  We present an overview of and first results from the OMEGA survey: the OSIRIS
Mapping of Emission-line Galaxies in the multi-cluster system A901/2. The
ultimate goal of this project is to study star formation and AGN activity
across a broad range of environments at a single redshift. Using the
tuneable-filter mode of the OSIRIS instrument on GTC, we target Halpha and
[NII] emission lines over a ~0.5 X 0.5 deg2 region containing the z~0.167
multi-cluster system A901/2. In this paper we describe the design of the
survey, the observations and the data analysis techniques developed. We then
present early results from two OSIRIS pointings centred on the cores of the
A901a and A902 clusters. AGN and star-forming (SF) objects are identified using
the [NII]/Halpha vs. W_Halpha (WHAN) diagnostic diagram. The AGN hosts are
brighter, more massive, and possess earlier-type morphologies than SF galaxies.
Both populations tend to be located towards the outskirts of the high density
regions we study. The typical Halpha luminosity of these sources is
significantly lower than that of field galaxies at similar redshifts, but
greater than that found for A1689, a rich cluster at z~0.2. The Halpha
luminosities of our objects translate into star-formation rates (SFRs) between
~0.02 and 6 Msun/yr. Comparing the relationship between stellar mass and
Halpha-derived SFR with that found in the field indicates a suppression of star
formation in the cores of the clusters. These findings agree with previous
investigations of this multi-cluster structure, based on other star formation
indicators, and demonstrate the power of tuneable filters for this kind of
study.
