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The two principal classes of information needed about photometric instruments concern filters and detectors. We need two kinds of information for filters and detectors: physical information about the instrument itself, and logical information about the way it is represented in data files -- for example, the codes used to identify filter positions.
In addition, it is useful to know the condition of the telescope optics: clean optics give not only better signal/noise ratio than dirty ones, but also more stable zero-points from night to night. Sometimes it is possible to have the optics cleaned before a critical observing run; observers should consider this possibility.
Some, but by no means all, of the required information is available in the ESO Archive -- mostly in the archive log files [5]. This can be stripped out and used when it is available; when it is not, the observer will have to supply the information.
In addition to information about the instrument itself, the reduction program
needs to know the structure of the data in their table file (see
section 
, ``Observational data'').
Thus, you must make sure that information such as the number of filter-code
columns and the filter codes that correspond to particular passbands, which is
in the instrument-description table, agrees with the actual data tables.
Usually, this is determined by the program that converts raw data files to
table format.