Re: [ATM] Meade 'Fesses up

["vladimir sacek" <vlad@copper.net>]

Hi John,

> I guess I was thinking of the classic Cass which uses a >parabola for a 
> primary (or do I have that wrong too? - >all my books are in storage, life 
> has not been fun lately), >and thinking that the commercial SCT's do the 
> same.

Sorry to hear that things not quite right; hope will turn better soon. I 
seem to be having problem with sending email: as much as I can trust my 
senses and memory, I changed the address when replying, but it somehow ended 
up sent to the original address (a couple of times).

You are right, classical Cassegrain does have parabolic primary, and its 
secondary has to have negative conic as determined by -(m+1)/(m-1) in order 
not contribute any spherical aberration ("m" is secondary magnification).
SCTs are no different: for the secondary with conic given by the above 
formula, corrector/primary combo has to  imitate parabola in order to keep 
(third order) spherical aberration zero. With lower absolute secondary 
conics, the secondary will become increasingly overrcorrected, requiring 
corrector/primary to act as an oblate ellipsoid. For higher absolute conic 
values, secondary will be inducing increasing undercorrection, requiring 
corrector/primary to act as a hyperbola.

It is nice to have anything perfect, but I'm glad I'm not as picky 
optics-wise as your friend :)

Have (more) fun,

Vlad



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