"Chuck" wrote in message ...

That brings up a related question. If you can make out the central
obstruction in the daytime with, say a 7mm exit pupil, It's still there at
night decreasing contrast right? It's just harder to see. What exit pupil
shouldn't you go above with a 33% CO if one doesn't want contrast decreased
on axis?

That's a very interesting question; I'd never thought of it that way.
After thinking about it for a while, I've concluded that the answer
probably varies from one individual to another.

If the eye were optically perfect, then it would be easy to compute
the loss of contrast due to the magnified CO. The way I figure it,
if your eye opens to 7mm and the central 2.5mm of the light cylinder
coming out of the eyepiece is obstructed, it is just like a
2.5/7 = 35% CO in a 7mm telescope.

However, hardly anybody can resolve anywhere near the theoretical
limit of a 7mm telescope. If a 7mm eye were optically perfect, we
would be able to split a 0.3' double naked-eye, but in fact very
few people can split even a 2' double and fewer than 50% can split
the Double-Double (4'). My own limit is around 6'.

OK, so why are we so far from perfect? Insofar as it's due to the
granularity of the retina, it won't be affected by CO at all.
Insofar as it's due to imperfections in the eye's lens, CO will
have a particularly deleterious effect, because the CO masks the
part of the eye lens with the best resolution -- the part that's
used for daytime viewing -- leaving only the sloppy outer portion
that's used under low-light conditions.

In practice, I bet that the effect of the CO for low-power images
is negligible for almost everyone up to a CO of 3mm or more. And
if you *really* wanted to see high contrast in small features,
you would increase the magnification, rendering the whole question
irrelevant.

- Tony Flanders