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At 09:20 AM 8/26/97 -0400, you wrote:
>Thanks to Joe Cali, Gary Becker, and Veikko Makela for their kind
responses. I
>will forward a paraphrase of these answers to the original requester. I
did
>have one question, however: both Joe and Gary seemed to agree that
increasing
>the magnification (e.g., from unity to 7x) on an extended source like a
lunar
>halo should decrease its contrast. However, my experience in observing
faint,
>extended deep-sky objects is that increasing magnification (up to a
certain
>point) will generally actually INCREASE the contrast of the faint source
>against the dark-sky background. Do observers of bright light phenomena
(such
>as lunar halos and sundogs) generally find that this conventional
"deep-sky
>wisdom" is actually reversed?? Again, thanks for forwarding any comments
to me
>directly, as well as to the 'meteoptic' list!
>
>Clear skies (or hazy as you prefer... ;>),
>Lew Gramer
>owner-meteorobs_at_latrade.com
Lew Gramer:
When viewing extended objects, it is the focal ratio of the system which
is
paramount; when trying to capture point sources aperture is supreme.
However, a halo and the sky are both extended sources. The problem is to
make the object which we are interested in viewing stand out against
another
extended source, the sky. Consider the sky, the noise which you want to
reduce. By using a fast system such as binoculars to view a halo, the sky
(which is hazy to begin with when a halo occurs) is brightened to the
point
where it measurably interferes with the signal that you want to see--the
halo. Contrast is reduced. Using the same optics on an extended source
in
dark sky conditions does not noticeably increase the noise, so the object
becomes more apparent. But view that same object with the same instrument
during a full moon, and the object in question is more difficult to see
because contrast is reduced. The sky brightness now interferes. The sky
brightness is now enhanced to the point where it becomes obtrusive. There
is much more which could be said about this subject, but that probably hit
upon your question.
Then, of course Joe Cali noted the magnification question which decreases
brightness. Double the magnification and your extended image is 1/4th as
bright. However with night binoculars (7 x 50's) the
brightness/magnification situation pretty well evens out and the issue
should be one of contrast once again. See if Cali agrees with this
reasoning.
Bottom line might be that you just don't view diffuse extended objects
with
magnification. Now the object is bigger, but the contrast from point to
point is less because of the increase in size making the whole phenomenon
less easily perceived.
It's past midnight here and bed beckons.
Gary A. Becker
www.astronomy.org
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