  A method is proposed for combining multi-frequency maps in order to produce a
catalogue of extragalactic point sources using data from future high-precision
satellite experiments. We have found the optimal way for combining maps at
different frequencies in order to maximize the signal (point sources) to noise
(rest of the signal) ratio. Our approach is a natural multi-frequency
generalization of the band-pass filter introduced by Tegmark and de
Oliveira-Costa (1998). We show that combination of different frequency maps
gives us the possibility of creating a more complete catalogue of point
sources.
