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Date: Fri, 15 Feb 2002 12:36:23 -0700 (MST)
From: Brian Skiff <Brian.Skiff@lowell.edu>
Subject: [vsnet-chat 0] Re: Airmass, extinction and seeing at moderate altitude (h) above the horizon
The simple linear formula (secant z) is good enough down to about
80 deg zenith distance. After that, to be correct, one needs to account
for the curvature of Earth (or more exactly, the curvature of the
atmosphere).
The canonical "lore" is that seeing degrades as zenith distance to
the power 0.6. Don't be surprised if it gets worse faster thand this,
mainly due to local effects.
Re the effect on limiting magnitude as you go to higher airmass
(i.e. lower in the sky) is a complicated function of extinction, seeing,
and background sky brightness, all of which act to lower signal-to-noise.
\Brian
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