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.