[AZ-Observing] Re: Mag sequence was:(Saturn's Rings)
- From: Brian Skiff <bas@xxxxxxxxxx>
- To: az-observing@xxxxxxxxxxxxx
- Date: Thu, 18 Oct 2001 19:36:45 -0700 (MST)
Padraig Houlihan (who is on the list, and also at Lowell---oh, and a
real PhD astronomer) and I were talking this pm about the effects of
atmospheric extinction on magnitude limits compared to seeing and the
increassed sky brightness as you observe away from the zenith. As we tried
to roughly quantify the effects, it became clear that extinction is
definitely the smaller of the effects. Going from the zenith to 60 deg
from the zenith reduces star brightness by only 0.15 mag. from a 7000-foot
site, and about 0.25 or so from a low-desert site. A change of 0.2 mag. or
so is hardly discernable visually, yet your magnitude limit is cut by far
more than that. The other effects must include seeing and sky brightness.
One could start getting a feeling for how seeing changes the limit
by making the magnitude-limit checks at the zenith (only) under different
seeing conditions (estimated using double stars). The sky brightness part
is harder to measure without instrumentation, and without having an
exceptional night, the change in limits as a function of zenith distance
can't be assessed separate from seeing.
Russell Chmela mentioned his preference in re site altitude. I've
read that one's response to altitude is not a function of age, gender, or
physical condition (i.e. the buff mountain-climber can get pulmonary edema
as readily as the couch-potato). Yes, I agree some acclimation to even
modest alitude is best, but probably 24 hours or so before observing is
sufficient for coming from roughly sea level to 7000 feet.
\Brian
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