The Photodetector Array Camera and Spectrometer (PACS) on the Herschel Space Observatory

The Photodetector Array Camera and Spectrometer (PACS) is one of the three science instruments on ESA's far infrared and submillimetre observatory. It employs two Ge:Ga photoconductor arrays (stressed and unstressed) with 16 x 25 pixels, each, and two filled silicon bolometer arrays with 16 x 32 and 32 x 64 pixels, respectively, to perform integral-field spectroscopy and imaging photometry in the 60-210 mu m wavelength regime. In photometry mode, it simultaneously images two bands, 60-85 mu m or 85-125 mu m and 125-210 mu m, over a field of view of similar to 1.75' x 3.5', with close to Nyquist beam sampling in each band. In spectroscopy mode, it images a field of 47 '' x 47 '', resolved into 5 x 5 pixels, with an instantaneous spectral coverage of similar to 1500 km s(-1) and a spectral resolution of similar to 175 km s(-1). We summarise the design of the instrument, describe observing modes, calibration, and data analysis methods, and present our current assessment of the in-orbit performance of the instrument based on the performance verification tests. PACS is fully operational, and the achieved performance is close to or better than the pre-launch predictions.