Re: change of airmass across can be important

[<aah@nofs.navy.mil>]

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