Re: Testing old mirrors

paul valleli

Remember that James Short was making Gregorian Telescopes of a crude nature before there
was a full understanding of the properties of the Gregorian Configuration.
If he was matching secondary mirrors to the primary, then your interferograms will have limited value.
The best you could do is relate the Primary surface figure to the desired prolate spheroid and then 
either make a Null Test setup for Conjugate foci or just report the shape of the Primary as related 
to a spherical reference at the Center of Curvature. Diameter and Radius of Curvature could be reported
but results of star imaging could be very poor if the mirrors are mis-spaced compared to a nominal design.
A missing secondary could mess up the imaging performance or introduce erroneous Spherical Aberration.
Personally, I would record a set of interferograms for each Primary and RoC, Best Fitting Conic, and 
amount of Astigmatism if it is a property of the test setup.
What to do, if the mirrors are flexing as mounted? That is where the stand-alone interferograms provide
a reference point, depending on gravity caused distortion, vs. mounting problems. 
To keep this analysis as practical evidence of Short's work would require amassing a large, but 
incomplete, database for each mirror. 
It might be reasonable just to provide the serialized interferograms with a manual profile reduction. Same for
the secondary mirrors. 
Is a convex test plate available ?
Interferograms of Secondary surfaces, made to avoid making erroneous measurements at incorrect 
spacings could simplify the analysis.
For the cases where the complete telescope is available, a qualitative evaluation of image quality at 
intended range and at infinity could help define relative quality.
In my opinion, the surface profiles obtained as topographic surface maps would be most meaningful.
Paul V. 

On Fri, May 7, 2021 at 10:36 PM Yuri via <> wrote:
Dale and All,
I have to put this project on hold. Just need to study more about testing of a non-spherical mirrors.

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