Introduction.

WFCAM¹and VISTA² will enable the next generation wide-angle sky surveys to be undertaken in the UK. They follow on from the hugely successful UK Schmidt photographic surveys of the last decades of the twentieth century, the major difference between the old and the new being the data rates and volumes that will be produced. WFCAM employs 4 2k$\times$2k Rockwell devices and has an instantaneous field-of-view of 0.21 square degrees. WFCAM is expected to be on-telescope for the greater fraction of all available UKIRT time, and will have average/peak data rates of 100/230 Gbytes per night. It will commence operations in the final quarter of 2003. VISTA, on the other hand, is a dedicated survey telescope with an IR camera employing 16 2k$\times$2k devices in a 0.44 square degree FOV. The data rate for VISTA will be $\sim400$ Gbytes per 10 hour night, and this facility is expected to begin operations in the third quarter of 2006. In terms of both timescale and scope, WFCAM therefore represents a natural `stepping stone' to VISTA, and in the overall scheme of UK wide-field astronomy the WFCAM project can be thought of as `VISTA phase A' (see later).

There is, of course, a clear need for 4m survey facilities in the era of 8m-class telescopes; the relative performance of VISTA and WFCAM (as measured by their `grasp', or information gathering product A$\Omega$) shows<sup>3</sup> that they are the worlds leading IR survey instruments, even when including other non-dedicated survey facilities such as VLT-IRMOS. The combined science case<sup>4</sup> proposed by the UKIDSS consortium for WFCAM, for example, details a programme that is unrivalled in terms of depth, field of view and therefore survey volume. UKIDSS proposes a nested series of surveys ranging from the Large Area Survey (`LAS', 4000 sq. de.g. to K=18.4), the Galactic Plane Survey (`GPS', 1800 sq. de.g. to K=19), the Galactic Clusters Survey (`GCS', 1600 sq. de.g. to K=18.7), the Deep Extragalactic Survey (`DXS', 35 sq. de.g. to K=21) to the Ultra-Deep Survey (`UDS', 0.8 sq. de.g. to K=23). The image data alone for these amounts to $\sim50$ Tbytes of data, while the object catalogue and ancillary information are likely to be many Tbytes in size. VISTA survey data volumes will likely be more than $5\times$ those of WFCAM.