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Re: [ATM] Focal reducers

Paul Bradley wrote:

>... if one were to use a focal reducer with a fast (f5) 20" newtonian by 
>moving the
> mirror forward, would aberrations coma etc. be within reasonable bounds 
> across the whole field of a 2/3" ccd?
> I presume coma could become unacceptable because you'd be working at f/2.5 
> or faster...

Reducer won't mimic faster mirror coma-wise. Unless is induces its own 
significant coma
(unlikely), it will actually reduce linear coma for given angular field, in 
this case by a
factor of ~2. This means that at the edge of the detector linear coma size
wouldn't change  (3*8/16F^2 = 0.06mm for the full comatic blur length).

However, variuos reducers have various amounts of astigmatism and field 
curvature,
(chromatic and spherical too) and they can be significant. It is advisable
to consider how well any particular reducer would match the Newtonian field.

Vlad 

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