Re: Ronchi Test

Chuck Grant (grant@aretha.llnl.gov)
Mon, 1 May 95 18:25:09 -0700

> From: "Tony Cecce, Corning, NY" <CECCE_AJ@corning.com>
>
>
> I finally worked mirror to a fairly flat sphere under the knife edge test.
> However, I still had a turned down edge. (I know how I got it and I swear
> never to worry about a turned up edge again) It didn't look to be that
> bad, extending maybe 1/8" into the mirror. I decided to try out a new
> Ronchi grating I made with a CAD package and a HPLJ4. What a shock! The
> Ronchi test was very sensitive to the TDE. In actuality it was severe
> on the outer 1/8" but extended to a lesser degree almost 3/8". So, back
> to polishing. I've been using MOT 1/4 centered strokes for 10 minutes
> at a stretch. Using the ronchi grating I have been able to watch my
> progress and see the edge improve. However, I have very faint zonal
> defects under the focault test that I can't see with the ronchi. So it
> seems that both tests really are useful.

The at-center-of-curvature Ronchi test can be difficult to do interpret for small TDE because of diffration around the edge of the mirror. The grating size should match the focal ratio of the mirror. Narrow TDEs I have looked at with a Ronchi test all looked worse than they were because of diffraction. Then again, a perfect mirror does look pretty nice in a Ronchi test.

Chuck