Re: ATM armchair theorizing.

["Bob May" <bobmay@nethere.com>]

A lot of philosophical discussion here Charles!
The thing about supporting a mirror in a cell is to try and get it to be
fully relaxed and without stress.  This is why you see all of the mirror
supports basically going down to 3 points of support.  The 3 point support
will always allow for the object to be supported to be stress free as far as
the support is concerned - 4 points of support will cause 3 of them to take
most, if not all, of the support and leave the rest to the last support,
stressing the object being supported.  Thus, the rug support will be a
random support at the best and will be wavy with only a few really supported
areas at the worst.  You need that 3 point support for stressfree support.
The higher number of support systems that are used are all basically stacks
of those 3 point (or its other brother the 2 point bar support at some
points) supports and are allowed to move according to the needs of the
object being supported.
When it comes to the looking sideways, there will indeed come a time when
the glass is mostly supported by a sling or side stresses against the cell
but at those points, the seeing really isn't good anyway so a misalignment
of the glass on the cell really won't matter that much.  I will note that
there is a video sequence of John Dobson kicking the back end of a telescope
of his to "relax" the mirror in its cell.
As to the bending of glass in a vacuum support system, the actual shape is a
catenary curve rather than the parabolic one.  The differences are minor
when applied to the stuff that has been done with the stressed mirrors but
there is a difference.  Schmidt used the curve obtained to inverse the
amount of glass on a surface so that he could get a plate for which his name
is attached.  The difference he calculated was of such a small difference
that he wouldn't be able to see the difference in the images he'd get from
the scope.  The glass, he calculated, would act as a regular simple solid
material and, as you now know, it did.  All basic solids (including good
rubbers) act this way although the amount of deformation may vary by a lot
(you can't make glass stretch out like rubber does!).
Bob May
http://nav.to/bobmay
bobmay@nethere.com
NEW! http://bobmay.astronomy.net