[Aavso-photometry] Bruce L. Gary's algorithm for Airmass

[Yenal Ogmen]

Bruce L. Gary gives nice algorithm in his book
"Exoplanet Observing for Amateurs" about calculating
Airmass from JD, Site coordinates and RA/DEC.
Unfortunately, I am unabled to apply this algorithm in
Excel. This is a really helpful tool especially when
doing time-series, where I couldnt find a way to
calculate airmass for a large number of observations.
Is there anybody who already applied such algorithm or
have an idea how to calculate it for time-series?

Thank you.

--- arne <arne at aavso.org> wrote:

> Jim Roe wrote:
> > I'm doing differential photometry.  Ie, my comp
> stars are on the same 
> > image as my target and, usually only a few arc
> minutes away.  Ergo, I 
> > expect delta X to be zero.  According to Hall &
> Genet, the second order 
> > extinction coefficient in V band is essentially
> zero.  So, should I see 
> > a brightening of my target as it rises from a low
> elevation (High air 
> > mass) to a high elevation (low air mass) (in V)? 
> The B-V values of the 
> > target and comp star are very close, too.
> > 
> This problem is not stated concisely enough to have
> a good answer.
> In general, stars will get brighter as they go from
> high airmass to
> low airmass.  In general, blue stars will get
> brighter quicker than
> red stars, in the same filter.  Beyond that, the
> amount of change
> depends entirely on your circumstances.  Which star
> is redder than
> the other.  Which star is at higher altitude in your
> frame.  Whether
> seeing is important for the problem.  And remember,
> you are phrasing
> the problem in terms of differential photometry, not
> raw instrumental
> values.
> 
> The easiest solution is to use two comparison stars
> of similar color
> to the variable, and see if that (K-C) difference
> changes systematically
> the way the (V-C) difference does.  In all
> photometric projects, you
> look for systematics - they guide you as to whether
> a given feature
> is possibly real or an artifact.
> Arne
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Yenal Öðmen
  Green Island Observatory Homepage
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North Cyprus