Re: [ATM] 25" mirror possibilities

[Jan Bentz <jbentz@inreach.com>]

Dennis -- Just something to keep in mind if you try to quantify the 
figure on the mirror is that in a Wright the figure on the mirror and 
the depth of correction on the corrector are a function of the 
separation of corrector and mirror, with k = +1 for a separation of R/2 
for the mirror.  k being 0 for a sphere and -1 for a parabola.

Jan Bentz

dmikk2@netscape.net wrote:

>  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.   
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