From: "Vladimir Galogaza" 
To: "ATM shore" 
Reply-To: "Vladimir Galogaza" 


While reading the list I have noticed very controversial approaches.
 Depending on context, mirrors are considered to be "infinitively" stiff
or "infinitively" soft.

Examples for the "stiff faction" are abundant  when talk  is
about supporting
the mirror in grinding , polishing and testing circumstances.

"Soft faction" talks emerge while discussing mirror cells.

Here are two recent examples from this thread:

>. If the mirror is supported WITHOUT FLEXURE, and you
>are rotating both the mirror and the tool, as you get rid of the TDE the
astigmatism should clear up.

(This is curiously enough written by "mother of all flexing"  our Bill.)

While Mutalib wrote:

>Can you guarantee that the terry cloth provides equal support to within
1/10
>ounce in all sectors of your mirror?  I think not.

>Your biggest mistake was probably failing to realize how flexible your
glass
>is.   I suspect that if you apply a load by hand to it, you will be able to
>SEE the warping in the reflection of distant objects... no tester needed.
I
>also suspect that the figure will be distorted in worse ways than
>astigmatism.   Remember the fairy story of the princess who was identified
>as such because she complained about the discomfort of a pea under her
>mattress?  That mattress is your glass.  It tends to pass the smallest
lumps
>underneath to the surface.

Amount of flexing which we are talking about is far beyond our
"feeling" or "instinct" about it (based on everyday
experience). Forces we are exerting on mirror while grinding and polishing
are enormous compared to gravity forces
in the cell  both in intensity and "eccentricity". It is miracle
that we can grind the mirror to Raleigh limits or better at all using
simple  ATM methods. Therefore in ATM hands this is more sort of art than
science.

What puzzles me is that our bellowed mirror while in the process of making
and testing is treated as utterly insensitive infinitely stiff princess,
happy to sleep on carpets. But when finished they are instantly transformed
into hypersensitive fairy princesses as
Mutallyb beautifully compared them with in his post.

Regards
Vladimir.

PS
To my knowledge first serious ( quantitative and ATM measurable ) hint that
usual test stands
for mirrors indeed induce deformations ( so commonly ignored) is to be
found in recent
James Burrows post on the latest developement of his Hartman webcam test method .

--- BBBS/NT v4.00 MP