From: "Stathis Kafalis" 
To: "ATM" 
Reply-To: "Stathis Kafalis" 


Hello optic design friends,

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? This way reasonable magnifications would be possible
and the eyepiece would not have such a hard job to deliver good images.

- How could such a corrector look like? May be a 3 lens design similar to the
photographically optimized Wynne corrector?
- How far away from focus it has to be positioned? This will lead to the
necessary diameter to illuminate a 31 mm Nagler field.
- Can a corrector in front of the secondary be an alternative? This way the
secondary could be made much smaller (~6" mm instead of ~8")

I think, it would be sufficient, if off axis aberrations are no worse than for an
f/5 Newtonian. Though unlike a photographical design it has to be really sharp on
the axis. I am not firm enough to calculate such a design with OSLO myself.

The background of my question is, to avoid the tall ladder that would be
needed with a 32" f/4.5 or f/5 Dobsonian.

Any ideas?

Stathis Kafalis
http://www.geocities.com/dobsonstathis

--- BBBS/NT v4.00 MP