Keywords: class-weighting

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# 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.

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