The Photodetector Array Camera and Spectrometer (PACS) for the Herschel space observatory

The Photodetector Array Camera and Spectrometer (PACS) is one of the three science instruments for ESA's far infrared and submillimeter observatory Herschel. 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 imaging line spectroscopy and imaging photometry in the 60 - 210 mu m wavelength band In photometry mode, it will simultaneously image 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 will image a field of similar to 50 '' x 50 '', resolved into 5 x 5 pixels, with an instantaneous spectral coverage of similar to 1500 km/s and a spectral resolution of similar to 175 km/s. In both modes the performance is expected to be not far from background-noise limited, with sensitivities (5 sigma in 1h) of similar to 4 mJy or 3 - 20 x 10(-18)W/m(2), respectively. We summarize the design of the instrument, describe the observing modes in combination with the telescope pointing modes, report results from instrument level performance tests and calibration of the Flight Model, and present our current prediction of the in-orbit performance of the instrument based on the ground tests.