Re: [ATM] Entrance, exit pupils-

[Alexis Cousein <al@sgi.com>]

Dominic-Luc Webb wrote:
> On Wed, 16 Apr 2008, David Harbour wrote:
> 
>> For an interesting discussion of entrance and exit pupils, with some very
>> nice hand drawn illustrations, go here:
>>
>> http://www.atmsite.org/contrib/Harbour/StoppingDown.html
> 
> Yup! And this is essntially what I have done. Does this not
> exemplify an entrance pupil effectively at the aperture stop,

Yes. On the other hand, it's usually not rational to build such
a scope only for a use with that entrance pupil
(it's a waste of aperture, and if you actually plot out
MTF functions contrast is usually lowered on every frequency
of the frequency domain, except perhaps in *very* bad seeing).

The asymmetrical placement of the aperture also leads
to very funky spot sizes and diffraction patterns, which tend
to be worse than those of an equivalent long Newt (which means
you'd usually not *build* a Newt to be used only like this;
the equivalent "classical" Newt with a small mirror and large
f/ratio would have a very small central obstruction and much
better off-axis images.)

I confess I also have a mask like that for my scope - you never
know. But even in bad seeing, hat maks usually doesn't outperform
the full aperture with a suitably designed apodising mask.

-- 
Alexis Cousein                                  al@sgi.com
Senior Systems Engineer/Solutions Architect     SGI/Silicon Graphics
--
<If I have seen further, it is by standing on reference manuals>

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