Re: [ATM] Re: Maksutov vs. Schmidt

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

vladimir sacek wrote:

>Richard Snashall wrote:
>
>
>
>>>As Dominic-Luc suggested, the curve needed for Schmidt corrector would
>>>
>be
>
>>>very 
>>>shallow: ~1/25mm at the 0.866 zone, and less than that everywhere else.
>>>Is it really
>>>much harder to make such a slight curve, as opposed to Maksutov
>>>corrector, which also 
>>>requires very tight tolerances?
>>>
> 
>
>>Actually, I get about 1/3 of that (but I do like the shift of the
>>corrector to 2.8 m out quite a bit better)...  but that is
>>equivalent to parabolizing a 500 mm f/2.4 mirror.  If that is
>>within your capability, good.  One historic advantage of the
>>Maksutov has been the spherical surfaces.
>>
>
>That's correct: it is ~1/72mm. But I wouldn't agree on the equivalent
>being parabolizing an f/2.4 mirror. It comes out like that with a
>numerical 
>consideration, but there's more to it than just numbers. Flat surface is 
>easier to aspherize than one that is deeply curved. Moreover, strokes are
>
>much shorter when making the Schmidt curve, than those used for
>parabolizing 
>concave surfaces.
>

However, this is the second time I've seen a symptom.  I know the
theory suggests that putting the neutral zone at root(3/4) "minimizes"
color, as the slope, equal at the 0.5 and 1.0 zones is equal (but opposite
in sign)... however, it stinks!(IMO)  If the neutral zone is  root(1/2),
the RMS wavefront error appears halved.  In fact, in the figured secondary
version you mentioned, the system is nearly over the 0.8 Strehl limit for
entire the i-t band and a +/-21 mm field (too bad it's not flat).

>
>Vlad
>
>  
>

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