Re: [ATM] Aplantic folded Gregorian variety

["vladimir" <svla@isp.com>]

Mike Peck wrote:

> I proposed some folded Gregory designs with similar geometries about 6 
> years ago -- here's the ATM list archive link: 
> <http://astro.umsystem.edu/atm/ARCHIVES/AUG99/msg00093.html>. As specified 
> the design is an all reflective Schmidt: it eliminates 3rd order SA, coma, 
> and astigmatism leaving only Petzval field curvature.

I replied to Mike directly, accidentally, (hopefully forgiven),
so this is mainly a re-post to the right place.

Yes, excellent correction of three major aberration makes Paul's
configuration tempting,
but it just gets too complicated in Gregorian arrangement. Also, it requires
relatively fast parabolic primary. The system with spherical primary has
disadvantage of strong astigmatism in the focal plane, but it is much less 
of disadvantage in
visual use. That goes for use of any positive lens system - reducer 
included -
which should make it viable for photographic applications.
What the two really have in common is their mechanical/collimation
difficulty.

Nice thing about "focal diagonal" is that it is small enough to be made 
easily
interchangeable. That would allow choice of c.obstruction sizes from bellow
20% (small high-power field) to ~50% (wide low-power/photographic field).
If a practical mechanical solution for achieving and maintaining good 
collimation
could be found (a challenge, but not impossible), this kind of system with
easy to make mirrors would become an interesting alternative.

I'll note that an all-reflecting Schmidt alternative (somewhat slower) for
amateurs is Gregorian-Loveday variety, which can also be made
an anastigmatic aplant with moderately
aspherized primary (~-0.4) and strongly
hyperbolic concave secondary (~-6). It is near excellent
over curved field down to about f/4, and still quite useable
with ~10 micron spot at ~f/3 (mostly due to uncorrected
spherical; still not sure whether it is really uncorrectable,
or it is some software glitch).

Vlad

----- Original Message ----- 
From: "Michael Peck" <mpeck1@ix.netcom.com>
To: <atm@atmlist.net>
Sent: Sunday, August 14, 2005 8:50 AM
Subject: Re: [ATM] Aplantic folded Gregorian variety


> At 17:37 8/14/05, vladimir wrote:
>>Played with my favorite 2-mirror design, and come up
>>with folded 3-mirror+diagonal design that uses two
>>spherical (including primary) and one moderately ellipsoidal
>>mirror. The specs for an effective 300mm f/4.15 system are:
>>
>
> I proposed some folded Gregory designs with similar geometries about 6 
> years ago -- here's the ATM list archive link: 
> <http://astro.umsystem.edu/atm/ARCHIVES/AUG99/msg00093.html>. As specified 
> the design is an all reflective Schmidt: it eliminates 3rd order SA, coma, 
> and astigmatism leaving only Petzval field curvature. The original idea 
> was due to Baker (see e.g. figure 3.75 in Wilson vol. I).
>
> Needless to say there's been no rush to build one of these.
>
> Mike Peck
>
>
> _________________
>
> Michael Peck
> email mpeck1@ix.netcom.com
> Wildlife photoblog! http://wildlife-pix.com
> Amateur telescope making 
> http://home.earthlink.net/~mlpeck54/astro/astro.html
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> 

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