ATM aargh! maybe

[Mark Holm <mdholm@telerama.com>]

Just finished running my most recent Foucault data through Sixtests.  I knew I 
was in trouble (maybe) before even plugging the data in, because the inner zone 
numbers just weren't right.  I have been parabolizing a 205 mm BVC mirror of 
focal ratio slightly less than 6.  Am on my second parabolization attempt.  It 
was going very well and I was sneaking up on 1/20 wave P-V.  Both center and 
edge were high, with mostly nice smooth curves going toward both.  Each session 
was moving me about 5 nm closer and the changes were going in a very predictable 
pattern.  Everything seemed well under control.

The most recent Foucault data show the center inexplicably went just the 
opposite way from what it had been, up instead of down, and there is evidence of 
a new zone that hasn't been there through the last six or more sessions of 
figuring and testing.  The last figuring session was no longer or more vigorous 
      than preceding ones.  I used a little wider strokes, because the center 
had been a little higher than the edge, thinking that wider strokes would bring 
the center down a little faster.  That is just the way it had been working, as 
well as what Texerau recommends.  I am testing in 8 zones, and repeated several 
of the zones to be sure, once I saw the direction the numbers were going.  Boy I 
sure hope another round of testing tomorrow shows this was just one of those 
crazy testing flukes, because I had a pretty good figure (or at least it sure 
looked that way) and was on the way to having a really great figure.  I'm going 
to be some sort of pissed if it has really gone bad.

The only thing I can think might have gone bad was if the lap wasn't in good 
contact.  That seems unlikely as I always let the mirror cold press a good long 
time before each figuring session, and this one was no exception.  (Cold is 
relative, the room temp is about 70F and I am using Gugolz #64.)  I have had no 
sign of the lap not being in contact up to now.  No zones developing or 
unexpected behavior.  Slight zoneiness from earlier corrective work was slowly 
fading away.  The mirror "feel" on the lap was good.  Nice and even, not too 
slippy, not too draggy, less draggy than for real aggressive polishing.

Sorry to vent like this.  Perhaps sharing frustration will help others realize 
they are not alone when they have problems.  Perhaps tomorrow will dawn a 
brighter day with better Foucault shadows.  I am sure going to retest before I 
put that thing back on the lap.

Mark Holm
mdholm@telerama.com