Calculating Proper Motions in the WFCAM Science Archive for the UKIRT Infrared Deep Sky Surveys

The ninth data release from the UKIRT Infrared Deep Sky Surveys (here-after UKIDSS DR9), represents five years worth of observations by its wide field camera (WFCAM) and will be the first to include proper motion values in its source catalogues for the shallow, wide area surveys; the Large Area Survey (LAS), Galactic Clusters Survey (GCS) and (ultimately) Galactic Plane Survey (GPS). We, the Wide Field Astronomy Unit (WFAU) at the University of Edinburgh who prepare these regular data releases in the WFCAM Science Archive (WSA), describe in this paper how we make optimal use of the individual detection catalogues from each observation to derive high-quality astrometric fits for the positions of each detection enabling us to calculate a proper motion solution across multiple epochs and passbands when constructing a merged source catalogue. We also describe how the proper motion solutions affect the calculation of the various attributes provided in the database source catalogue tables, what measures of data quality we provide and a demonstration of the results for observations of the Pleiades cluster.