Re: [ATM] two mirror folded Newtonian

["vladimir sacek" <vla@toast.net>]

David Harbour wrote:


> One time, I talked to Thomas Cave about a Classical Cassegrain; he said
> there was no justification in building a Cassegrain, which is more twitchy
> with the amplification requuired by a convex secondary;

That is probably what most people think, but doesn't seem to be justified.
As long as we compare same F# primaries for both, Cassegrain and Newtonian,
there is no considerable difference in miscollimation sensitivity between 
the two.
For instance, 1mm decenter/despace of the diagonal in a 16"  f/4 Newtonian
will result in nearly 1/10 wave RMS of center-field coma, about as much as
0.1 degree diagonal tilt error. On the other hand, same amount of secondary
miscollimation in a 16" f/4/14 classicial Cassegrain will result in about 
1/11
wave RMS and 1/16 wave RMS of center-field coma, respectively
(the despace error wouldn't cause any appreciable error in the Cassegrain).

Vlad 

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