For a beam-switched observation lasting 
, obtained
by observing 
times the source and a reference
position ( = 2 say, means two ``round trips" source-
reference for a total number of 4 independent pointings), each
consisting of the average of 
elementary images
with integration time 
, the rms noise for a single
pixel is obtained as the quadratic sum of:
where the rms of the exposure is given, as before, by 
, the HF term of the detector noise
appropriate for each array.
The low frequency noise contribution is given by:
The flat-field noise applies to the differential signal, i.e. Source minus Reference:
and 
represents the residual fluctuations of the
background at the source and at the reference positions.