Re: [ATM] Constraints on design

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

Richard wrote:

>Hi Richard,
>
>Wednesday, April 27, 2005, 11:30:44 PM, you wrote:
>
>RFLRS> Actually, only ATMOS indicates that this as a poor design.  Both OSLO
>RFLRS> and ZEMAX indicate visually diffraction-limited performance.  The
>RFLRS> constraints are that the primary is spherical while the secondary
>RFLRS> is parabolic with a BFL of 3/4 aperture.
>
>Yes, I meant primary, sorry, although the cost of materials and
>labour on making that huge optical corrector must outweigh the
>hyperbolic figuring of the mirrors.
>
>As for difraction limited performance, I see a best case
>geometric spot radius of 20.19um and an Airy disk of 13.8um.
>What do you see?
>
>  
>
At 555 nm, the spot size is 10.7 um diameter.

>I'm not meaning to criticise Richard, I just don't understand
>the thinking behind this design. About the only thing that would
>make me consider it is if someone gave me a whopping great disk
>of BK7 for free and even then it would be a difficult decision
>between that and a standard RC. Actually, I'd sell the BK7
>and buy the blanks for the mirrors with it and have a good
>holiday with the change :)
>
>  
>
I have heard it is difficult to make a fast paraboloid.  In a strict
RC design, the primary is [roughly] parabolic.  At 24 inch diameter
f/2.35, I assumed that might be difficult.  The secondary is
now also on that order (f/2.93), but only 6 inches in diameter.
Given that it is also parabolic, it could also be somewhat easier
than the corresponding RC hyperbolic.

The corrector would presumably be done with the vacuum method.

Given all this, some might consider that this design might be easier
than the strict RC; don't know for myself, though... but from the
buzz on this list, it sounded reasonable to me.  From what you're
saying, though, maybe not.
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