On Sat, 18 Sep 2004 23:44:08 -0600, "Chuck"
wrote:

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?

The thing that counts in this case is the entrance pupil of the eye
rather than the exit pupil of the telescope. In daylight your pupil
is 1 or 2 mm across so the shadow of the secondary may fill or nearly
fill the entrance pupil of the eye, so there is a dark spot. At night
the entrance pupil of the eye is 5 - 7 mm so the shadow has less
effect.