From: "Bob May" 
To: "atmlist" 
Reply-To: "Bob May" 


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{at}nethere.com
NEW! http://bobmay.astronomy.net

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