IMPORTANT: This does not apply to refractors.
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.
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 f5 spherically figured telescopes! Now, f5 (the focal length is five times the diameter) parabolic mirrors produce excellent bright images and can stand high magnifications on the best nights. But, spherical mirrors of less than f10 will produce truly awful images!
Some well-known and respectible manufacturers produce non-parabolised telescopes. Watch out and only buy the parabloised models.
Spherical focus: Some light focused at A, some at B, and some at all points inbetween!
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).
The spherical mirror always produces out of focus blurry stars and smudged planetary images.
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 images are pin sharp.
A telescope with a parabolic mirror produces the best images a telescope of its size can produce.
If you want to get the best out of your telescope, and to be able to use the optimum magnification and have the best images, you MUST have a telescope with a parabolic mirror.
The difference in monetary outlay for a parabolic telescope can be as little as £30 for a telescope of the same diameter. If that means that you have to save up for another few weeks, do it! Waiting a few weeks is far better than spending the rest of your life wishing you'd bought a decent scope in the first place! '£30 more', is a small price to pay, for a telescope that will perform much, much better than one with a spherical 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. Some telescopes that are very good sellers are not parabolic telescopes and will give inferior images.
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 specification and ensure your telescope will have a parabolic mirror.
< Shockingly BAD image in this toy.
The spherical f4 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 Syetem.
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 a refractor.
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?
If you're buying a reflecting telescope...
MAKE SURE IT'S A PARABOLIC MIRROR!