Re: [ATM] Exit pupil and brightness

["Bob May" <rmay@nethere.com>]

Paul, As I see it, you have several errors.
When you're doing a star test, you're seeing the objectivepassing
the light on to the eyepeice.  It is, of course, out of focus
because that is the intention here to see what the telescope is
doing with the light before or after it gets to the focus point.
Normally you rack in as this makes for less reverssals of the
light path efffects on the light.
Thus, if you defocus and you see the black hole in the middle of
the star's image, you're using a scope with a central obstruction
and that obstruction is what is causing that black center.
In the typical Newtonian scope, the entrance pupil is at the
primary mirror and if you put an annulus on it, you are just
shrinking the size of the mirror!  What this will mean is that
the outer size of that doughnut will get smalleras that part of
the incoming light won't make it to the eyepi4ece.  The light
that does get to the eyepiece won't be diminished in any way by
this annulus, just the total amouint of the light which is
something different.  Of course, if you're looking at the focused
image, you will see the light from the star diminish in intensity
but that is because you're not using a F5 scope anymore but
rather something more like a F10 scope with the same focal lenght
as the original scope (you haven't chnaged the scope's shape, did
you???)
For Sc type scopes, the entrance pupil is at the Corrector plate
so the mirror itself is a bit back from there but the same thing
applies.
If you doubt this, defocus a star and stick your hand into the
aperture of the scope.  You will see the hand waving about in the
doughnuyt and not see the brightness of the whole image of the
defocused star dim.Pleae wexcuse typing errors as my eyes are bad
tonight.
Bob May

rmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net


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