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

[Mark Suchting <masuch@dap.CSIRO.AU>]

Bratislav wrote:

> >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.

I don't think this is quite right. You may not put your eye directly in
the center of the exit pupil, but moving your eye around you will sample
the unobstructed portions of the exit pupil. This is the effect Dobson
described when he found the view of M57 to be no brighter in a 60" scope (
fitted with much too low magnifivcation )on a public open night, than in
one of his 18 inchers. He described moving his head around to avoid the
obstruction.

Perhaps the effect could be examined by putting a paper disc on a camera 
lens with say a .33 ratio obstruction and an 18mm pupil, or something to 
that effect. i'm curious....

Mark