Telescope Reviews: Roughness and its complement

I'm one of those insane people who believe smoothness and an accurate figure add up to the best performance at the eyepiece, to the extent that more times than I'd care to count I've taken an otherwise finished mirror that exhibited surface roughness and reworked it from scratch until satisfied. I know it pays off - the hardest part is getting the techniques down to avoid adding roughness while figuring aspheres. I've no faith in claims that mirrors can be "smoothed" after figuring - it only takes a moments inattention to put roughness on the glass, but serious extended work to take it off again.

This brings to mind something I've discussed with several opticians, present company included, and that has to do with "perfect mirrors." They do happen, they have happened, and when they happen they go on to amaze people, as in drool-bucket time.

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The difficult part is figuring out what the difference is between the ordinary fine mirror and these. Several examples of these come to mind, and I usually check them out pretty carefully to verify, but one thing stands out - in every case they showed Ronchi patterns while passing a handheld grating through the caustic of focus at ROC that was textbook and flawless. Ronchi is not supposed to be that sensitive (we're talking here differences at the 20th wave level or so, judging by the corresponding Foucault testing) but in motion I believe it yields information that's very hard to ignore, and when it shows no flaws at all the mirror inevitably performs like nobody's business.

A large part of the automation effort I've been engaged in the last few years is to obtain those kind of mirrors every time out.

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Here's an example of smoothness on fused silica - this is a personal 14" f/4.6 I made last year for the ongoing prototype scope, and later sold, it was done fairly quickly and turned out only about 1/14th wave PVW, but was noticeably smooth in high-res Foucault imaging (reduced here). The stuff at the top is some water marks, the spot in the center the reflection off the back surface. I've found, in comparing Lyot and high-res Foucault imagery done by Dale Eason, that both capture the same level of detail, but the Lyot is like a contrast boost and of course it drops out the SA on the glass, so it's easier to see the whole surface. Under visual inspection with Foucault and grazing illumination there was nothing to see. What it is.

Best,
Mark

Edited by mark cowan (10/13/09 04:48 PM)