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Re: [ATM] Re: Schmidt-Cassegrain
Once more thanks Vlad, the greg you suggest sounds
VERY interesting, and doable within my atm background.
20 to 30 microns is what i get from atmospheric turbulence
anyway, so i think it would make a good camera, and i like
the light baffling possible with this configuration.
Overall f/3 is fantastic !
Think i will go for it. I promise you the first pictures,
but don't hold your breath ;-)
Jean-Guy
----- Original Message -----
From: "vladimir sacek" <vlad@copper.net>
To: <atm@atmlist.net>
Sent: Wednesday, January 12, 2005 11:40 PM
Subject: Re: [ATM] Re: Schmidt-Cassegrain
>
> At 13mm off-axis ~35 micron only from field
> curvature (~1200mm). But the field can be flattened
> with a plano-convex lens with the curved surface radius
> given by R=f(n-1)/n, with "f" being the mirror f.l. and
> "n" the corrector refractive index. The price is introduction
> of some spherical aberration.
>
> If you are willing to go "long", you may consider Gregorian arrangement. In
> general, aplantic Gregorian
> has somewhat better off-axis quality than a comparable
> RC. Primary's conic is lower for lower secondary magnification and larger
> secondary relative size. Secondary conic is always low, about -0.2, give or
> take.
> If you'd accept inaccessible image, secondary magnification can be bellow 2.
> The tube structure doesn't have to go all the way to the secondary, which
> can be
> mounted as an extention. The nice thing about Gregorian
> is that the light bundle diverges towards secondary, so that nearly entire
> interspace between the secondary and
> the image location can be obscured by some sort of
> cilindrical baffle - something that can't be done against
> converging Cassegrain cone. The result is nearly perfect baffling in the
> Gregorian.
>
> At certain combinations of secondary size and magnification, the astigmatic
> field in the Gregorian
> has nearly flat median (best image) surface, so that no
> flattener is necessary. Unfortunately, it requires rather
> large secondary sizes. One with only mildely curved (~4300mm) best field
> surface and not too big secondary would look like this (with D=600mm f/2
> primary):
>
> primary: R=-2400, K=-0.746, 1680mm to secondary
> secondary: R=576mm, K=-0.177
>
> Secondary minimum size is 0.4D and magnification is 1.5, for an f/3 system.
> The astigmatic blur is nearly 40 microns
> 11mm off-axis (gets bellow 25 microns at ~8.5mm off-axis). Assuming ~50%
> obstruction, allowed front detector
> diameter in the light converging from the primary would be less than ~60mm.
>
> With smaller secondaries and higher magnifications, placing the detector
> gets more comfortable, but a plano-convex lens is needed to flatten the
> field.
>
> Vlad
>
>
>
>
>> Vlad, thank you again for your analysis.
>> With all those difficulties and limitations
>> a Schmidt plate is beginning to look attractive ;-)
>> and i could mount the camera directly at the f/2 focus.
>>
>> One last question : with a flat detector 22.7 x 15.1mm,
>> what would be the image blur size at the edge ?
>>
>> Jean-Guy
>>
>> ----- Original Message -----
>> From: <vlad@copper.net>
>> You are right, the three mirror systems,
>>> especially with this fast primary, are sensitive
>>> to anything one can think of. It is hard to
>>> have them "synchronized" on the raytracing board,
>>> let alone real telescope. After taking a closer look
>>> at the (better) three mirror system showed that it
>>> still have more than negligible astigmatism/field
>>> curvature, although its performance is still good, nearly as described. I
>>> checked out simplified form of this arrangement:
>>> two-mirror w/three reflections (concave secondary reflects back onto the
>>> primary, which then focuses
>>> through the secondary). For a flat Petzval aplant,
>>> required parameters are:
>>>
>>> primary/tertiary: R=-2400mm, K=-1.138, 828mm to secondary
>>> secondary: R=-1200, K=-6.806
>>>
>>> Resulting system is an f/4.7, with 15 micron blur 12mm
>>> off-axis on ~600mm curved astigmatic field. It is on par
>>> with the comparable RC, with somewhat easier to make optics (K1~-1.3,
>>> K2~-10 for the RC). It may be possible to adjust
>>> mirror radii so that the best image surface is nearly flat, which
>>> would make it even more viable. It's sort of odd that this
>>> variant hasn't been mentioned.
>>>
>>> But, after all the talk, there's still no "right" answer:
>>> a wide-field corrected arangement with spherical or mildely
>>> aspherized f/2 primary w/o full aperture corrector. As Robin Williams
>>> said on the "Actors Studio" interview: "We're
>>> trying, we're trying..."
>>>
>>> Vlad
>>>
>>> _______________________________________________
>>> ATM mailing list http://www.atmlist.net/
>>>
>>>
>>>
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>>
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>
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