# Weighting modes

In CLASS, the SET WEIGHT command offers 3 weighting modes (see IRAM memo 2009-4):

- EQUAL: the weights are set to ,
- TIME: the weights are set to , where is the integration time in seconds, the frequency resolution in megaHertz, and the system temperature in Kelvin.
- SIGMA: the weights are set to , where is the rms noise in Kelvin. is usually pre-computed by the BASELINE on channel ranges with no signal. The is introduced to match the unit of the TIME weight.

These are relative weights between spectra to be combined in commands like AVERAGE, XY_MAP, etc. Absolute weights involve an additional constant factor which is not described here. The weighting modes can not be mixed: when combining spectra, all weights are EQUAL, or all weights are TIME, or all weights are SIGMA. Note that the TIME weight is a theoretical value based on parameter values taken at observation time, while the SIGMA is a measured value based on the actual noise property of the spectrum.