Re: ATM 3" 75mm or 4" 100mm eyepieces

[epabcc@epa.ericsson.se (Bratislav Curcic)]

>  Say you had a 10" diameter 245mm eyepiece giving a mag of 18X and exit pupil
>  of 30mm!  What would the view look like here with a Kelner?

This one is fairly easy to check yoursef. Get a ~50mm f.l eyepiece and
put it behind an f/1.4-1.8 camera lens. The difference between what you
see in that arrangement and what you would (or wouldn't, to be more
precise) in your Newtonian with 250mm f.l. eyepiece is that central
obstruction will be big enough to COMPLETELY cover your eye's pupil,
i.e. you will see nothing.

If you want wide field, use telescopes that are designed for that.
A short focal ratio Jones Bird, Wright or APO refractor with say 
20mm Nagler is the way to go.

If you use 100mm focal length eyepiece on an 22" f/8.5 instrument,
having a 5mm eye pupil (more or less average) you'll be using only ~9
inches of your mirror. Considering that your secondary is around 6" (i.e.
you'll be using only a 1.5" thin "anulus"), you will have greater field 
of view, better contrast and better images with a 4-6" instrument and 
an "ordinary" 2" eyepiece. Not to mention a pleasure of sitting low in the 
comfy chair instead of doing a balacing act 10 feet above the ground !

B.