FreeLists / az-observing / [AZ-Observing] Re: Weekend Observing

[Brian Skiff <bas@xxxxxxxxxx>]

>>  Are you following a star
>>  from the meridian down to higher airmass?

>>  Yes!

     OK, your negative extinction then almost certainly results from
the transparency changing during the interval.  Since the airmass
associated from the trough Thursday/Friday was receding during
Friday evening, there's good reason to think this is what was
going on.  At J and H, you'll be affected more directly by water-vapor
changes as well as just plain clouds because of the way the 
atmospheric 'windows' at these wavelengths change shape as a function
of, say, precipitable water-vapor.
     Instead of following just one star from high to low (or the 
reverse), you can avoid the temporal changes in extinction by
measuring several stars in as short a time as possible at both
high and low airmass.  If you get negative extinction from this
procedure, then it's simply cloudy.  Presumably you have some list
of bright JHK standard stars you can use for doing the extinction.
     ...it occurs to me that the patchy nature of extinction at the
near-IR wavelengths may cause it to come out with a negative value
even if it's "clear" in the usual meaning of this term.  This might
be a good question to ask on the AAVSO photometry discussion list,
for instance.

\Brian
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