The Star Test

Assuming everything else—collimation, cooldown, and seeing—checks out, but images remain poor, it might be wise to test the CAT's optical quality. That is relatively simple to do via the star test, in theory at least. In theory, the star test is easy to perform and very informative. On a night of good seeing—this is very important—point a telescope at a medium-bright star such as Polaris. Use an eyepiece that gives a magnification of about 150x and rack the star out of focus slightly until four or more diffraction rings are visible; the star should now look like a little bull's-eye, as seen in Figure 4a.

Compare the way the diffraction rings of a slightly out-of-focus star look on each "side" of focus, intrafocal (inside focus), and extrafocal (outside focus). In an SCT or other moving mirror-focusing scope, intrafocal images come when the focus knob is turned counterclockwise through sharp focus and beyond; extrafocal images happen when focus is passed by turning the knob clockwise. If the patterns are identical on each side, then the scope's optics are perfect. Slight differences indicate optical problems. Figure 4a shows how a perfect telescope's intrafocal and extrafocal star diffraction patterns should look.

Star test tolerances are stated in terms of wavelengths of light, and a good optical system will have errors in its figure no larger than a quarter of a wavelength of light. The fact that the star test can easily detect such tiny irregularities gives an idea of how sensitive it is. Fortunately, star testing a new scope's optical quality does not require quantifying exact mirror figure deviations. The basic job is merely to examine the appearance of a star's diffraction rings; how they look will tell what is wrong—or right—with a new scope's optics. Note that a telescope can suffer from more than one problem, and that a real-life star test may show diffraction patterns that are the combinations of several different aberrations.

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