  We apply the linear filter for the weak-lensing signal of dark-matter halos
developed in Maturi et al. (2005) to the cosmic-shear data extracted from the
Garching-Bonn-Deep-Survey (GaBoDS). We wish to search for dark-matter halos
through weak-lensing signatures which are significantly above the random and
systematic noise level caused by intervening large-scale structures. We employ
a linear matched filter which maximises the signal-to-noise ratio by minimising
the number of spurious detections caused by the superposition of large-scale
structures (LSS). This is achieved by suppressing those spatial frequencies
dominated by the LSS contamination. We confirm the improved stability and
reliability of the detections achieved with our new filter compared to the
commonly-used aperture mass (Schneider, 1996; Schneider et al., 1998) and to
the aperture mass based on the shear profile expected for NFW haloes (see e.g.
Schirmer et al., 2004; Hennawi & Spergel, 2005). Schirmer et al.~(2006)
achieved results comparable to our filter, but probably only because of the low
average redshift of the background sources in GaBoDS, which keeps the LSS
contamination low. For deeper data, the difference will be more important, as
shown by Maturi et al. (2005). We detect fourteen halos on about eighteen
square degrees selected from the survey. Five are known clusters, two are
associated with over-densities of galaxies visible in the GaBoDS image, and
seven have no known optical or X-ray counterparts.
