Although Celsius or Kelvin degrees are a useful basis for judging whether the actual size of an apparent temperature coefficient is reasonable, the reduction program simply needs an independent variable to work with. The temperatures of filters and detectors were discussed in previous sections dealing with instrumental parameters. If temperatures are measured only occasionally, and not with every observation, the values should still be recorded in columns of the data table file. For example, in the ESO Archive logfiles, DOME TEMP is recorded every 15 minutes. The times and temperatures can be stripped out of the logfiles and stored in the observational-data files.
Relative humidity is treated exactly the same way.