Re: [ATM] Testing for Smoothness?

[mdholm@telerama.com]

The Foucault test is reasonably sensitive for roughness, especially when the
figure is spherical.  Now that you are parabolized, look at each zone in turn,
without the mask.  When the zone is gray, it should be evenly gray and the
shades of gray should fade evenly .  Irregularity is a sign of lumpiness.

If you really want to see roughness, there is the Lyot / (some other Frenchman)
phase contrast test.  It is (probably) written up in "Best of ATM
Journal",
available from Willman-Bell.  The test isn't a whole lot more difficult than
Foucault, but requires a quite bright light source, as well as a special filter
plate you have to make.  I think this test pretty much ignores spherical
aberration, so you can use it on a parabolized mirror.  (Is that right, guys? 
I'm thinking based on wave optics and diffraction theory, not based on personal
experience.)


The most troublesome aberration that the Foucault and Ronchi tests are bad at
picking up is astigmatism.  It is a good reason to star test, because it will
show up there if your magnification is fairly high, and the seeing is steady
enough.

If you have reasonably thick, well annealed glass, and have been following the
usual recommendations for avoiding astigmatism, then the likelyhood of bad
astigmatism is fairly low.

Mark Holm
mdholm@telerama.com

Mark Holm
mdholm@telerama.com
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