  It is common practice to estimate the errors on the angular power spectrum
which could be obtained by an experiment with a given angular resolution and
noise level. Several authors have also addressed the question of foreground
subtraction using multi-frequency observations. In such observations the
angular resolution of the different frequency channels is rarely the same. In
this report we point out how the ``effective'' beam size and noise level change
with ell in this case, and give an expression for the error on the angular
power spectrum as a function of ell.
