Atmospheric extinction

From: Matson, Robert (
Date: Mon Aug 27 2001 - 10:58:49 PDT

  • Next message: JAY RESPLER: "Re:"

    Hi All,
    Chris responded to the question:
    > > Does Heavens-above take atmospheric extinction into account?
    > We also (for the time being) ignore atmospheric refraction, at least as
    > far as satellites are concerned, but not when calculating Sun/Moon rise
    > and set.
    This is a reasonable approach; neither IRIDFLAR nor SkyMap
    computes atmospheric attenuation when computing visual
    magnitudes.  The reason is that stars at low elevation angles
    are just as attenuated as satellites, so magnitude comparisons
    only remain valid if extinction is applied to both (or
    neglected for both).
    When you attempt to estimate an ordinary satellite's visual
    magnitude without stellar references, you'll probably be lucky
    to get it within 1 or 2 visual magnitudes.  But put a known
    set of bounding stellar references nearby, and you can improve
    your photometric accuracy signficantly -- perhaps 0.3 magnitudes,
    sometimes better.  Thus it's the *relative* brightness that's
    important -- not the absolute brightness.  So if you see an
    Iridium glint low on the horizon (just to the left or right of
    Vega let's say), and it looks twice as bright as Vega, then
    you'd know that the glint would have reached almost mag -1 if
    it had been high in the sky.  Your observation would have
    been photometrically dimmer than this, but there is little
    practical value in knowing the apparent magnitude unless your
    goal is to measure atmospheric extinction.
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