  Observations of the high redshift Universe through narrow-band filters have
proven very successful in the last decade. The 4-meter VISTA telescope,
equipped with the wide-field camera VIRCAM, offers a major step forward in
wide-field near-infrared imaging, and in order to utilise VISTA's large
field-of-view and sensitivity, the Dark Cosmology Centre provided a set of 16
narrow-band filters for VIRCAM. These NB118 filters are centered at a
wavelength near 1.19 micron in a region with few airglow emission lines. The
filters allow the detection of Halpha emitters at z = 0.8, Hbeta and [OIII]
emitters at z ~ 1.4, [OII] emitters at z = 2.2, and Ly-alpha emitters at z =
8.8. Based on guaranteed time observations of the COSMOS field we here present
a detailed description and characterization of the filters and their
performance. In particular we provide sky-brightness levels and depths for each
of the 16 detector/filter sets and find that some of the filters show signs of
some red-leak. We identify a sample of 2 x 10^3 candidate emission-line objects
in the data. Cross-correlating this sample with a large set of galaxies with
known spectroscopic redshifts we determine the "in situ" passbands of the
filters and find that they are shifted by about 3.5-4 nm (corresponding to 30%
of the filter width) to the red compared to the expectation based on the
laboratory measurements. Finally, we present an algorithm to mask out
persistence in VIRCAM data. Scientific results extracted from the data will be
presented separately.
