FreeLists / az-observing / [AZ-Observing] Re: Image Question

[Brian Skiff <Brian.Skiff@xxxxxxxxxx>]

     The effects Stan is describing, and the various (mostly correct)
suggestions folks have offered, are attributable to a large number of
phenomena in the light-path that can be generically put under the heading
of "scattered light".  They are sometimes hard to trace, and working
out the sources of each effect can be complicated.  Consider the things
that have been mentioned along the light-path.  First, you have the
25-year-old optics.  Is the coating reasonably new and is it/they _smooth_?
Just that is enough to cause all kinds of scattered light.  Then the
supposedly "better mirrors" of the 25-year-old Coulter optics.  Putting
wavefront accuracy aside, again how smooth are the surfaces?  Considering
the price they charged, I'd bet they aren't very smooth.  This is something
that can be examined, possibly measured, with fairly simple test apparatus.
(Warning:  the results may not be particularly flattering.)  Stan also
mentions 20-year-old eyepieces.  Almost guaranteed that those eyepieces
have lots of scattered light from scratched surfaces by now, and probably
weren't all that good to begin with.  When viewing deep-sky objects, or
anything lots fainter than Mars is right now, you won't notice such
problems.  Perhaps the edge effect Stan is sesing is related to the fact
that the edge is the only part of the lenses that aren't scratched up.
In any case, I'd recommend new eyepieces.  Then finally there's Stan's eyes.
As someone noted, scattered light in the eye, including reflections of
the very-bright planet off the retina, back off the lens/cornea, and then
into the fovae, can cause bizarre effects.  Can you make observations that
show the problem is not in your eye?  Are you even aware of the specific
effects your eye induces in your vision?
     Apart from actual optical devices (telescope, eyepieces, the eye),
what about baffling, grazing-incidence reflections in the tube assembly
(especially in/near the eyepiece drawtube)?  How 'bout Mars-light coming
right down past the secondary into the drawtube without hitting anything
before coming into the eyepiece?  All those sorts of things go zap-zap-zap
to contrast, and which become all too apparent (even if their source isn't)
when viewing Mars or other bright objects.
     So where's Pierre when you need him?  I'd bet he'd give you an earful
on this subject, and also be able to take a look through the back-end of
your telescope (sans eyepiece) pointed at the daytime sky and tell you
a dozen places where scattered light is coming into the focal plane, 
and what to do to fix it.

\Brian
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