Re: [ATM] Herschelian MMT (neat idea?)

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

Mike Carambat wrote: 

> What about using (2) 6" F12 Herschelian scopes ...

Too much aberrations. The center-field aberrations
caused by mirror tilt are astigmatism:
W=(Dt^2)/8F p-v wavefront error,
and coma:
W=tD/32F^2 also p-v wavefront error
(D is the mirror diameter, F =f.l./D and "t" is the tilt angle).

The tilt angle "t" is 1/4F when the focus coincides with the incoming 
marginal ray, and larger for the focus located out 
of the incoming light. Even taking t=1/4F gives 1.25 wave p-v of 
coma and astigmatism (each) for a 6" f/12.

A 6" Herschelian needs to be slower than f/20 for acceptable 
center-field quality.

Vlad
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