[ATM] If it's in a supremely stiff cell, that does not twist,why would a thin mirror sag?

[bobmay at nethere.com (Bob May)]

If you place a mirror against a sort of flat surface (the carbon fiber
surface) and glue the two together you are in deep trouble as the two
materials (the glass and the carbon fiber plate) will expand differently and
you will get a change of ROC as a result at the least.
Place the two pieces together and don't glue them together, the surface
shape will change as the temp rises and falls and that will cause a problem
with the shape of the mirror.
Basically, you have to provide a 3 point base to the supports so that the
mirror substrate itself controls the shape of the surface and this also
means that you have to control the temp variations on both the top and
bottom of the glass or the difference in temps will cause a change in the
surface shape.
Bob May
http://nav.to/bobmay
bobmay@nethere.com
NEW! http://bobmay.astronomy.net