The star test is a great way to check telescope optics. It's free, doesn't take very long and it's not as difficult as some would suggest.
In a nutshell, you use the defocused image of a star to reveal aberrations within your telescope's optical system. The star test makes even minor, unimportant aberrations obvious to the eye. This is both good and bad. Aberrations are easy to identify however, if you're not careful, you may become obsessed with eliminating optical aberrations that have negligible impact on your telescope's performance.
The star test allows you to determine if the optics of your new telescope are as advertised. Good optics are called diffraction limited. This means they are limited in performance only by the physical laws of optics and the observing conditions. Even the best optics will suffer when the seeing is soft, but poor optics will not perform to their theoretical limit even under the best of conditions.
The star test can also be used to make final adjustments to the collimation of your telescope. Often, you will only need to make minor adjustments to one collimation screw to correct the misalignment. Once you've gained some experience with this procedure, you'll find a five minute star test at the beginning of each observing session will allow you to check and tune your telescope's optics for maximum performance.
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Revised: February 11, 2002 [WDF]