[ATM] Folded Gregorian, was 25"mirror possibilities

[Jarvis Krumbein <jkoptic@juno.com>]

Hi Hugues,

The idea of a folded Gregorian was a direct result of the Stellafane tent
talk but didn't really get started until the 20" slumped blank made its
appearance.  The blank started life as a tool for one of John Vogt's 32"
mirrors.  Due to a mix up, the glass was slumped to the focal length
instead of the ROC of the 32" and was not used.  When the 32" was done, I
got the glass with the purpose of doing a folded Gregorian.  Since then,
retirement, a move to the Pacific Northwest from NY, and more years than
I can remember have delayed the project.  I did build a new machine when
I got out west that would easily handle the 20" (and considerably larger)
and have worked at it on and off for the past 10 years.

The f3 was dictated by the slumping and an actual 120" ROC after
grinding.  One of the attractions of the Gregorian was the possibility of
doing an Aplanat that would be easy to null test all the components.  A
final system f8 was to have an optical tube that wouldn't need tall
ladders.  I've been frightened by clinging to the eyepiece while 16'
above the ground while viewing through several 40" DOBs at Table Mountain
over the last ten years.

One of the other intriguing possibilities of this system is to make it
truly multi-functional.  Imagine a f8 folded Gregorian that can converted
to a very fast Newtonian (with the help of a fairly simple corrector lens
system) or to a long focal length Dall-Kirkham.  Once the correction for
the Gregorian primary is arrived at, it should be fairly easy to design
the other elements to match.  I have all the glass to do this and all I
need is a long life to go along.  I'm in my 70s now but my family has
always been long lived so there is hope.

Jarvis



On Mon, 22 Nov 2004 09:56:39 +0100 Hugues.Laroche@ses-astra.com writes:
> Hi Jarvis and others...
> 
> >At the time he wasn't talking about a super
> >fast primary but was going to be content 
> >with an f3 or there about. 
> 
> About 3-4 years ago I had seriously envisaged about this
> setup. 26" f/??, 1" flat secondary, 10" (oversized) f/1.4 tertiary.
> I wisely started with the tertiary (keeping the 26" in reserve
> for a newtonian in case of failure) then finally gave up.
> One day maybe...
> 
> The aplanatic gregorian has the advantage to be allow to null
> test the primary and tertiary. 
> 
> Hugues
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
> 
> 

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