RE: [ATM] 25" mirror possibilities

["Scott Milligan" <starzkey@charter.net>]

  I recently obtained a large (25" full thickness) mirror on Ebay.
(Aperture fever struck.)  This is a unique piece, and I would really
appreciate some ideas from other atm'ers as to the best way to put it to
use.  

  

Dennis:

Short of reconstructing the Wright-Schmidt corrector plate and using as is
(a time consuming and/or expensive proposition), I suggest one of two
possible uses for large, thick glass:

1: Make into a flat, and use the flat to test other optics.  Unfortunately,
this option doesn't make much sense if all you want is one, large telescope.

2. Pay someone to lightweight the substrate by generating a conical profile
on the back and coring a hole through the center.  Then, refigure the
optical surface into whatever kind of 'scope you are looking for, and use
the perforation in a center mount concept.

A full thickness blank can be an asset to an optical shop, since the
thickness will help minimize self-weight deflection when mounted in the test
stand: therefore, full thickness blanks of moderate aperture make excellent
flats when used in a temperature stabilized environment.

The extra thermal mass, however, is a drawback when used in a telescope,
since it will take MUCH longer to come to thermal equilibrium as the ambient
temperature drops around it.  The result; a turbulent layer of air near the
mirror surface is always present, preventing the 'scope from attaining it's
theoretical resolution and contrast transfer.  Of course, this problem could
perhaps be adequately managed by installing the scope in a permanent
observatory, with some degree of temperature control inside the building.

A third option is to ignore all of the above, wing it, and put it into a
telescope.  But my recommendation would be to not spend a lot of money doing
that, until I had discovered whether or not the thermal lag problem is
significant.

Scott Milligan



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