Re: [ATM] Entrance, exit pupils-

[Alexis Cousein <al@sgi.com>]

David Harbour wrote:
>> 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).
> 
> I guess you didn't read my article (whoever answered Dominic's question with 
> "yes".

Oh, I did.

> However, the end of the tube of a Newtonian is not the entrance 
> pupil; the primary mirror is; 

In the case of your stopped down aperture (at the front of the tube),
it most certainly is. And that's why the off-axis spot diagrams and
diffraction patterns are so different from those in a Newt, and different
from those of a Newt with an off-axis mask right on top of the mirror,
precisely because the entry pupil is somewhere else.

 > Guys with big Dobs don't need one either, for looking at little smudges
 > of light at very low magnifications.
> (Just pinprickin' you guys; not a genuine insult- 

But quite a misunderstanding anyway - if you presume Dob users don't look
at planets, think again. Mars looks just fine at 720x in a large Dob
when the seeing allows it, and better than in many dedicated "planetary"
instruments.


-- 
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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