To: "Stathis Kafalis" 
From: Mark Suchting 
Cc: atm{at}shore.net
Reply-To: Mark Suchting 


At 08:04 PM 4/29/2003 +0200, you wrote:

provided one can figure a nice 32" f/3 parabola. Is it possible to
make a coma corrector optimized for visual use that entends the focal
length to let's say f/5 at the same time?


Hello Stathis

A simple solution might be to use a Televue Big Barlow, and just move the
lens spacing closer to the eyepiece for say 1.5X or less magnification.  I
have mounted a 2X barlow in my F4 Astroscan permanently in the drawtube for
1.5X and I now have pinpoint stars to the deg with a 32mm 1.25" Erfle
EP.

Some of those guys building the big fast ( F3.3?) dobs in France  seem to
get great results with the Televue Paracorr even with F3 mirror. Maybe a
combination of Big Barlow and Paracorr will get you to F5 and pinpoint star
images.

I'd recommend looking at folding the light path back down at 45 degrees
with a 33% diameter folding flat too. That way you can get similar tube
length of an F3 , but using an f4.5 mirror.   The 15 wave asphericity of
the F4.5 will be much easier to achieve than the 54 waves of an F3.

~Mark

--- BBBS/NT v4.00 MP