To perform a star-test on a telescope is relatively easy, this page is a stepwise introduction to Startesting. More information can be found on the other-sites (click in left upper-panel).
Step 1. Set up your scope.
If you own a SCT or MCT telescope be aware that for star-testing your scope should be sufficiently cooled down. If not the star-test will be severerly hamperred by tube-currents. Also choose a night where seeing seems to be perfect, a clear sky with no twinkling in the stars.
Step2. Aim at a star
Choose a high-placed (altitude) star that is bright and use high magnifications at least 40-50 times your telescopes diameter (in inches). Some people say a magnitude 1 star is sufficient but a brighter star is probably also good or even better. The brighter a star is the more light (absolute) will be put into diffraction rings making them more obvious. The choice of a high-placed star will reduce the thermal effects from the surroundings (roofs) and you will also suffer less from turbulence. The latter because you simply have to look through a thinner atmospheric layer.
Step3. Deciding to do a star-test or not.
At this point you should considere whether or not a star-test is possible. Often everything looks perfect, your scope (if MCT or SCT) has been out for hours and is nicely cooled down, skies look great but through the eyepiece ..... its like a dancing fire-fly. For proper star-testing you want the image to be as stable as possible, some amount of turbulence is unavoidable but it should be really minor. The more dynamic the star-images in/out focus look the less interesting it is to proceed.
Step 4. Star-Test in focus
The most important part are the in focus images. So keep the star centered in your eyepiece (axis-drives are very usefull) and see if the image is according to theoretical expectations. You can first study this with the aberrator, simply generate a non aberrated image for your scopes diameter and obstruction (incl spiders). If you see a perfect round airy-disc surrounded by concentric well defined rings your scope is definitely collimated. If you dont see the airy-disc or concentric rings somethings is not perfect, the question will be: what is not perfect, your scope or the environment surrounding your scope.
Step 5. Star-Test out of focus.
Whilst In-focus images tell us how stars will look during normal star-gazing. The out of focus images will reveal more details on the nature of the aberrations than the focussed image does.
Shadow: For those who have an SCT,MCT or Newtonian when you go out of focus you will see a darkning in the center of the images. This is called the secondary shadow and is caused by the secondary obstruction (mirror).
Symmetry: There are two out of focus sides you can study, the intra-focal (pre focalpoint) and extra-focal (post focalpoint). Ideally they should appear completely the same. If you defocus 5 waves intra-focal, the image should be like the image you see at 5 waves extra-focal.
Non symmetrical: Several types of aberrations lead to non-symmetry. A common type is Astigmatism But also Lower and Higher Spherical aberrations do lead to non-symetrical image where one side of focus is brighter than the other.
If you have had a good look at the images at your eyepiece you can turn to the Aberrator to find a comparable difference. First read all about the different aberrations and their effects.