On Tue, 12 Aug 2003 08:23:24 -0400 (EDT), (G=EMC^2
Glazier) wrote:

Beniot My main point is do we block out planet and star light??
Bert
Sorry, i did not read carefully the first time. The light entering a telescope
is "all mixed up". It is the main mirror ( or the front lens or a refractor
scope ) that "sort it up" to send each photons at the correct place. If you
place a blocking screen "in front" of the telescope, you just reduce the overall
level of light entering the scope. That technique is sometimes used to get more
contrast on bright objects like the Moon.

What we need is to place the screen where the photons are focused and most of
the time it is in the eyepiece. You don't need a circular screen anyway. I've
heard of an amateur who could observe the companion of Sirius ( a white dwarf )
by blocking the light of Sirius with his reticle! In fact, most pros use a
special wire in the focal plane or their scope to do that.

Benoît Morrissette