[ATM] Roddiers' curvature sensing

[Michael Peck <mpeck1@ix.netcom.com>]

This probably wasn't a fair test of the method, but at the moment I don't 
have a telescope of known quality, whereas I do have a well tested uncoated 
mirror. I also have a SBIG camera that isn't getting much use for it's 
intended purpose. So, I decided to try out "roddier.exe" 
(<http://www.astrosurf.com/tests/roddier/projet.html>) using an artificial 
star at the center of curvature of the 6" f/5 mirror that I've used many 
times for my explorations of various optical tests.

Here's the basic result: the wavefront estimated by curvature sensing bears 
a vague resemblance to what I measured interferometrically, but that's 
about it. The overall net estimated wavefront error was almost twice as bad 
as interferometry says it is. roddier.exe says the mirror is significantly 
more undercorrected than I estimate from either interferometry or Foucault 
testing, and it also found large amounts of trefoil which I didn't see 
interferometrically and which I don't think the test stand could have 
induced. On the positive side the mirror is quite rough, and it did see the 
surface roughness. But the surface roughness can be seen easily in a knife 
edge test and even (less easily) in interferometry, so that's not a 
comparative advantage.

As I said, this probably wasn't a fair test. I had to mount my light source 
separated by about 45mm from the center of the sensor, and I didn't take a 
lot of care with equipment alignment. The resulting wavefront had 
significant amounts of tilt, astigmatism, and coma that had to be removed 
from the net wavefront. Also this is a (almost) paraboloidal mirror, so 
there's about 1/4 wave RMS of spherical aberration relative to a sphere.

Based on this I'd say that curvature sensing probably isn't a substitute 
for conventional optical shop tests. It may be fine for it's intended 
purpose though, which is testing of telescopes "in function." YMMV.

Mike Peck

_________________

Michael Peck
email mpeck1@ix.netcom.com
Wildlife photography page http://home.netcom.com/~mpeck1/index.html
Amateur telescope making http://home.earthlink.net/~mlpeck54/astro/astro.html 

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