[ATM] Re: Aplantic Gregorian

[vladimir sacek <vladis.2@juno.com>]

Hi Mauritz,

"Yes one can think of such ("compensating") designs if one gives up the
coma-free condition. One can lower the conic constant of the primary to
get to a reasonable null focal distance..."

If one is willing to let more coma in, Dall-Kirkham is hard to beat.  
A 200mm f/11 DK with f/4.5 primary and same minimum secondary
size (33% in Cassegrain arrangement) would have about as long tube 
as the Gregorian with f/2.25 primary, only with -0.61 primary conic, 
spherical secondary and 1/3 of coma. 

Gregorian with spherical secondary and similar amount of coma is 
also possible, but impractical. It requires larger secondary and 
hyperbolical primary.

Btw, thanks for posting those foci figures; it made me realize  that I 
was using those formulas in some weird way - don't ask me why -
and coming up with incorrect (greater) foci distances.

Vlad
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