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Hi Lew & meteoptic list,
> Do observers of bright light phenomena (such
>as lunar halos and sundogs) generally find that this conventional
"deep-sky
>wisdom" is actually reversed??
I can't answer the bright light question however I agree with Lew's point
that :
>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.
however, in my experience this seems to work best when you are observing
from and area with some light pollution or skyglow. I always thought that
the explanation was that by magnifying the image & the sky fog, the sky
fog
becomes too faint to detect and hence the contrast appears to increase.
I'd be interested in any other explanations of this phenomena.
On considering the question of magnification and contrast, we just don't
see lunar halo effects often enough down here for me to comment
specifically on those. I've seen one since meteoptics formed about three
years ago.
As I think of more examples they all begin to contradict eachother :
- The Ring nebula is high contrast object to an experienced observer.
But
show it to an untrained observer at moderately high magnification through
a
4" - 8" telescope and they won't think it's high contrast because it's
"too
faint".
- Detail on planets eg Jupiter & Mars. Due to seeing effects, the best
magnification is often a middle magnification unless the seeing is
excellent.
I dropped out of "hard core" astronomical observing about 8 years ago. In
a blind test, I'm not sure that I would perceive a 1 magnitude difference
between say mag 11 & 12 stars as the same contrast as between mag 1 & 2
stars any more.
So "apparent contrast" is possibly a mixture of observing experience,
seeing conditions, sky transparecy light pollution &
aperture/magnification=(exit pupil)
In regard to the question yesterday, I think it's just the inability of
a
brief text passage to accurately convey a visual concept and the reading
between the lines that we all subconsiously do to fill in the gaps. I
took the person to be an untrained observer & understood that she meant
brightness not contrast. She didn't refer to contrast except by using the
word vivid. She asked directly about colour and brightness. So I assumed
she equated colour & brightness to vividness.
Reading the original question again she may have been refering to
contrast.
The drop off of intensity with mag. squared explains both colour vision
loss & brightness drop. I don't know how it affects contrast on a halo
against a bright sky. I don't recall ever looking at a large halo
through
binoculars.
I had written all of the above reply when Gary & Brian's posts came
through. I have to agree with Brian that it's the exit pupil & aperture
not
f/ratio which is important on extended objects. The exit pupil is only a
function of aperture & magnification. Take two telescopes of same
aperture
but one at f8 and one at f4. Stick a 12mm eyepice on the f4 and a 25mm on
the f8 and you have the same magnification & exit pupil size. The only
difference in brightness will be due to any difference in light
transmission that might exist between the two eyepieces and any vignetting
etc due to the secondary mirrors. Theoretically, they will show exactly
the same brightness image because the exit pupils are identical. A rich
field telescope is one that has an exit pupil equal to the entrance pupil
of your eye when dialated (7mm). You could do that with an f10 if you
could get all the light into a long fl eyepiece efficiently. If I recall
one of the 3 volume ATM series books had a very good explanation of this
concept. I don't own the set and read them almost 20 years ago so I can't
tell you which volume. But it's under richest field telescopes(RFT). The
myth that Brian referred to is that an f4 is an RFT. An f4 tends to
conveniently give you the RFT exit pupil using a 25mm eyepiece. with high
f
ratios you need longer fl eyepieces. the myth probably devloped many
decades ago when you couldn't get longer than 40mm eyepieces. With my
6"f7
I'd need something like a 50-60mm eyepiece to reach RFT and a much bigger
diagonal than I presently have.
I don't quite follow Gary Beckett's explanation of contrast. I might post
again after I've thought about his reply. As I've said above there are a
number of physical and optical principals at work and then some
subjectivity regarding "apparent contrast" and observer experience.
Finally, Meteoptics has awoken from a long hibernation.
Joseph A. Cali | |
Environmental Geochemistry Group | |
Research School of Earth Sciences | Phone : +61 2 6249 3246 |
Australian National University | Fax : +61 2 6249 3683 |
Canberra, Australia, 0200 | Email : Joe.Cali_at_anu.edu.au |
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