The Herschel Space Observatory, formerly known as FIRST (Far-infrared Space Telescope), will be ESA's fourth cornerstone mission and will conduct far-IR and submillimeter imaging photometry and spectroscopy from 60 ^m to 670 ^m. The telescope is due for an Ariane 5 launch in early 2009 and it will be placed into orbit about the Second Lagrangian (L2) point of the Earth-Sun system (1.5 million km from Earth in the direction directly away from the Sun), where it will operate for a period of about 3 years. The L2 orbital position has been chosen because of its low and stable background radiation field, and orbital stability.
The Herschel spacecraft is approximately 7.5 m high and 4 x 4 m wide, with a launch mass of around 3.3 tonnes (Figure 8.5). It carries a 3.5 m radiatively cooled
Ritchey-Chretien telescope (the largest ever built for space operation) and three scientific instruments: PACS, SPIRE, and HIFI, housed in a superfluid helium cryostat and thus cooled to approximately 2K. Herschel's long wavelength range will primarily allow it to investigate the formation of stars and galaxies, since it is only at these long wavelengths that radiation may penetrate the circumstellar and circum-galactic dust clouds in detectable quantities. In addition, the wavelength region contains features useful for the study of interstellar medium (ISM), astrochemistry, and the atmospheres of the giant planets. Herschel should prove particularly useful in providing new data on the D/H ratio of Uranus, Neptune, and the abundance of water vapor in the giant planets.
The three planned scientific instruments of Herschel will now be reviewed.
Photodetector Array Camera and Spectrometer (PACS)
PACS is an imaging photometer and spectrometer which covers the wavelength range 60 ^m to 210 ^m. For photometry, radiation is imaged in both a "blue" and a "red" channel, each covering a FOV of 105 x 210". The blue channel is composed of a 32 x 64 bolometer pixel array covering the wavelength ranges of either 60-90 ^m or
90-130 ^m. The longer wavelength red channel is composed of a 16 x 32 bolometer detector array and covers the 130 ^m-210 ^m wavelength range. In spectroscopy mode, PACS will image a FOV of about 50 x 50", resolved into 5 x 5 pixels. The light is then dispersed by a grating onto one of two 16 x 25 GeGa arrays covering the ranges 40-120 ^m and 100-210 ^m, respectively. The resolving power of the instrument will be approximately 1,500 and the detectors will be cooled to 300 mK by an internal 3He sorption cooler to improve its sensitivity.
Spectral and Photometric Imaging Receiver (SPIRE)
SPIRE comprises an imaging photometer and a symmetrical Mach-Zehnder imaging spectrometer. The instrument has five bolometer detector arrays, which are cooled to 300 mK by an internal 3 He sorption cooler.
In its broadband photometry mode (R ~ 3) SPIRE will simultaneously image a 240 x 480" field of the sky in three bands centered at 250 ^m, 350 ^m, and 500 ^m with angular resolutions of 71", 24", and 35", respectively. This mode will not be useful for giant planet observations. However, in its spectrometer mode SPIRE will operate as a Mach-Zehnder configuration Fourier Transform Spectrometer (FTS) with a wavelength range of 200 ^m to 670 ^m, and a circular FOV of diameter 156". The spectral resolution is tunable between 0.04 cm-1 and 1 cm-1. The FTS has two detector arrays and may operate in either the 200-300 ^m or 300-670 ^m ranges. SPIRE should provide very useful observations on the far-IR disk-averaged spectra of the giant planets.
Although SPIRE has been optimized to address the questions of how stars and galaxies form it should also provide important measurements not only for the giant planets, but also for comets and galactic ISM.
Heterodyne Instrument for the Far Infrared (HIFI)
HIFI is a very high-resolution heterodyne spectrometer offering resolving powers in the range 1,000-1,000,000 combined with low noise detection using superconductor-insulator-superconductor (SIS) and hot electron bolometer (HEB) mixers. Although not an imaging instrument, HIFI will provide continuous coverage in five bands over the range of 480 GHz to 1,250 GHz (240-625 ^m) in five bands and two additional bands between 1,410GHz and 1,910GHz (157-212 p.m).
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