[ATM] Re: Fast primary unobstructed telescope

[vladimir sacek <vladis.2@juno.com>]

Kevin Whitefire wrote:

>I have a primary that's 13.25" in diameter with f/D of
3.125, give or take a little.  Its exact ratio isn't
closer than a quarter inch accuracy, using a solar
focus on a white surface.  Its surface figure is
smooth, slightly zonal, and hyperbolic in general.  So
far, I have intent to use it as a Gregorian if I can't
get an unobstructed design for it.<

I will echo Jarvis. The closest you can get to 
unobstructed is folded Gregorian. But this primary 
is still somewhat slow, and would require field
restriction. In this particular case, you would 
have nearly 50% c.obstruction for 1 degree 
field diameter. C. obstruction would diminish
in proportion with the flat size, but so would 
field diameter.

Hyperboloidal primary can work with spherical 
secondary in Gregorian arrangement, with corrected 
spherical aberration and similar 
amount of coma to a comparable Dall Kirkham.
Advantage of folded design is that it allows low
secondary magnification, keeping the coma at the
possible minimum. 
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