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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