Star Test Caveats

Sitting in a comfortable den reading about it, the star test seems to be simplicity itself. Aim the scope at a star, defocus, and the exact optical condition of a CAT is

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a: Inside/outside Focus

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a: Inside/outside Focus

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Poor Seeing o a: Inside/outside Focus

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Figure 5. (Star Test 2) Star test diffraction patterns page two. Credit: Author.

Figure 5. (Star Test 2) Star test diffraction patterns page two. Credit: Author.

revealed. Move outside into the real world, and it becomes more difficult. If the air is not steady—and it is not perfectly steady on most nights—diffraction patterns will be dancing around. They might just as easily indicate "terrible" as "perfect." Even on nights of good seeing, it is never as easy to tell what is going on as it is in the nice, clear, computer-generated diffraction patterns in Figure 4a. The software that produced these pictures, Cor Berrevoets' freeware program Aberrator, is a fantastic tool for star testers, but it cannot duplicate exactly what you will see with your scope and your eyepieces under your sky.

There is also the question of the basic validity of the star test for CAT owners. As has been noted by many CAT fanciers (and telescope makers, including Astro-Physics' Roland Christen), unlike a Newtonian or a simple achromatic refractor, a CAT's intrafocal and extrafocal images are rarely identical, even when the optics are perfectly made. The complex arrangement of lenses and mirrors in these scopes means the in-focus images' "wavefront" can be great, while slightly out-of-focus images are strongly degraded. That does not hurt the scope a bit in normal use, but it is a killer for the star test.

If the star test is not always a good way to test a CAT's optical quality, what is? Go back to planetary images. As is shown in Figures 4 and 5, introducing optical aberrations has a severe effect on the planets. The effect in the real world is just as striking. A look at Jupiter and Saturn at medium to high magnification easily reveals a scope's optical quality. Does Jupiter display plenty of cloud bands and, on good nights, detail within these cloud bands? Does it show subtle extensions and rifts? Does Saturn show off the Cassini division? How about brightness and color variations across the rings? Even more difficult, are a fair amount of Saturn's subtle atmospheric features visible on the disk? If these things are visible in a cooled and collimated CAT, rest assured that the telescope is up to snuff.

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