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.
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 truely awful images!
Spherical focus: Some light focused at A, some at B.
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 need to 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.
This 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.