16.25” Cassegrain design for several F/ratios

Please read all of this carefully!  I have updated the designs as of 2pm on Oct. 13.

Recent developments:
Mike Conron came by recently to check out my interference tester, and I asked him to do a Focault test of the Cass primary while he was there.  He obtained results very close to mine, confirming the obvious undercorrection of the primary with respect to its parabolic ideal.  Therefore, the only conclusion is that the primary was fabricated improperly.  I will not mince words here - the surface of the primary is smooth, but the wavefront error is around 2/3 wave, which I call unacceptable, and this is especially bad for the primary of a compound scope.

We now know that the original design was a Classical Cassegrain, thanks to the confirmation of several members, and the examination of reports written in 1993.
Therefore, I'm working out possible designs for the scope using the Classical Cass design, as documented in Texereau's "How to Make a Telescope".
The design dimensions would also work with the Dall-Kirkham design, but we would likely need to use F/15 or higher for the F/ratio.

Below I have listed the results of running the numbers for the design of a Cassegrain telescope of varying F/ratio.  The three quantities listed at the top of the results below are enough to start the design as long as you have an F/ratio in mind.  I ran the design for F/10, F/12.5, F/15 and F/18.  Results were originally calculated with a calculator, and then checked with a quick computer program that I wrote.

My opinion:  (This is my personal opinion.  Others' opinions may vary, and I would love to hear any input from them.)
My system of choice is F/12.5 Classical Cassegrain, the first design in red below.  I believe this is a good tradeoff for deep sky and lunar observing vs. planetary observing, and a good choice for a telescope that will be used by club members.

I am not in favor of the Dall-Kirkham design since it will have more off-axis abberations, and will probably need to be of higher F/ratio, such as F/16 or greater.  Based on recent developments, the scope was not intended to be of this design.  I believe that an F/12.5 instrument will be more useful than a higher-power version to the general membership.
The central obstruction for an F/12.5 design is quite manageable at ~30%.  The low power view is 83 X with our club-owned 55mm plossl, which will yield a reasonably wide field of view for larger objects, like the moon and large deep sky objects.  The scope can easily go to high power.  The other calculated parameters are reasonable.  The F/15 design is another alternative, with smaller central obstruction and a lowest power of 100 X.

Interestingly, the deviation from a sphere of the convex hyperboloid secondary is not much less for the F/15 system.  So, I am not at all inclined to lengthen the system due to difficulty of figuring the secondary, because it simply won't make it much easier.  (For comparison, my first mirror, and 8" F/3.9, deviated from a sphere by about 40 microinches.)

A backup plan:
   For completeness, I have obtained a quote for such a secondary mirror from a fine optician, Dick Wessling, of Milford, Ohio.  That quote was $900, including coating.  So, after refiguring the primary if I were for some reason unable to complete the secondary, the club could obtain a high-quality secondary mirror to complete the system.  Dick is my optical mentor, a first-rate optician, and the author of several articles in Telescope Making magazine.  He's also the instrument chair for ALPO.  Dick noted that the classical design would be preferable to a Dall-Kirkham, particularly if Naglers were used in the scope.

Necessary changes after the optics are completed:
I am volunteering to fix the optics for the scope because I enjoy working on optics, because I wanted to make a Classical Cassegrain, and because I believe having good optics available will jump start efforts to make the telescope more user-friendly.  For me, the first two steps in this process are obvious, and necessary:
Bill Cochran asked us to contact him if we ever want to install solar panels and batteries at the dome to power the fans or a clock drive.

Here are the design details.  Any of the systems below will fit into the current tube, and are designed with the existing mirror cell in mind.  FIF means "Fully Illuminated Field", and 0.6" is a reasonable size for visual use.

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Cassegrain Design for CUAS for varying F/ratio

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  Primary diameter:                16.25"
  Primary focal length:            61.13"
  Focal plane to primary surface:  14.75"
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Design for F/10.0
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Focal length:                     162.5",  4128 mm
Lowest power (w/55mm plossl):     66.6 X
Secondary power (gamma):          2.66  
Secondary ROC:                    66.50"
Secondary diameter, 0.6" FIF:     5.74"
Percent obstruction:              35.3 %
Secondary conic constant (b2):   -4.9
Secondary deviation from sphere: -29.9 microinches
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Design for F/18.0
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Focal length:                     292.5",  7430 mm
Lowest power (w/55mm plossl):     119.9 X
Secondary power (gamma):          4.78  
Secondary ROC:                    33.17"
Secondary diameter, 0.6" FIF:     3.61"
Percent obstruction:              22.2 %
Secondary conic constant (b2):   -2.3
Secondary deviation from sphere: -18.5 microinches
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