| Star testing is a very sensitive technique for checking telescope optics. It is easily performed, but requires careful examination to reach a correct interpretation. When a point source is viewed through a telescope, many optical aberrations can be determined by carefully inspecting the patterns of the diffraction images. The telescope is focussed at a very high magnification, if possible at least 10x per cm of aperture. A good quality eyepiece is used but it is best not to use a Barlow lens since it might introduce its own aberrations. The patterns are then observed both in focus and very slightly inside and outside of the focal point. In a perfect diffraction image, the focussed pattern has a sharp, bright Airy disk, surrounded by increasingly faint concentric circles, and what is more important, the patterns at similar points inside and outside of focus, are round and match each other exactly. The perfect out-of-focus image for a refractor is a series of alternating light and dark, concentric circles, while that of a centrally obstructed telescope shows a dark central shadow. Specific problems can be determined from both the overall shape of a diffraction pattern and by comparison of the pattern differences between the inside and outside focus images. |
The star test is usually carried out at night using a bright star but it can be done in daylight using an artificial star, such as the reflection off of a Christmas tree ornament placed at some distance away. Before trying to identify aberrations, the telescope must be carefully collimated, and its temperature must be allowed to come to equilibrium with the surrounding air, to avoid tube currents. Testing must only be done when atmospheric turbulence is minimal so that the results are not masked. For the same reason it should not be done while sighting over buildings or parking lots where rising columns of air can interfere.
Some of the patterns will indicate things that you should be able to correct. For example, severe atmospheric turbulence means that you should try another night, and the presence of tube currents means that you should be patient and wait a bit longer for your scope's temperature to come to equilibrium. Poor collimation of a reflector, or of a new Sky-Watcher refractor model which has an adjustable objective-lens cell, means that you should get out your tools. Similarly, coma may be caused by poor collimation. However, if some coma still exists around the edges of the field after collimating, its effects should not be seen at the field's centre. Any one of several "pinched optics" patterns means that you should check your system for sources of stress on the optics, such as collimation screws which have been over tightened, tube-ring clamps which are too tight, and so on. Finally, astigmatism can have many causes, but two of the most common are poor collimation and a secondary mirror or a diagonal mirror which is not perfectly flat. Some aberrations result from poor shaping and polishing during the mirror-making process and cannot be readily corrected by the user. These include mirror over-correction and under-correction (which can result during the parabolizing process), some causes of astigmatism, edge defects, zonal defects and rough surfaces.
An excellent, advanced reference, for interpreting the results from this technique, is "Star Testing Astronomical Telescopes" by Harold Richard Suiter, (Willmann-Bell, Inc., Richmond, VA, 1994).