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Sky Darkness and the Contrast Illusion
by Tim Hunter and James McGaha
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How dark is your sky? This is not an easy question to answer
with exactitude. You first have to decide what you mean by
"dark" and then provide a definitive way of measuring darkness.
It's fairly easy to say the sky in downtown New York City is not
very dark, while the sky atop Kitt Peak is fairly dark, and the
sky atop Mauna Kea is very dark.
One convenient way to judge sky darkness is to visually estimate
the stellar limiting magnitude overhead and at 45 degree angles
above the horizon in various directions away from obvious areas
of light pollution. The limiting magnitude can be judged with
the naked eye or with optical instruments.
It is most important when comparing one site with another or one
night with another to be meticulous and consistent in your
visual estimates. You should be fully dark adapted and refrain
from smoking or drinking for several hours beforehand. It is
best to use good star charts that will allow estimates accurate
to about 0.1 to 0.2 of a magnitude. You will be very inaccurate
if you just look up and say " it seems like a 6.0 magnitude sky
tonight."It is not known if there is a direct relationship
between the stellar limiting magnitude for a given night and
site and the limiting magnitude for faint, extended deep-sky
objects, such as nebulas and galaxies. Some persons have had the
experience of being able to see very faint stars on a given
evening but not seeing deep sky objects as well as might be
expected. The "seeing" quality or steadiness of the atmosphere
may be quite important. A site with steady seeing may allow
better visualization of faint objects than one that is
intrinsically better (higher altitude and less light pollution)
but with poorer seeing. |
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