[ATM] 25" mirror possibilities

[dmikk2@netscape.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.  

  The mirror is from an 18" aperture wide field collimator manufactured by Tinsley Laboratories for Boeing in the 1960's.  The collimator was a modified Wright-Schmidt design, with a 5" flat field.  I only have the mirror, not the other optics from the collimator.  (The Schmidt corrector plate was won by another bidder.  Also, the optical tube for the collimator was a closed tube, from which the air was pumped out to form a vacuum.  Consequently, the corrector plate was designed to deform slightly and "be right" when one side was in a vacuum.  I did not think that that would ever be practical for an astronomical telescope.)  The entire collimator was 13.5 feet long and weighed over 3000 lbs, before it was "parted out".

  I just recently received the mirror and have not had a chance to test it carefully.  (It's not easy to "safely" handle a 180 lbs piece of glass.)  The mirror surface is in excellent shape, since it spent most of it's life in a sealed tube.  I believe that the mirror's "figure" is an oblate ellipsoid.  I'm gueesing that the focal ratio is about f6, though I have not had a chance to make a test stand and measure it.

  So...how can I make a telescope out of this?  The only options I see are:

1. Refigure it into a paraboloid.  I would need to pay to have this done, since I'm sure I can't handle 180 lbs of glass.  For example, Galaxy Optics lists a refiguring and recoating service for about $2000.  I haven't asked them if they would do that much refiguring on a full thickness mirror for that price, so it might be much more expensive.  This would work, but, it seems like a shame to take what is probably a very high accuracy optical surface and destroy it.  Any suggestions as to what companies might do this sort of work would also be appreciated.

2. Try to use it as is.  Is it possible to construct a corrector lens that would go near the focal plane for an oblate ellipsoid?  I had originally thought that a Dall-Kirkham design might work, but that requires a prolate ellipsoid.  As I understand it the oblate ellipsoid actually has worse spherical abberation than a sphere, whereas a prolate ellipsoid is part way to a parabola.    

  I did contact Tinsley Laboratories to find information on the mirror, but they did not have any info on projects more than ten years old.  I'm not even sure what material the mirror is made of.  If any one knows of a good way to determine the substrate, or can direct me to references for testing oblate ellipsoids, I would appreciate the info.  This should at least be an interesting learning experience.  Thanks in advance for any info and/or suggestions.

Regards,
Dennis

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