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 μm wavelength regime. In photometry mode, it simultaneously images two bands, 60−85 μm or 85−125 μm and 125−210 μm, 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 −1 and a spectral resolution of ∼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. Key words. space vehicles: instruments -instrumentation: photometers -instrumentation: spectrographsHerschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.

The constituents of the cosmic IR background (CIB) are studied at its peak wavelengths (100 and 160 μm) by exploiting Herschel/PACS observations of the GOODS-N, Lockman Hole, and COSMOS fields in the PACS evolutionary probe (PEP) guaranteed-time survey. The GOODS-N data reach 3σ depths of ∼3.0 mJy at 100 μm and ∼5.7 mJy at 160 μm. At these levels, source densities are 40 and 18 beams/source, respectively, thus hitting the confusion limit at 160 μm. Differential number counts extend from a few mJy up to 100-200 mJy, and are approximated as a double power law, with the break lying between 5 and 10 mJy. The available ancillary information allows us to split number counts into redshift bins. At z ≤ 0.5 we isolate a class of luminous sources (L IR ∼ 10 11 L ), whose SEDs resemble late-spiral galaxies, peaking at ∼130 μm restframe and significantly colder than what is expected on the basis of pre-Herschel models. By integrating number counts over the whole covered flux range, we obtain a surface brightness of 6.36± 1.67 and 6.58± 1.62 [nW m −2 sr −1 ] at 100 and 160 μm, resolving ∼45% and ∼52% of the CIB, respectively. When stacking 24 μm sources, the inferred CIB lies within 1.1σ and 0.5σ from direct measurements in the two bands, and fractions increase to 50% and 75%. Most of this resolved CIB fraction was radiated at z ≤ 1.0, with 160 μm sources found at higher redshift than 100 μm ones.

We report accurate new wavelengths for 29 mid-infrared ionic Ðne-structure lines, based on observations with the Short Wavelength Spectrometer (SWS) on board the Infrared Space Observatory (ISO). Our results originate from observations of NGC 7027, NGC 6543, NGC 6302, the Circinus galaxy, Sgr A West, and W51 IRS 2. The obtained accuracies (j/*j) range from 3 ] 104 to 1 ] 105, depending on instrumental mode and uncertainty in radial velocities.

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 × 25 pixels, each, and two filled silicon bolometer arrays with 16 × 32 and 32 × 64 pixels, respectively, to perform imaging line spectroscopy and imaging photometry in the 60 − 210µm wavelength band. In photometry mode, it will simultaneously image two bands, 60 − 85µm or 85 − 125µm and 125 − 210µm, over a field of view of ∼ 1.75 × 3.5 , with close to Nyquist beam sampling in each band. In spectroscopy mode, it will image a field of ∼ 50" × 50", resolved into 5 × 5 pixels, with an instantaneous spectral coverage of ∼ 1500 km/s and a spectral resolution of ∼ 175 km/s. In both modes the performance is expected to be not far from background-noise limited, with sensitivities (5σ in 1h) of ∼ 4 mJy or 3 − 20 × 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.