Notes on Star Testing Refractors

by Roland Christen

Identical inside/outside rings in an achromat is normal. This is due to the nature of chromatic aberration, which take all colors, except green, far away from focus, where they cannot interfere with the diffraction pattern. In an Apo lens, red, yellow, green and blue are all very close to focus, but not perfectly, exactly at the same identical focus. These colors tend to interfere with the perfect diffraction pattern. Every Apo lens I have ever star tested, even ones that tested close to 1/20 wave P-V, shows different inside and outside patterns of interference. This is normal. By contrast, I have an 8" SCT that shows perfectly identical inside vs. outside diffraction patterns, but tests only 1/4 wave. The real test of an optic is not so much how the diffraction pattern looks outside of focus, rather, how much extraneous junk is floating around a star when it is in focus. (in my 8" SCT there is quite a lot of that). In an achromat with perfect optics, there is a lot of unfocused blue and red light around any object. The brighter that object is, the more that light interferes with the image. One saving grace for achromats is a set of filters. It does not "fix" the chromatic aberration, rather it lets you see the image in one color in monchrome very sharply. =================================================================== If the outer rings are identical on either side of focus, there is no significant spherical aberration. What you see in contrast difference on the inner rings is due to chromatic effects. Use a green or yellow filter (in fact, use them both at the same time) to isolate the middle of the visible spectrum. Try different eyepieces if the refractor is faster than f10, you may see different contrast effects. Plossls and some Orthos tend to add undercorrection to the image. When you find the eyepiece that gives the most equal out-of-focus images, use it in your observing. Hint - multi element eyepieces like the Naglers have the best spherical correction for fast scopes. Roland Christen, ASTRO-PHYSICS
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