## Consequences

The most critical parameter of the calibration is the Forward Efficiency . This parameter is a function of frequency (antenna surface accuracy), but also of the receiver used (illumination) and of the Focus with a tendency to be too low if you are defocussed. If is underestimated, is underestimated (See equation 8 and note that is always greater than except in very bad sites...) and you may obtain anomalously low vapor content, and vice-versa.

The sideband gain ratio is also a critical parameter. is not only a scaling factor as obvious in Eq. 8, but is also involved in the derivation of the atmospheric model since the contributions from the atmosphere in image and signal bands are considered. This effect will be important only if the opacities in both bands are significantly different, as for the J=1-0 line of CO.

On the 30-m telescope, because of the design of the cold load, is also a function of frequency and receiver (it varies from 87 K at 230 GHz to 116 K at 75 GHz). The values of are fairly well known by the receiver engineers : ask them...