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Are there ways to 'cheat' with the setup of the interferometric analysis (aside from fraudulent reports)?

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Hmm, maybe.

Even if you're trying to do it right (nontrivial), stepping outside of the comfort zone (say by testing thin mirrors and supporting them vertically on a sling) can introduce enough test stand error to swamp the mirror's errors. And that's before you do the analysis of compound errors to set some sort of bounds on the overall accuracy. If you take multiple IFs with a mirror rotated systematically, reduce each one, then average the results properly, test stand errors can be eliminated. But a 10 minute test stretches to hours that way.

If you're making mirrors, any test regime is there to ensure you meet your quality targets. And those mirrors go on to be examined critically in the field, so if the quality target was met and the mirrors perform, the actual testing regime is moot.

That said, I'm building a Shack-cube interferometer just to get full surface analysis on large mirrors down to f/2.5. But it's the last step in a long chain of metrics used for fabrication control, and only to validate production control. But granted, if you accept the test methodology, having the full surface metric showing errors on the glass can be considered "more objective" than just knowing that it passed muster with the optician's usual care in fabrication.

Best,
Mark

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A ronchi can show alot . post a ronchi pic and people will see all kinds of things

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I seeeee a small rabbit on an elephant..running through the clouds....

Not really.....

The center needs more pushing for sure.....Huge mirror

Rob

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www.goldmtobservingcenter.com
A great place for amateur astronomers, and ATM's to come and enjoy their hobby.

http://tech.groups.yahoo.com/group/HomemadeRefractorTelescopes/ My homemade refractor group.

www.vimeo.com/6014031

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Greg,

I see what you're saying. I'll be making an f/2 reference sphere for testing the IF out with, I'll send that out for independent looks and get some traceability. Small plano-convex lens as sourced by, say CVI can be acquired coated/uncoated and one could build two cubes, one for coated mirrors, one for not. The quality is likely to be very good near the center for small lenses with lambda/10 specs, I'll be finding that out.

Using the "outboard" reference sphere would provide an optional method of making it work.

John,

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anyone have any thoughts on using a webcam instead of the human eye for evaluating grey values in a Coude mask? I would have thought using 255 shades could give a numeric value for better matching?

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I've been working up a high-res digital (unmasked) Foucault tester (not the way RoboFoucault does it, BTW!) that in manual digitization tests showed something <1/40th wave PVW agreement with careful manual measurements. It will measure hundreds of points across the mirror, if desired, stepping equally through the caustic and then back-computing the "zonal" radius.

Best,
Mark

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