to save up for another week, do it!
Waiting a few days more is far better than spending months wishing you'd bought a decent scope in the first place! (See example view photos below)
Surely, 'just a few pounds or dollars more', generally about an extra 10%, is a small price to pay, for a telescope that will perform much, much better than one
with a spherically figured mirror.
Please be careful when choosing your astronomical telescope as different models have different mirrors and it's not always obvious from the model number, focal
ratio or focal length. Some telescopes that are very good sellers are not parabolic telescopes and will give inferior images, but have the same apparent perameters as parabolic
models!
Eg: The 130mm x 650mm f5 mirror supplied in a SkyWatcher Explorer is parabolic, but in a Celestron Astromaster is
spherical.
You owe it to yourself to get a telescope that can show the best images you can to help you in your
hobby.
So, please check the telescope's specification and ensure your Newtonian telescope will have a parabolic mirror.
The Bird/Jones design, also called Catadioptric-Newtonian, gives awful views.
These telescopes are a compromise, as mentioned above. They have spherical mirrors and a Barlow lens in the focuser to optically lenthen the focal length. The Barlow is laughingly called a 'corrector lens'. It is supposed to sort the problems out by lengthening the focal length of the primary, but in reality the cheap, often single element, Barlow makes a poor image even worse!
To spot a Bird-Jones "Catadioptric/Newtonian" just check the focal length and compare it to the tube length. If the focal length is about twice the tube length then you'll need garlic and a wooden stake!
FACT: There is NO getting around using a Parabolic mirror for the Newtonian design of telescope!
PLEASE NOTE: No manufacturer refers to their telescope as a 'Bird-Jones' design. They all claim their instruments are 'Catadioptric / Newtonian'. If you Google "Bird-Jones Telescope", you'll see why they never admit to this!
Commonly Found Examples:
The SkyWatcher 130P
Spec: Diameter = 130mm Focal Length = 650 Focal Ratio = f5
This is an excellent telescope with a parabolic mirror.
It can utilise 250x magnification with no degredation of the image on the best nights.
Detail on the planets and Moon is good and the magnitude limit on stars is the best for its size.
Because this telescope mirror puts all the light into one point there is no smudging in extended images such as planets, Moon and DSOs, and no loss of light in the faintest star images. This gives truely bright, crisp image quality, and shows the most detail and faintest stars theoretically observable. Consequently, the parabolic Newtonian is 'top dog'.
The Celestron Astromaster130
Spec: Diameter = 130mm Focal Length = 650 Focal Ratio = f5 (Note the 'same' spec as above!)
This telescope utilises a spherical f5 mirror and gives poorer image quality because it doesn't put all the light into one focal point.
It can't untilise magnifications over about 50x without losing definition and detail.
Details on the planets and Moon are slightly smudged and the stellar magnitude limit is reduced because of light scattering at 'focus'.
The Celestron Powerseeker127 and PS1000
Spec: Diameter 127mm Focal Length = 500 + 2xBarlow (1000mm) Focal Ratio f7.8
This Bird-Jones telescope utilises a spherical f3.9 mirror with 2x Barlow and gives poorer image quality because it doesn't put all the light into one focal point.
It can't untilise magnifications over about 50x without losing definition and detail.
Details on the planets and Moon are slightly smudged and the stellar magnitude limit is reduced because of light scattering at 'focus'.
The Bresser Pollux 150/1400
Spec: Diameter 150mm Focal Length = 700 (f4.7) + 2xBarlow (FL = 1400mm) Focal Ratio f9.4
WATCH OUT! Even some larger telescopes have spherical mirrors!
This impressive looking 6" Bird-Jones telescope utilises a spherical mirror with 2x Barlow and gives poorer image quality because it doesn't put all the light into one focal point. This telescope comes with a 'highest magnification' of 1,050x, (See box in pic) but is all out of 'useful' magnification at 75x - Poor spec!
It can't reasonably untilise magnifications over about 75x without losing definition and detail.
Details on the planets and Moon are slightly smudged and the stellar magnitude limit is reduced because of light scattering at 'focus'. To date this has been the 'best' Bird-Jones telescope I have used, but is still poor overall!
IT REALLY DOES PAY TO CHECK IF YOUR NEWTONIAN HAS A PARABOLIC MIRROR - NOTHING ELSE WILL PRODUCE THE PERFECT IMAGE!
Genuine Example views:
The photographs below are all taken with the same Samsung Galaxy J6+ phone, using a phone mount. They are all single shot images (not stacked). That is to say, they are directly comparable as they have been produced in the same way. In this comparison I took time to get the best focus and selected the best image I could take. These images clearly demonstrate the optical difference between the different mirror figures.
These images represent the best image quality I could get from the respective scopes.
Pic: Example of Bird/Jones view. 127mm x 1,000mm f7.9
(Celestron Powerseeker 127 Bird-Jones type telescope)
A very common telescope, I'm sorry to say. It is very difficult to get one of these telescopes to focus correctly. (Also branded PS1000.)
I am disgusted at the manufacturer's website that describes the image from these telescopes as 'crisp and bright'. Have a look. What do you
think? Is an image like this acceptable to you? Does this image look 'crisp'?
As a telescope expert with years of experience, I collimated and focused carefully before this pic was taken. Awful image quality - Celestron "SHAME ON YOU".
Ironically, these telescopes produce worse images than the spherical mirror telescopes they suposedly supercede!
Pic: Example of spherical mirror view. 130mm x 650mm f5.
(Celestron AstroMaster130. Newtonian - Spherically figured.)
This is the best focus I could achieve with the spherical mirror f5 Newtonian. (It's supposed to show the features of the Moon!)
You can see that, because the light is always unfocused in part, there are few contrast features, no proper defined edges, few detailed markings on the surface and light is dispersed into the 'sky' around the image (Blurry edge). All this is because the mirror is not putting all the light into one focus!
Celestron I have a question for you - "Don't you think people deserve good telescopes at all?"
LEFT: Example of a parabolic mirror. 130mm f5
(SkyWatcher 130P Parabolic Newtonian telescope.)
That's more like it! Sharp features, delicate shading, detail, nice black 'sky'.
All three of these pictures were taken with aproximately the same size Newtonian - It's not the slightly larger mirror that produces the much better image - It's the parabolic figure on the mirror's surface.
Do yourself a big favour: If you buy a Newtonian telescope, make certain it has a parabolic mirror!
SkyWatcher - "I have to say, thank you for treating your customers with respect and for your excellent parabolic telescopes!"
These example shots show all you need to know about spherical and parabolic mirror images... I can't change the quality of these manufacturers, what
you see here is what you will get!
You get to choose which image quality you'd like when you buy a telescope! Choose wisely.
You get a shockingly BAD image in this supposedly 'entry level', Newtonian.
Many companies sell these 'starter scopes': The spherical f3.9 mirror in this supposedly 'entry level telescope' is so poor that it's just about good enough to be used as a demonstrator for the Newtonian optical system. (Which I how I came to have a couple in my possession!)
If you are thinking of getting such a telescope for a child to start astronomy - Please don't!
Buy them a proper telescope, with a parabolic mirror or, better still, a refractor. See 'Supporting Parents' section for full information.
Pic: This telescope produces images about as bad as the 127mm f7.9 above - This one has a spherically figured 75mm f3.95 = AWFUL IMAGE OVERALL! (See below). SkyWatcher also produce 76P (Parabolic) f3.95 reflectors - But, really, they're too small to see enough to be useful.
If you're buying a telescope for a child, you want their experience to be a good one, don't you?
You don't want them to give up astronomy because of disappointment, do you?
So, why would you buy them something that will not give great images? (See below)
Pic: Here's an image of the moon taken through the 76mm f4 (f3.95) 'First Scope' above!
You can see that, because the light is always unfocussed in part, there are no detail in the contrast features, no proper defined edges, no clearly defined markings on the surface and light is very much dispersed. All this is because the mirror is not putting all the light into one focus and at under f4 that is a huge spread of focus!
Do you think this is good enough?
Say no more!!!
Pic through a similarly sized Parabolic Newtonian:
Far better, though aproximately the same size, is the SkyWatcher 100P. The P means "Parabolic" and the image quality in this telescope is far superior to even the 130mm f5 spherically figured Newtonians and Bird-Jones telescopes you get!
The larger diameter 130mm spherical mirror's potential detail advantage is wiped out by not being parabolic! The 100mm parabolic scope will give more detailed images than a 127 Bird-Jones or Spherically figured telescope. However, of course, a 130mm parabolic scope shows even more detail!
Pic: Once again the image is taken by the same mobile phone with a single shot (not stacked). The 100P doesn't have an equatorial mounting and the AZ it is on isn't computerised or driven. The telescope is therefore unable to take video for stacking. This is a single shot image!
The Moon pic was taken through the SkyWatcher 100P pictured here. It is a 100mm parabolic telescope. The image quality speaks for itself. (Click to enlarge as usual) If you must have a small Newtonian, make sure it's parabolic, like the 100P.
You would get the same image quality from the Orion Skyscanner100 as it uses the same optical components.
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 (Regardless of the magnification used on the smaller).
So, the limit for seeing fine detail in an image is dependent on the accuracy of the telescope mirror or lens. A lens that is perfect, optically, is said to be 'Diffraction Limited'. Because the only thing that is limiting it is optical physics. That is: lt's as good as it can be, given the size.
So diffraction limited optics are the best.
Always look for "diffraction limited optics" and look for the word PARABOLIC (or 'aspherical') when buying your telescope. If it doesn't say in the blurb or specification page -
ASK! (Or assume it's not parabolic.)
In a Newtonian Reflector, for example, the mirror MUST be parabolic to achieve diffraction limited images. A spherical figured mirror is always out of focus at some point! A Bird-Jones design will ALWAYS have a spherical mirror.
NOTE: Some suppliers claim that spherical mirrors over f8 are, in fact, 'diffraction limited' - Having seen the quality of images from these telescopes, I remain to be
convinced! SkyWatcher produce f8 parabolic Newtonian's - Why would they do that if it didn't matter or make a difference?
Sad to say that SkyWatcher produce the 1309 Newtonian (130mm x 900mm f6.9) - This is not parabolic and should be avoided.
The Rayleigh Limit
This is the formula for finding the resolution limit of your telescope (But ONLY if it has diffraction limited optics).
This, or a very close approximation of it, is also known as 'The Dawes 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 is 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 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 is the atmosphere. Whilst your telescope may be diffraction limited, it will also, and always, be limited by the atmospheric seeing too. There are bad nights, good nights and very good nights. It's all part of the game!
I sincerely hope that I have sufficiently demonstrated the difference between the awful spherical mirror and the perfect parabolic or 'aspherical' one! Please make sure you get a parabolic telescope if you buy the Newtonian design.