From: "CSC" 
To: "Atm" 
Reply-To: "CSC" 


What do you think the 6 depressions are from?  Are they very close to the
center?  If so, remember the small contribution to the image from the inner
zones. If you get a sphere with a hint of the depressions, it may be o.k.
to parabolize from there, and have a fine mirror.  I've been too picky at
times, and ruined a sphere.

Remember that long strokes to parabolize will tend to turn the edge, so a
slight tue before parabolizing might not be such a  problem.  Deepening the
center might also mostly polish out the remaining defects.  Have you
quantified the depressions?  Measure the actual radius of the depression
with the KE and do the calcs.  They are probably a millionth deep or
something crazy like that.

Congratulations of correcting these microscopic problems.  What an artist!!!

Colin

-----Original Message-----
From: owner-atm{at}shore.net [mailto:owner-atm{at}shore.net]On Behalf Of
Aditya Munshi
Sent: Wednesday, May 21, 2003 10:00 AM To: atm{at}shore.net
Subject: ATM figuring question



Hi everyone

I am figuring my 8" f/7 mirror which had a severe TUE (ROC diff 10 mm)
and a central hole. When I attempted TUE correction with a TOT Chordal
Stroke it created a severe surface irregularity ( I could see 6 small
depressions arranged in a circle around the centre in the Foucault test).
However I have come a long way since then, hot pressing and polishing out
defects with only slight variations in stroke length. The Central hole is
nearly gone and the TUE ROC diff is about 2 mm. The problem is that the
surface irregularity has also disappeared with about 4 hours of MOT and TOT
work. The surface is pretty smooth as a whole but I can just spot the 6
depressions in the Foucault test. As a next step I will probably try
micro-faceting and slow gentle polishing but any help is greatly
appreciated.

The irregularity is very minor as compared to the previous figure but still
I think it would be best to iron out all visible defects.

Thanks for any help

Aditya Munshi

--- BBBS/NT v4.01 Flag-4