Re: ATM 11.2" F/2 Primary testing questions :)

[Aplanatic@aol.com]

Hi James,

>Schmidt Camera Positive comments:
> #1 No coma
> #2 Easier to Fabricate?

If you make a good Schmidt camera you can use it with a field flattener for larger CCD imagers in the future, or for some great medium format film images with a curved film holder.  An uncorrected f/2 paraboloid will always be limited in field size due to coma and is really hard to correct over a decent field using lenses.  Something to consider.  

Easier to fabricate?  I personally would not attempt to figure an f/2 paraboloid.  Using the vacuum method of Schmidt plate fabrication, the correction is placed on the plate by grinding and polishing a very long radius sphere.  Not hard at all.  The correction is fine tuned by changing the deflection of the plate slightly and polishing some more.  I consider this method one of the most elegant ways of making a high quality optical system.  

There are interesting subtlies in making a good plate.  If you decide to do so, please contact me.

The only drawback is making the vacuum pan.  I happen to have one for an 11.25" clear aperture corrector.  Where are you located?  I might be willing to loan the pan to you if you decide to make a Schmidt.


> Schmidt Camera Problems:
> #1 Have to fabricate a corrector plate 
>    (1/4" plate glass would work, yes?)

I've tried plate glass and it's not worth it.  As a test, I ground and polished both sides of a piece of plate glass roughly flat and tested the optical characteristics of the plate by placing it in front of a spherical mirror.  The Foucault test of this situation revealed that the plate contained lots of striae and large regions of inhomogeneous material.  I estimated that the plate had as much as 1/2 wave OPD inhomogeneity, not including the highly refractive but thin regions of striae.

I think it would be much better to spend about $100 on a decent BK7 blank.  See, for instance, Newport's COC1225, a 12.25" diameter corrector blank.

> #2 How to Focus CCD?  (not impossible, 
> but more difficult)

Why more difficult?  You just need a means of moving the CCD longitudinally a wee bit.  A simple, homemade three-screw mounting plate will work.  Same problem with the parabolic primary.  If you use a truss structure to hold the corrector then access to the focusing mechanism is easily achieved in such a design.

 #3 Longer OTA (50" vs 25")

Yes, optics is a tradeoff. That's the price that we pay for a fast aplanatic system.

Dave Rowe