The telescope is regularly switched on and off source. It can be shown (see
e.g. Kramer, 1997) that the brightness temperature of the source can be
related to the measured counts (on and off-source) by
(12)
where
, the calibration factor, is given by
If the cosmic background power (
) is negligeable compared to the
atmospheric emission (
), then the last equation reduces to
(17)
From the previous steps, all the parameters are known to compute
and thus
.