[ATM] Ultrathin glass and mirrors

[vorblesnak@peak.org]

Dale has exposed a number of issues That I want to look at locally, i.e.
on my work bench.  I believe much of his testing distortion came from his
support of the mirror while testing.  Although the interferometer is many
times more sensitive than the common tests that I use.

As each flaw has come to light I have worked on it at my place using a pie
plate for the thin mirror.  More and more heat seems to be the enemy. 
Regular, even half thickness blanks have enough mass to resist the minor
heating that accompanies polishing and figuring.  The ultrathins have no
mass and respond instantly to heat.

A trick I have used on a poorly matched lap, one that should be OK, that
has already been pressed but for some reason skips or jumps a bit, is to
warm it a bit in water and then polish.  That quick dip softens the pitch
just a bit and smooths that last bit of unmatched surface.  That tiny bit
of warmth will distort the glass on contact and start an astigmatism. 
Small laps warm the glass where they are, but no where else, and cause
astigmatism.  Figuring has become a challenge.

I think the foam buttons are a good idea, that was why I used them.  There
is too much variation in the COE of glass and metal for a standard cell to
work with ultrathin glass.  That which touches the glass has to be dead to
heat and cold.  At least that is my opinion.  Cell design options will
need to be looked at also.  PLOP designs may not be the best approach.

I have made a screw adjusted platform for the ronchi testers.  This will
allow me to dial in the offset instead of the usual way I test.  Once the
16 inch is back  I will run a full set of ronchi tests on my test stand
and compare them with what Dale found using his.  They will all be posted
here.

David Davis
Toledo, OR 97391

_______________________________________________
ATM mailing list http://www.atmlist.net/