Re: [ATM] Mirror Cell Question

[Mark Holm <mdholm@telerama.com>]

Jeff told you the dimensions that we need in order to design a flotation 
system for your mirror.  The most critical one is edge thickness.  Try 
to get that to a millimeter accuracy.  While you are at the mirror with 
a ruler, might as well get the diameter to a millimeter or so also.  For 
the diagonal diameter, measure across the narrow direction of the 
ellipse.  Focal length is the least critical.  Measure roughly from the 
front of the mirror to the diagonal and from the diagonal to just inside 
the focuser.  An inch one way or the other on the FL won't make much 
difference.  For glass type, we may be able to guess if you can't find 
out.  If for instance the thickness to diameter ratio is 1:6, then Pyrex 
is a good guess, especially if it is a couple of decades old..  Most 
Pyrex blanks were made with that ratio.

While you are looking at the mirror, examine the back side closely.  It 
may be possible to guess the glass type based on the appearance of the 
back.  If it is shiny, like window glass, all over, then it is more 
likely plate glass.  If there is a pattern that looks like it came from 
the inside of a mold, then it is probably Pyrex.  If it is ground all 
over, then we will have trouble guessing.  If we can't guess the glass 
type, we will design in between the two sets of properties and be a bit 
more conservative in design choices to make the design more error tolerant.

Plop will let us design a layout for the back supports that will give 
acceptably low residual deformation.

Edge support is actually a somewhat more demanding problem.  Jeff, I 
think, overlooked the fact that your scope is on an equatorial mount.  
You won't be able to use a simple sling.  If you have some modest 
machine tools available, mostly a drill press, the list can probably 
help you design a cell structure that will let you glue the mirror to 
the rear supports with silicone.  Then no edge supports are needed.  
This has worked quite well for several atms.  The risk of silicone 
letting loose appears to be small, but non zero.  If you go the glue 
route, you will probably want some safety clips in front and on the 
sides.  They don't actually need to touch the mirror.  They just keep it 
from going too far if the glue lets go.

Ordinary mirror clips work the same way.  They are just there to keep 
the mirror from tumbling out if it goes face down for some reason.  A 
millimeter gap between mirror and clips is plenty.

If gluing doesn't appeal, then you will need to have some kind of edge 
support.  Dobsonian builders are spoiled because the mirror only needs 
edge support in one direction.  With an equatorial mount, you need to 
support in all directions.  On a 12.5 inch full thickness mirror, list 
members should be able to point you toward a decent edge support scheme 
that will not be too complicated.

Mark Holm
mdholm@telerama.com

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