Re: [ATM] Flexed Mirrors

[G Gagne <gagne51@clearwire.net>]

The recent posts by Bill Kelley and others have got me interested again 
in my own flexed mirror project. Some time ago, I investigated making a 
light weight 12" telescope. I procurred a 12" x 1" plate glass disc and 
proceeded to grind and polish for a f/5.6 mirror. So far, it is almost 
fully polished. The back has also been fine ground to a 135" radius with 
a maximum wedge of .0011". The required tension for a paraboloid figure 
is 69.7 lbs. or 0.62 psi.

The main problem with a thin 12" flexed mirror is the varying slump with 
altitude which compromises its otherwise perfect figure. I decided to 
apply air pressure to the back while polishing and polish back to a 
sphere. But, how to pressurize the back? First, we need an airtight 
enclosure attached to the mirror back that does not distort the glass in 
any way. Since air pressure must be maintained over the entire mirror 
back, and the mirror front must remain available for polishing, this 
leaves only the mirror edge for attaching the enclosure. For discussion 
purposes only, consider a shallow tin can with the same inside diameter 
as the mirror, slipped over the back of the mirror and cemented to its 
edge with silicone rubber ( RTV). Adler’s Flex.exe program can be used 
to calculate the shear forces in the silicone rubber between the mirror 
and enclosure, which are surprisingly low in a 12" mirror. In this 
instance, the 0.62 psi air pressure results in a shear force of 2.46 
lbs./sq. in, well below the yield strength of RTV. Air pressure to this 
enclosure can be introduced through a small barbed nipple, soldered to 
the can bottom, for connecting an air hose. This air pressure is the 
same value as vacuum applied to the back for flexing the mirror into a 
parabolic shape and will bulge the front surface an equal amount. It 
just remains to polish about 12.6 wavelengths off the front surface back 
to a sphere while maintaining air pressure. I contemplated doing this 
using a water column 16.97" high with a small PVC tank and some clear 
plastic tubing. Testing for a sphere while maintaining air pressure can 
be done by the usual Foucault method. Releasing the air pressure should 
result in a perfect paraboloid and the mirror can now be supported with 
a proper mirror cell. Voila! no more slumping with altitude!

As soon as my current project, adding 'goto' to my 12.25" truss dob 
telescope is finished, I'll have to find time to get back to the flex 
mirror. Meanwhile, I hope this correspondence will stimulate some more 
thinking on flexed mirrors.

Clear skies,
Gil Gagne
Port Orange, FL

_______________________________________________
ATM mailing list http://www.atmlist.net/