The Photodetector Array Camera and Spectrometer (PACS) on the <i>Herschel</i>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 16×25 pixels, each, and two filled silicon bolometer arrays with 16×32 and 32×64 pixels, respectively, to perform integral-field spectroscopy and imaging photometry in the 60–210 <i>μ<i/>m wavelength regime. In photometry mode, it simultaneously images two bands, 60–85 <i>μ<i/>m or 85–125 <i>μ<i/>m and 125–210 <i>μ<i/>m, over a field of view of ~1.75'× 3.5', with close to Nyquist beam sampling in each band. In spectroscopy mode, it images a field of 47” × 47”, resolved into 5×5 pixels, with an instantaneous spectral coverage of ~1500 km s<sup>-1<sup/> and a spectral resolution of ~175 km s<sup>-1<sup/>. 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.