Re: [ATM] Anastigmat

["Richard F.L.R. Snashall" <rflrs@rcn.com>]

Richard wrote:
> Hi Richard,
>
> Thursday, December 6, 2007, 9:17:28 AM, you wrote:
>
> RFLRS> Richard F.L.R. Snashall wrote:
>>> yes, it only works for this diameter at
>>> f/7.863.
> RFLRS> That may not necessarily be the case.
>
> Wouldn't it be more correct to say "it only works at approximately
> f/7.8". Diameter shouldn't enter into, as you should be able to prove
> by scaling the whole design in your optics design program.
>
Not really.  Astigmatism is corrected in the field, with an
S-T crossover usually somewhere just inside the edge.  The
field diameter will not scale with the 'scope.  The maximum
field, in degrees, will vary inversely with the 'scope size.
The net residual astigmatism will also decrease as the 'scope
gets larger, since it typically varies as the cube (?) of
the field.
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