  Galaxy clusters are usually detected in blind optical surveys via suitable
filtering methods. We present an optimal matched filter which maximizes their
signal-to-noise ratio by taking advantage of the knowledge we have of their
intrinsic physical properties and of the data noise properties. In this paper
we restrict our application to galaxy magnitudes, positions and photometric
redshifts if available, and we also apply the filter separately to weak lensing
data. The method is suitable to be naturally extended to a multi-band approach
which could include not only additional optical bands but also observables with
different nature such as X-rays. For each detection, the filter provides its
significance, an estimate for the richness and for the redshift even if photo-z
are not given. The provided analytical error estimate is tested against
numerical simulations. We finally apply our method to the COSMOS field and
compare the results with previous cluster detections obtained with different
methods. Our catalogue contains 27 galaxy clusters with minimal threshold at
3-sigma level including both optical and weak-lensing information.
