[ATM] Second Thoughts on Mirror Support

[stuart452 at earthlink.net (Stuart Hutchins)]

To All,

If all the stars look like tiny round dots under high power, and the owner
is happy, then it is a good telescope.

However, one mirror cell that seems to work well does not make a sound
principle for design.  If one inch glass deflects under one pound of force,
then rest assured that 12 gauge steel sheet will look like a trampoline on
the scale used by  photons.  A bent edge that feels stiff is deflecting a
thousand wavelengths.

Internal stresses vary from one piece of formed steel to the next. Under
load and thermal expansion, one piece will change size and shape differently
than the next, and none will match the glass.  The "compliance" of nine RTV
pads just reduces the forces produced from the hundreds of pounds to ten or
one.  It is only a crude approximation to the flotation or whiffle tree
system.  Mainly it is good glue, fast and cheap, and sometimes it may work
OK.

A sound design principle is one that can reliably give predictible and
measurable results.

Stuart Hutchins