There is a fairly simple test you can do to test your optics. This test is applicable for all types of telescopes, reflectors, refractors or even binoculars.
Set up your telescope to view a star. Make sure the star is close to the centre of the field of view. Find the best focus position and then rock the focus backwards and forwards through this point. At the best focus, the star should be a small point of light, but as the focus is moved, the star will expand into a disc.
The enlargement should be even and the disc should be evenly lit throughout with no brighter rings or centre forming within the disc as the focus changes. The intensity will fade as the disc gets larger. The disc should be circular at all times. Any deviation from these conditions indicates a problem.
If this is the view you get when you rock backwards and forwards through focus with a low power eyepiece then congratulations! There is nothing much wrong with your telescope!
If you use a high power eyepiece, you may see a fixed ring appear round the central pinpoint just on focus. This is the first Airy Ring. If you can see this, it is yet another good indication there is nothing wrong with the telescope.
Testing at Home
Here is something you can do at home!
If one or more bright rings are forming and disappearing as focus is rocked, or there is a ring that appears to move, then this indicates a problem with the mirror/lens correction. Certain zones of the mirror have slightly differing focal lengths.
If the image changes shape as the focus is rocked, especially close to focus, this indicates astigmatism in the system.
First check the lens, mirror and the elliptical flat is/are not clamped too tightly in their cells before blaming the optics.
Over-tight clamping of the optics is often the cause of Astigmatism being seen by this test. If a mirror or lens really has astigmatism, then it can usually be proved by rotating that element, which will move the astigmatism by the same amount.
(Please don't adjust the optics yourself unless you are absolutely clear what you are doing!)
If you are getting views that suggest you might have a problem, please try a different eyepiece before worrying about the main optics.
If you are getting a good result from a star in the centre of view and you have a Cassegrain or Schmidt-Cassegrain, then repeat the test with a star towards the edge of the field of view. A Cassegrain or Schmidt-Cassegrain Telescope with Ritchey Chretien correction should give the same good result. If the view does deteriorate significantly, it indicates poor correction of the mirror and/or corrector plate combination.
With a Newtonian telescope, the view will always deteriorate as the star moves towards the edge of the field of view. The effect will worsen with faster focal ratios. But watch out! - If you are using a eyepiece without a wide angle capability, then astigmatism in the eyepiece may be far more of a problem! - Try another eyepiece.
A final note: - If you are not clear yourself what you are seeing - then get a more experienced person to look through the telescope with you.
IThe actual view you see does depend on the telescope size, eypiece focal length and magnification the system is running at. You are guaranteed to see coma in a Newtonian if looking off axis. If you are using low magnification, then a typical star will look like a comet with a tail. (Which is where the name of Coma comes from.) If using high magnification sufficient to see the Airy disc, then it will appear with sections of the Airy rings. Usually only the first one or two rings are seen. The two images next illustrate those two possible views.
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But to confuse things further, Coma may also appear in other forms. Below are two more examples of what to look for.