Temperature and humidity data

The most temperature-sensitive parts of most photometric instruments are the filters, the detector, and the electronics. The likely temperature coefficients are of similar orders of magnitude: generally several tenths of a percent per degree. As temperatures usually vary by several degrees during a run, temperature effects are likely to exceed 0.01 magnitude.

Although the ESO Archive standard is to record temperatures in kelvins, this is often inconvenient. Temperature data may be recorded in Fahrenheit or Celsius, or in some scale with perfectly arbitrary units, such as the output of some uncalibrated thermistor sensor. 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. Therefore, if the temperatures are not in kelvins, the appropriate units should be provided in the table file (if temperatures are in a column), or as a comment.

The temperatures of filters and detectors were discussed in previous sections dealing with instrumental parameters. If they are regulated, such data should be stored in Real*4 descriptors in the instrumental ``.tbl'' files (see above). If they are measured, they should be in data columns with the labels FILTTEMP and/or DETTEMP.

If temperatures are measured only occasionally, and not with every observation, the values should still be recorded in columns of the data table file. In this case, the temperature measurements are essentially asynchronous with the photometric measurements; then the OBJECT column should contain the word FILTTEMP or DETTEMP, as appropriate, and the temperature columns will contain the ``undefined'' value for actual observations.

For example, in the ESO Archive logfiles, DOME TEMP is recorded every 15 minutes. This should be used for FILTTEMP if the filters are at ambient temperature, and for DETTEMP for an uncooled detector.

If filter and detector temperatures are otherwise regulated or recorded, it may still be useful to record DOMETEMP in the data file as a Real*4 column. 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. The column label is RELHUM, and this should be put in the OBJECT column for sporadic readings.

Note that the times must be converted to Real*8 because of the MJD format required! As data in the ESO Archive logfiles are stored in hh:mm:ss form, they will have to be converted.