From: "Aditya Munshi" 
To: 
Reply-To: "Aditya Munshi" 


Hi mark

Is testing with an intermediate mask equivalent to testing with a mask of
7-8 zones, what I mean to say is that because I'm not an expert at Foucault
testing ; testing with a 8 zone mask will no doubt have doubtful readings.

Anyway tell me what you think about it. I tested the mirror with an
intermediate mask yesterday, the surface is quite similar with a pv of
1/4.2 or so, didn't have much time yesterday so I will take more readings
to confirm the results today.

Thanks for your help

Aditya

-----Original Message-----
From: mdholm{at}telerama.com [mailto:mdholm{at}telerama.com]
Sent: Thursday, July 24, 2003 6:21 PM To: amul49{at}eth.net
Subject: Re: ATM Figuring Update

Aditya,

I have just run your numbers in Sixtests.  It shows 15.2 nM RMS, Strehl ratio =
0.887, very slighty overcorrected (hyperbolic) with b = -1.075.  Peak to valley
error from the best fit parabola is about 68 nM.  The edge is about 30 nM above
the best fit parabola.  There is a low zone at about 86 mm radius, a higere
zone crests at about 57 mm radius and the center is about 38 nM low.

These RMS and Strehl numbers are without a secondary.  If I add in a 38 mm
secondary, the RMS goes to 14.2 and the Strehl to 0.900

I am at work now, and cannot upload the Sixtests images.  I will try to do it
this evening, so you can see the graphs and double check the numbers.

My advice.  If these numbers are good, and there is not roughness or narrow
zonal defects, your mirror is already reasonably good.  It can be better, and
it may well be worth putting in the effort to make it better, but you will need
to be careful, slow and cautious.  In mirror making, the first hundred miles of
the journey is easy, the last few feet (or nanometers) are the really difficult
ones.

Do yourself a favor at this point.  Test in more zones.  7 or 8 would be good.
Remember that between your measured values, Sixtests, or any other evaluation
program is just guessing about the shape of the mirror.  The guesses are based
on good math and reasonable assumptions, but they are still guesses. Only more
data points can make the guessing process more accurate.

Average at least 4 measurement sets between each figuring session.  You are at
the point where your ability to measure the Foucault readings is barely good
enough.  (Not a comment on your skill.  Everybody has this limitation.)

Averaging really helps improve the reliability of the test results.

As I recall, you have a thin mirror, so you prefer to work MOT to help prevent
astigmatism.  For the moment, I am going to be silent regarding figuring

strokes.  This is because I don't feel qualified to give advice in this
situation.  I hope other list members, more experienced with figuring will
add
their advice.

Mark Holm
mdholm{at}telerama.com

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