Re: [ATM] Astigmatism and Ronchi questions

[vorblesnak@peak.org]

Much has gone on off list and there may be someone interested so I thought
I would post a brief update.

I have learned much about astigmatism in the last few days.  Between Mel
and Craig and as much printed reference as I could find I have a better
understanding of what I am seeing and why.

There is some positional sag but it is miniscule compared to the actual on
mirror astigmatism.  The thing that threw me initially was, by the wildest
chance I put the mirror in the cell with the high points lined up
vertical.  This meant that when I check the figure with a vertical ronchi
grid the figure looked pretty damn good.  When the line orientation was at
an angle, (45 degrees in the case of the 85 line grid), I had very bad
astigmatism.  The vertical grid was sampling the best of the figure; the
45 degree grid was sampling the worst.

Based on my discussion with Mel and what I have seen in the pattern, the
shape of the glass on the surface is a cross.  As I approach the ROC the
lines sweep up the incline from the low point to the top of the ridge that
forms the cross and then drop back as I cross the point of confusion.  By
looking carefully I can see one line go almost horizontal then 'jump' back
to an angle orientation as I cross the ROC.  Moving outside to inside the
ROC the lines twist counter clockwise and moving from inside to outside
they twist clockwise.

The fix then would seem to be work the cross ridge and bring it down to
the same level as the rest of the mirror.  I know exactly where it is so
it should be fairly easy to target and then blend.  I suspect it will be
like parabolizing, several strokes and then stop and test.  In ATM 2 there
is a nice article on astigmatism and its correction.  They advise caution
and be prepared to switch course if you find you have read the mirror
wrong.  I do that alot.

David Davis
Toledo, OR



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