The Herschel Space Observatory is an ESA cornerstone mission, for high spatial resolution observations in the FIR and sub-millimetre regime, launched on 14th of May 2009 aboard an Ariane 5 rocket together with Planck. It entered a Lissajous 700000 km diameter orbit 1.5 million kilometers away from Earth at the second Lagrange point of the Earth-Sun system.
The mission is named after Sir William Herschel, who discovered the infrared radiation in 1800.
It is be the first space observatory to cover the full far-infrared and sub-millimetre waveband. It performs photometry and spectroscopy in the 57-671 µm range, with its 3.5 m diameter radiatively cooled telescope, while its science payload complement of three instruments is housed inside a superfluid helium cryostat.
Herschel is operated as an observatory facility offering three years of routine observations, which is available for the entire scientific community. Roughly two thirds of the observing time are "open time", and will be offered through a standard competitive proposal procedure.
The Spectral and Photometric Imaging Receiver (SPIRE) is one of the three science instruments of the Herschel Space Observatory. It contains an imaging photometer and an imaging Fourier Transform Spectrometer (FTS), both of which use bolometer arrays operating at 300 mK and are coupled to the telescope by hexagonally close-packed conical feedhorns. The two sub-instruments offer two basic, mutually exclusive modes:
Three bolometer arrays are used for broad-band photometry (λ/Δλ ~3) in spectral bands centred on approximately 250, 350 and 500 µm. A 4'x8' field of view is observed simultaneously in the three bands through the use of two fixed dichroic beam-splitters. Signal modulation can be provided either by SPIRE's two-axis Beam Steering Mirror (BSM) or by scanning the telescope across the sky. An internal thermal source is available to provide a repeatable calibration signal for the detectors
The FTS has spatially separated input and output ports. One input port views a 2' diameter field of view on the sky and the other is fed by an on-board reference source, in order to null the telescope emission. Two detector arrays at the output ports cover overlapping bands of 194-324 µm and 316-671 µm. The FTS spectral resolution is set by the total optical path difference, and can be adjusted between 0.04 and 0.83 cm-1 (corresponding to λ/Δλ = 1000 - 40 at 250 μm). The same internal thermal source used for the photometer is seen by FTS detectors as well.
The Photodetector Array Camera & Spectrometer (PACS) provides Herschel with the capabilities for spectroscopy and imaging/photometry in the 57-210 µm range. The PACS instrument is equipped with two sub-instruments:
Imaging dual-band photometry (60-85 µm or 85-130 µm and 130-210 µm) over a field of view of 1.75'x3.5', with full sampling of the telescope point spread function (diffraction/wavefront error limited). The PACS photometer has two monolithic bolometer matrices operating at 300 mK temperature. The blue channel has a 32x64 pixels arrays, while the red channel is equipped with a 16x32 pixels array.
Integral-field spectroscopy between 57 and 210 µm with a resolution of ~75-300 km/s and instantaneous coverage of ~1500 km/s, over a field of view of 47"x47" projected onto 5x5 spatial pixels. The spectrometer employs two Ge:Ga photoconductor arrays (low and high stressed) with 16x25 pixels, on which the 16 spectral elements of the 25 spatial pixels are imaged.