setairmass

       images =                 Input images
 (observatory = )_.observatory) Observatory for images
      (intype = "beginning")    Input keyword time stamp
     (outtype = "effective")    Output airmass time stamp\n
        (date = "date-obs")     Observation date keyword
    (exposure = "exptime")      Exposure time keyword (seconds)
     (airmass = "airmass")      Airmass keyword (output)
    (utmiddle = "utmiddle")     Mid-observation UT keyword (output)\n
        (show = yes)            Print the airmasses and mid-UT?
      (update = yes)            Update the image header?
    (override = yes)            Override previous assignments?
        (mode = "ql")          

This task is prelude to computing the atmospheric extinction
correction for a spectrum, which depands on the airmass.

IRAF will find the name of the observatory in the data header
and then look up the observatory coordinates in its database.
Using those, the coordinates of teh target, and the time of the
observation it will compute the airmass at mid-exposure.