  In this short paper I present the results of a calculation which seeks the
maximum, or optimal, signal-to-noise energy band for galaxy group or cluster
X-ray emission detected by the Chandra and XMM-Newton observatories. Using a
background spectrum derived from observations and a grid of models I show that
the "classical" 0.5-2 keV band is indeed close to optimal for clusters with gas
temperatures >2 keV, and redshifts z<1. For cooler systems however, this band
is generally far from optimal. Sub-keV plasmas can suffer 20-60%
signal-to-noise loss compared to an optimal band, and worse for z>0. The
implication is that current and forthcoming surveys should be carefully
constructed in order to minimize bias against the low mass, low temperature end
of the cluster/group population.
