David Knisely wrote:
Jerry Warner wrote:
I think I was told once its Gregorian - note the long tube for the 4' ap at f/6.
Only the Gregorians had such low focal lengths - ???
jerry
I meant long not low! sorry.
I think that some of them did have somewhat shorter focal lengths. The
Gregorian might have been easier to make, since it used only concave optics
and these were easier to test than convex surfaces.
My understanding also David.
I recall a club program
put on by Dr. Eugene Rudd of UNL (Antique Telescope Society) who brought
several of these wonderful brass instruments to our meeting. One was a 3 or 4
inch Gregorian, and although the instrument looked wonderful, I understand
that the performance wasn't all that hot.
again my understanding also for most of these old optics, but it shouldnt be any
surprise. I was also told once that it had something to do with the way they ground
optics and the whole theory of optical fabrication in former times - that the older
fabricators thought concave surfaces were easier to grind and control for some reason?
Gregs are rare now in the amateur market. Lomo used to make a small one?
Run the specs. Secondaries get very wide for a given ap as the fl decreases so
you quickly lose aperture in order to gain say f/6, f/5, etc - designers today would
not accept that tradeoff. Dr. Stritmatter at Tucson had a 16" f/10 gergorian that
Ed Plamondon had made for him - Ed said it was a "great performer". Ed thought
about building several more for amateur use but then changed his mind, building
three classical cass. instead (all went to universities).
To me at least the greg design has always been interesting. I wouldnt be surprised
to see it come back in some form ???
Thanks,
Jerry
Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/
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