T h e IN T P h o to m e tr ic H a S urvey o f th e N o rth ern G alactic P la n e (IP H A S )

A B S T R A C T The INT Photom etric H a Survey of the N orthern Galactic Plane (IPHAS) is a 1800 square degrees CCD survey of the northern Milky Way spanning the latitude range —5o < b < + 5 o and reaching down to r ' ~ 20 (10a). It may increase the num ber of known northern emission line sources by an order of m agnitude. Representative observations and an assessment of point-source d ata from IPHAS, now underway, are presented. The d ata obtained are Wide Field Cam era images in H a alpha narrow-band, and Sloan r ' and i' broad-band, filters. We simulate IPHAS (r' — H a , r ' — i') point­ source colours using a spectrophotom etric library of stellar spectra and available filter transm ission profiles: this gives expected colours for (i) solar-metallicity stars, w ithout H a emission, and (ii) emission line stars. Comparisons with Aquila field observations show th a t sim ulated normal star colours reproduce the d ata well for spectral types earlier than M. Spectroscopic follow-up of a Cepheus field confirms th a t sources lying above the main stellar locus in the (r' — H a , r' — i') plane are emission line objects, with very few failures. Examples of H a deficit objects - a white dwarf and a carbon star - are shown to be readily distinguished by their IPHAS colours. The role IPHAS can play in studies of nebulae is discussed briefly, and illustrated by a continuumsubtracted mosaic image of the SNR, Shajn 147. The final catalogue of IPHAS point sources will contain photom etry on about 80 million objects. (abridged for astro-ph))