re: ATM Building a Cassegarian

[GregJ888@aol.com]

>From: Paulo Manuel Simoes - LEF <pss@students.si.fct.unl.pt>

>Is it possible to build a cassegarian?
>The image quality is the same as the Newtonian?

I hope so, have a 6" in progress.  

It started with the discussion of a  Super-Planetary scope and the desire to
have something more compact than my 8" f4 equitorial.  Last summer at Table
mountain I had a chance to look through several large reflectors ("John Deer"
) and a Tri-S-S.....  The planetary views were very impressive.  Then the
discussions of a reflector with minimal obstruction.  So why not a special
purpose Cassegrain?

Underway is a 6" ~f-14 system.  Primary is f-4, secondary is 1" (17%
obstruction) will likely use a window rather than a spider (waterwhite
plate).  

Comments on the design-
1) the secondary may go down to 3/4" (13%) depending on the performance of
the 1".  3/4" is way below suggestions, but this is an experiment afterall.
2) the mirror perforation will need to be quite small, 3/4" or a little less,
that and the small secondary mean the field of view will be very limited.
3) I had planned on using an f-3 primary but plan to null test the secondary
using a spherical mirror.  This requires a test mirror with the RC =<
FL-primary.  I moved to f-4 so the test mirror could stay at f-2 (fine
grinding now, looks like an ash tray :-).  Per Bratislav's
comments/questions, the Cassegrain is more difficult to fabricate.  Without a
null test for the secondary I see no way to achieve the required performance
and it still may be elusive.
4) The Primary mirror will be fixed (ie moving eyepiece to focus).  Easier to
build and less likely to shift focus. 
5) Looked at a SCT, full aspherical design, looks intersting, but at F-13 to
F-15 did not see an advantage.  With the limited field of view, the whole
image is effectively on axis,  maybe next time...