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Testing optical flats redux

Started by tiafolla, 08/19/2005 07:38PM
Posted 08/19/2005 07:38PM Opening Post
Hi all,

Using your suggestions from last week I successfully altered my interference fringe-type flat tester so that it now works great (using a mercury compact fluorescent bulb). I now get very clear Newton's rings when I compare one polished flat against like a champ for my polished, unmirrored flats and diagonals.

One follow-up question though: how do I test a mirrored surface against the reference flat? If I place the reference on top of a face-up mirror the rings aren't visible.

Thanks again,

Tim Iafolla

Posted 08/20/2005 03:02AM #1
Hi Tim,

I have been told that to test a mirrored flat, it is usually tested in reflection against a known good sphere. I suppose if the the Ronchi pattern of the mirror does not change, then the flat is good. Not sure if this will get you quantitative numbers though.

Vladimir Sacek should have some info on this subject I would think ?

David Simons
Posted 08/20/2005 05:28AM #2
You can try a single layer of pantyhose mesh between the mirrored piece and test flat.

Dominic DiLeo
Posted 08/20/2005 11:34AM | Edited 08/20/2005 11:34AM #3
You could use the Raleigh water test with a colored dye in the water but you need a colimating lens for this test. See July 1990 Sky & Tel.

Ed Jones
Posted 08/21/2005 07:24AM #4
Tom, using Photoshop to increase contrast sounds like a nice technique, and one that I can try without too much work.

Depending on how that works, here's my next alternative: I have a 6.5" pyrex blank with a rough f/8 (pregenerated) curve. From that I can polish and figure a very accurate sphere and test my aluminized flats using it and a ronchi screen.

However I'm still open to less labor-intensive solutions (g).

Thanks again,

Tim Iafolla