Several other data are useful, or even essential. Some of these, like temperature and humidity, are independent variables that are likely to affect sensitivity and spectral response. Atmospheric pressure, apart from very small effects, is directly proportional to the Rayleigh optical depth of the Earth's atmosphere, which is the most wavelength-dependent part of the extinction. Additional instrumental parameters, like the size of the measuring aperture or the high voltage used on a photomultiplier, are essential and must be recorded for each observation if they vary. Some systems have gain steps that must be recorded for every observation. Quasi-neutral attenuators may be used to calibrate nonlinearity; such information must be provided to the reduction program. The Geneva quality-control parameters [3] may be available; again, they should have separate columns in the table. Measurements from a seeing monitor may also be available.
Table 
describes these additional columns in detail.