[ATM] repeatable mounting of thin mirror

["Jerry B. Hillman" <JBHillman@ev1.net>]

Hi All,
As for repeatability in the positioning of a thin mirror.  First, the cell
MUST be made accurately.  That is a given. If that is not done, then the
rest has no meaning.
Not a lot of attention has been given wedge, other than it shouldn't be very
great. .010" is nominally considered to be close to the upper limit.
In the case of a ultrathin mirror, whose thickness ratio is exceeding 25 to
1, wedge becomes an important factor in support. If there is no wedge, then
this is irrelevant, but especially with plate glass mirrors, there will be
wedge of some amount.  Now consider the pains we go to in attempting to
support such a mirror.  We consult Plop, and have it design the ultimate
support for this mirror but, unless I am wrong, Plop does not allow for
wedge.  In a situation where tolerances are measured in microns, why do we
ignore the effect that wedge will have on the mirror support?  If there is
so much as a few thousands of an inch of difference in the thickness of the
mirror, then that thicker part will be heavier than any other corresponding
part.
How to compensate?  First, you must determine exactly how thick your mirror
is at the various points around the circumference.  This isn't as hard as it
sounds.  If, as in my case, the mirror is thickest at only one point, then
it only becomes a matter of determining where to position this thickest part
in the mirror cell.  Logic dictates that the heaviest part of the mirror
should be at the lowest point.  In my case, this proved to be true.  If the
mirror has more variance, then this has to be calculated to find a balance
point that will place the heaviest part of the mirror below the Center of
Gravity.
I can not do the math that would prove this. The engineer that explained it
to me didn't attempt to show me the math. But logic alone says it should
work, and it seemed to in my case.  My mirror, when placed according to
these instructions, showed no sign of astigmatism. This will prove or
disprove itself in the long run.  I think it will prove to be true.
I hope this helps anyone that is having trouble supporting a thin mirror.
Clear skies, Jerry



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