The commands in the previous Section require the
airmass as input parameter. Some instrument/telescope
combinations provide raw data files with the proper values of right
ascension, declination, siderial time, geographical latitude, duration
of measurement and eventually even ``mean'' airmass. In most cases it
will however be necessary to compute an appropriate airmass using the
`COMPUTE/AIRMASS`. Refer to `HELP COMPUTE/AIRMASS` for the
details of the image descriptors which are needed. Otherwise, the
required information must be provided by the user on the command
line. It is important to keep in mind that ``mean airmass'' and ``mean
atmospheric extinction correction'' are different from the values at
mid-exposure, especially for larger zenith distances. This is so
because the airmass depends non-linearly on zenith distance ()
and extinction corrections depend non-linearly on airmass
(
). For
reasonable combinations of exposure time and zenith distance, the
weighted mean airmass supplied by `COMPUTE/AIRMASS` should be
appropriate.