RE: ATM Why?

[Andy Saulietis <iss@pvtnetworks.net>]

Richard writes,

>If I remember correctly, the field curvature is determined by the curvatures
>of the two mirrors in a cass system, not at all by the amount of aspheric
>correction in them.  Why would the field of an R-C be any more curved than
>the field of the equivalent cass or "sub-cass" (as I like to call the D-K)?

I'm not a optical 'guru', but I think the RC's main design advantages of
a large field and fast primary mirror result in sufficient field curvature
to require correction because of the intended use of this design..while
the radius of field curvature is not different the field angle is much larger
(compared to the typical classical cass) thus the depth of the curved field is
also larger. At the f4 and slower primary ratios & fields used by most
ATM's the
field curvatures are very similar (classical vs RC) so IMHO there would be
no advantage for one design over the other except for the difficulty of 
testing & figuring. The 2.4m SDSS scope has a field diameter of ~24" &
would not be possible in a classical cass, IMHO the main reason for going to
the RC design is the increased field size.

Another consideration for an RC is that, with a 'fast' primary (<f~2.5) the
secondary size is larger and sky baffling becomes more difficult, making
the visual use of these instruments less desirable.

I do agree that Ritchey was unjustly treated due to the optical
turf wars of that era..:)

 

Andy Saulietis
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