From: "Richard Schwartz" 
To: 
Reply-To: "Richard Schwartz" 


And to carry your argument further, a refractor with NO central obstruction
might do even better.   But even this can be improved on by APODIZING:
instead of a sharp chop off of the aperture at the edge, have a gradual
tapering.  That will take light out of the first ring and put it into the
center (while making the center slightly larger).  The result should be
improved ability to see low contrast dark features on a bright background
at the highest possible powers.

Why do you think they have refractors and shiefspieglers and yolos and
solanos?   But for ordinary observations at low power, none of this
matters.

John, when are you going to call me so we can get together and argue?

. . . Richard

----- Original Message -----
From: "John Sherman" 
To: 
Sent: Monday, March 03, 2003 6:40 PM Subject: Re: ATM Diffraction due to
secondary sizing


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> Hi Marco,
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> >Here is the problem though. Commercial telescopes of the cassegrain
veriety
> >often have central obstructions approaching 37% by diameter which
Texereau
> >may well have baulked at the idea of. There is however no question about
> >these telescopes' ability to resolve very fine detail on the planets.
They
> >are in fact among the top performers.
>
> I disagree strongly!!!
>
> Where'd you come up with such a statement?
>
> I dare you to come to Los Angeles and set any 8" Cass next to my
8" f/8
> Newt in excellent seeing.  (primary mirror by Richard Fagin, tube assy
> by Ed Grissom)    Have you ever seen an 8" Cass that shows detail on
> Ganymede, and resolves Titan into a disc? I haven't.
>
> If you want a hi-res scope you should have less than 17% obstruction,
> in my opinion.
>
> When you add light to the diffraction rings, the diffraction disc becomes
> smaller, not larger. And yes, the diffraction does damage your view of a
> planet.
>
> John
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--- BBBS/NT v4.00 MP