  For high-precision cosmic microwave background (CMB) experiments,
contamination from extragalactic point sources is a major concern. It is
therefore useful to be able to detect and discard point source contaminated
pixels using the map itself. We show that the sensitivity with which this can
be done can often be greatly improved (by factors between 2.5 and 18 for the
upcoming Planck mission) by a customized hi-pass filtering that suppresses
fluctuations due to CMB and diffuse galactic foregrounds. This means that point
source contamination will not severely degrade the cleanest Planck channels
unless current source count estimates are off by more than an order of
magnitude. A catalog of around 40,000 far infra-red sources at 857 GHz may be a
useful by-product of Planck.
