Re: [ATM] Re: 25" mirror possibilities

[Jarvis Krumbein <jkoptic@juno.com>]

It was probably Gene Cross who gave a tent talk at Stellafane sometime
during the mid ?1980s?.   At the time he wasn't talking about a super
fast primary but was going to be content with an f3 or there about.  He
was also not aiming at a spherical primary so that must have come later. 
I started on a similar project in the early 90s when I acquired a
20"diameter 1"thick blank.  Mine is an f3 20" dia. slumped primary and
with the secondary the end result should be f8.  I'm not thinking of a
back pack but instead a DOB that you can get to the eyepiece without
needing a ladder.  A 1" diagonal is more than adequate for this beast. 
The system can easily be made Aplanatic (see Astronomical Optics by Dan
Schroeder) but not with a spherical primary and would be a hell of a lot
easier than the equivalent RC.  If you really want to have fun, make the
secondary as an off axis ellipsoid so that the path from the secondary to
the eyepiece just misses the diagonal and is unobstructed.  I'm still
working on the primary off and on (mostly off) and have hopes of seeing
it done before I die.  What would be nice is a version of PLOP or some
thing similar that can handle slumped glass mirrors.

Jarvis Krumbein


On Sat, 20 Nov 2004 15:07:33 -0500 "Jean-Guy Moreau"
<jgmoreau@cablevision.qc.ca> writes:
>  
> > "Has anyone made a system like this?"
> > 
> > Gene Cross has. He tried to popularize the design back in early 
> '90s. He
> > used very fast primary,
> > close to f/1 (it went under something like "20" backpack 
> telescope"). He
> > was even talking about
> > some sort of "breakthrough" in regard to not having to aspherize 
> primary
> > - if I remember correctly - 
> > but all went silent afterwards.
> 
> 
> Now this is getting interesting, not having to aspherize the 
> primary  :O)
> 
> cross axis Gregorian with tiny diagonal to boot !
> 
> what figure would be the Gregorian secondary then, if we want
> a reasonable configuration with the secondary on one side of the 
> tube
> and the focus on the other side ?
> for example for a 24" f/2 spherical primary, while we are at it, 
> can it be an aplanatic ?
> field curvature ?
> 
> Jean-Guy
> 
> 
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
> 
> 

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