To: imasloth{at}bigpond.com, atm{at}shore.net
From: mdholm{at}telerama.com
Reply-To: mdholm{at}telerama.com


Hi again John,

The copy of your message that made it into the digest version of the list
only had two sets of readings, not three.  I averaged the two sets and fed
them to Sixtests.  The shape of the mirror looks similar to your previous
data, but the amplitude of the residual errors is reduced.  (That is a good
thing.)  The Strehl ratio calculates as 0.923.  The RMS is 12.3 nM (~ 1/41
wave).  PV is about 37 nM, ~ 1/13.5 wave.

You are right to be concerned about the variability in your data.  Practice
can improve it somewhat, but you will probably find that, when you are
working at the level needed to confidently get a Strehl ratio of 0.9 or
higher, the measurement uncertainty will continue to be larger than you
would like.  In this case, the only recourse is more testing and use of
statistics (chiefly average or median) to pull truth out of the data. 
Testing in more zones at least partly compensates for noisy data too.

One of the beauties of making multiple readings in each zone is that
probability predicts that readings serioulsy in error should be rare.  If
you make several readings, the average should quickly approach the true
value.  For small sample sets, statisticians say that the median is even
less likely to be affected by noise than the average (mean).  The sample
size has to be at least 3 for there to be any difference between the mean
and median.  (That is 3 per zone.)

The data show that your previously turned up edge has turned a little down.
This may be real, or may be an artifact of measurement noise.  You may need
to be careful not to have the edge go down more.  I find this one of the
hardest parts of figuring.  I really don't have a good way to deal with it.
 I am pretty sure cutting down the diameter of the lap is not a good idea. 
(Unless you go to a seriously small lap to do local correction.

According to Sixtests, you are pretty close to being done (Assuming the
data are a good representation of the mirror.)  Whatever you did in the
last figuring session was basically pretty good.  If you can do a little
more of the same, modifying it a little to avoid lowering the edge, you may
do well.  If you are working MOT with a full size lap, try a 1/3 to 1/2 w
stroke avoiding running the center of the tool near the center of the
mirror to a large degree.  (Put more of the stroke motion on the sides of
the stroke than in the center.)  At this point, you probably want to
shorten polishing sessions to certainly not more than 5 minutes.  Perhaps 2
minutes would be better.  You don't want to rush this.  Creep up on it
slowly and test a lot.

Be prepared to quit while you are ahead.  A Strehl of 0.923, if further
testing bears it out, is not a bad mirror at all.  If you can make Strehl =
0.95, that would be a very good mirror, and serioulsly time to think about
quitting.  If you are confident that the figuing process is under control,
you can shoot for better, but if figuring is seeming iffy, it might be
better to stop than risk having it go off in an unexpected direction.

Oh, the Foucault and Rochi tests clearly show that hole in the middle of
the mirror.  Learn to ignore it.  It will be in shadow anyhow.  Optically,
it doesn't exist.  The one concession to it would be not to use the central
opening of a Couder mask (such as the central opening in the Intermediate
mask printed by CouderMask).  With that much slope in the middle, the zone
would be hard to read accurately.

Mark Holm
mdholm{at}telerama.com

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