Re: [ATM] Schmidt Cassegrain

[Michael Peck <mpeck1@ix.netcom.com>]

At 10:50 1/9/05, Richard F.L.R. Snashall wrote:


>Michael Peck wrote:
>
>>
>>If you want to try an interesting guessing game how about reverse 
>>engineering Meade's new line of Ritchey-Chretien telescopes 
>><http://www.meade.com/rcx400/>? They claim to correct astigmatism with a 
>>front corrector element, but from the information available it's not 
>>clear if it's a Schmidt like thin plate or a meniscus. I'm guessing it's 
>>a thin plate.
>
>That's this week's guessing game.  But, as you say, with two oblate
>ellipsoids, it can hardly be considered a Ritchey-Chretien; but it
>does remove some of the astigmatism.
>
>The spot diagrams given in that ad are a bit out of scale (typical

Last night I slogged through the relevant section of Wilson (3.4), did some 
number crunching on equation 3.220, and the solution I came up with says 
both mirrors should be oblate! When I do calculations with pencil, paper, 
and my trusty old HP calculator the first two answers I get are usually 
wrong, but OSLO agrees (not that I necessarily trust it either).

This makes me wonder what Meade is up to with their new line of 
"Ritchey-Chretien" telescopes besides maybe some creative marketing. My 
optical design skills are pretty rusty and OSLO's optimization routines 
don't always work, but the best I could come up with starting from an RC 
design and adding a thin plate at the front was a very weakly aspheric 
plate that did some aberration balancing at the cost of adding some 
spherical aberration. Field curvature obviously can't be significantly 
affected with a Schmidt type plate (can it?) so they must have taken some 
creative license with that spot diagram. Assuming typical commercial SCT 
parameters field curvature has to be significant in these things.

Mike Peck


------
Michael Peck
mpeck1@ix.netcom.com

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