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

[Jeff Hopkins <phxjeff@xxxxxxxxxxx>]

Thanks Brian,

The infrared is certainly interesting.  I'll try the multiple stars 
techniques next change i get. Looks like the next few days will be 
cloudy, even some possible rain.

Jeff

At 13:41 -0700 2/27/06, Brian Skiff wrote:
>>>   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
-- 
Jeff Hopkins
HPO SOFT
http://www.hposoft.com/Astro/astro.html
Hopkins Phoenix Observatory
7812 West Clayton Drive
Phoenix, Arizona 85033-2439 U.S.A.
www.hposoft.com
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