Re: change of airmass across can be important

["Glenn Gombert" <glenngombert@hotmail.com>]

For most of the sites inb the Midwest..I doubt that the change in airmass 
would make any significnat difference in the photometry....

Glenn G.

>From: <aah@nofs.navy.mil>
>To: tass@listserv.wwa.com
>Subject: Re: change of airmass across can be important
>Date: Mon, 6 Nov 2000 08:44:32 -0700
>
>Airmass is a derived term, primarily reflecting the amount
>of atmosphere between your site and some program field.
>As Herb mentioned, it is somewhat variable, having dependence
>on the temperature, atmospheric presssure, humidity, etc.
>However, most people ignore these effects for three reasons:
>(1) they are small variations as long as you stay away from
>high airmass, (2) you can include such variations as just
>additional extinction and zeropoint contributions during
>your reductions, and (3) you shouldn't be observing at high
>airmass anyway due to a multitude of other problems.
>   So for the raw starlist, calculating airmass for every
>star seems overkill as I consider such detailed calculations as part
>of the later reduction pipeline.  You could, however, include
>an AIRMASS= keyword in the header that reflects an approximate
>value for the central RA,DEC as a diagnostic.
>   Herb is concerned about redundancy.  When I say that something
>is redundant in the starlist, what I mean is that it is derived
>using the same raw data and so is not adding new information.
>I'm not against adding redundant information to the header since
>that is a small part of the file length, but I oppose adding it
>to each star since every new byte per record adds ~10KB to the
>file length.
>Arne
>

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