[Raytrace] Stevich Paul in OSLO LT

[Michael Peck]

At 19:46 04-05-03 -0400, Henry Feinman wrote:
>I have come up with a design that appears quite good  - going by wavefront 
>analysis, peak valley / RMS OPD, spot diagram, MTF, Strehl ratio, but 
>there appears one major problem:  The distortion analysis shows a huge 
>percentage compared to other  designs -350 Distortion % at about 2 mm 
>distant from axis.

I think that distortion graph is probably wrong. Distortion shouldn't scale 
that way, although I suppose strange things can happen in a system with 
tilts. If you look at the estimated Seidel distortion coefficient it's 
fairly small, which I believe it should be.

An interesting feature of the "Stevick-Paul" that isn't at all obvious when 
you set it up in an optical design program is that it isn't really a tilted 
component system at all. What Stevick did was take the basic Paul geometry 
and *rotate* the two spherical mirrors around their respective centers of 
curvature far enough to get them out of the way of the light cone. What you 
end up doing is looking at off-axis light from the primary. The reason that 
works is because the Paul system is an all-reflective Schmidt - its only 
3rd order aberration is Petzval field curvature. I don't know for sure but 
I'd guess that distortion might be asymmetric around the axis at the focal 
plane, but it should be very small.

One thing that bothers me about your design is that if you straighten out 
the mirrors the primary should be exactly paraboloidal. I don't understand 
why you're getting a slightly hyperboloidal primary in your design. Maybe 
someone who understands how to set up tilts in OSLO can comment on that - 
I've never bothered trying to figure out in any detail how to do TCT's.

Mike Peck


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Michael Peck
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