Description of the calibration scheme

In order to disentangle the different effects, three steps are needed

1. Once in a while, skydips are measured to determine the telescope forward efficiency ( ) as a function of frequency;
2. Every 10 to 15 minutes, observations of 1) two references at very different temperatures (the hot and cold load) and 2) the atmosphere in a direction devoid of signal are used to deduce the opacity of the atmosphere and the calibration factor. In modern system, the hot load is a foam at roughly the room temperature and the cold load is the cryostat window of the receiver which is roughly at liquid nitrogen temperature;
3. The telescope is regularly switched on and off source to subtract the atmospheric contribution to the signal seen by the receiver.