TheHerschel-SPIRE instrument and its in-flight performance
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abstract
The Spectral and Photometric Imaging Receiver (SPIRE), is the Herschel Space
Observatory`s submillimetre camera and spectrometer. It contains a three-band
imaging photometer operating at 250, 350 and 500 microns, and an imaging
Fourier Transform Spectrometer (FTS) which covers simultaneously its whole
operating range of 194-671 microns (447-1550 GHz). The SPIRE detectors are
arrays of feedhorn-coupled bolometers cooled to 0.3 K. The photometer has a
field of view of 4' x 8', observed simultaneously in the three spectral bands.
Its main operating mode is scan-mapping, whereby the field of view is scanned
across the sky to achieve full spatial sampling and to cover large areas if
desired. The spectrometer has an approximately circular field of view with a
diameter of 2.6'. The spectral resolution can be adjusted between 1.2 and 25
GHz by changing the stroke length of the FTS scan mirror. Its main operating
mode involves a fixed telescope pointing with multiple scans of the FTS mirror
to acquire spectral data. For extended source measurements, multiple position
offsets are implemented by means of an internal beam steering mirror to achieve
the desired spatial sampling and by rastering of the telescope pointing to map
areas larger than the field of view. The SPIRE instrument consists of a cold
focal plane unit located inside the Herschel cryostat and warm electronics
units, located on the spacecraft Service Module, for instrument control and
data handling. Science data are transmitted to Earth with no on-board data
compression, and processed by automatic pipelines to produce calibrated science
products. The in-flight performance of the instrument matches or exceeds
predictions based on pre-launch testing and modelling: the photometer
sensitivity is comparable to or slightly better than estimated pre-launch, and
the spectrometer sensitivity is also better by a factor of 1.5-2.