H-ATLAS: PACS imaging for the Science Demonstration Phase

We describe the reduction of data taken with the PACS instrument on board the Herschel Space Observatory in the Science Demonstration Phase of the Herschel-ATLAS (H-ATLAS) survey, specifically data obtained for a 4 x 4 deg(2) region using Herschel's fast-scan (60 arcsec s(-1)) parallel mode. We describe in detail a pipeline for data reduction using customized procedures within HIPE from data retrieval to the production of science-quality images. We found that the standard procedure for removing cosmic ray glitches also removed parts of bright sources and so implemented an effective two-stage process to minimize these problems. The pronounced 1/f noise is removed from the timelines using 3.4- and 2.5-arcmin boxcar high-pass filters at 100 and 160 mu m. Empirical measurements of the point spread function (PSF) are used to determine the encircled energy fraction as a function of aperture size. For the 100- and 160-mu m bands, the effective PSFs are similar to 9 and similar to 13 arcsec (FWHM), and the 90-per cent encircled energy radii are 13 and 18 arcsec. Astrometric accuracy is good to less than or similar to 2 arcsec. The noise in the final maps is correlated between neighbouring pixels and rather higher than advertised prior to launch. For a pair of cross-scans, the 5 sigma point-source sensitivities are 125-165 mJy for 9-13 arcsec radius apertures at 100 mu m and 150-240 mJy for 13-18 arcsec radius apertures at 160 mu m.