The VST Photometric Hα Survey of the Southern Galactic Plane and Bulge (VPHAS plus )

The VST Photometric H alpha Survey of the Southern Galactic Plane and Bulge (VPHAS+) is surveying the southern Milky Way in u, g, r, i and H alpha at similar to 1 arcsec angular resolution. Its footprint spans the Galactic latitude range -5(o) < b < +5 degrees at all longitudes south of the celestial equator. Extensions around the Galactic Centre to Galactic latitudes +/- 10 degrees bring in much of the Galactic bulge. This European Southern Observatory public survey, begun on 2011 December 28, reaches down to similar to 20th magnitude (10 Sigma) and will provide single-epoch digital optical photometry for similar to 300 million stars. The observing strategy and data pipelining are described, and an appraisal of the segmented narrow-band H alpha filter in use is presented. Using model atmospheres and library spectra, we compute main-sequence (u - g), (g - r), (r - i) and (r - H alpha) stellar colours in the Vega system. We report on a preliminary validation of the photometry using test data obtained from two pointings overlapping the Sloan Digital Sky Survey. An example of the (u - g, g - r) and (r - H alpha, r - i) diagrams for a full VPHAS+ survey field is given. Attention is drawn to the opportunities for studies of compact nebulae and nebular morphologies that arise from the image quality being achieved. The value of the u band as the means to identify planetary-nebula central stars is demonstrated by the discovery of the central star of NGC 2899 in survey data. Thanks to its excellent imaging performance, the VLT Survey Telescope (VST)/OmegaCam combination used by this survey is a perfect vehicle for automated searches for reddened early-type stars, and will allow the discovery and analysis of compact binaries, white dwarfs and transient sources.