Far-Infrared Observations

Wavelengths in the infra-red (IR) region of the spectrum are difficult to observe from the ground due to absorption in the Earth's atmosphere. This is caused by molecules such as water that have rotational and vibrational bands in the IR region. Figure 1.1 shows the approximate atmospheric transparency at ground level. It is clear from this figure that observations in the far-infrared (FIR) region, which covers the approximate range 30-300 $\mu$m, are impossible from the ground. Some of the effects of atmospheric absorption can be reduced by using airborne telescopes carried by aeroplanes or balloons but uninterrupted spectral coverage can only be achieved from space.

Figure 1.11.1: Approximate transparency of the Earth's atmosphere.

The spectral region in the FIR is particularly interesting as it is where maximum intensity occurs for cool objects such as molecular clouds, proto-stellar objects and outer solar system bodies. It is a very important region for studies of interstellar chemistry as it contains many low-lying rotational transitions of light molecules, radicals and molecular ions. FIR emission lines are an important method of cooling for molecular clouds and the associated photo-dissociation regions (PDRs) at their edges. Gas cooling in these PDR regions is dominated by atomic fine structure lines such as [OI] 63 $\mu$m and [CII] 158 $\mu$m (e.g. Wolfire et al., 1990). These lines can be used as a diagnostic of the physical conditions within the clouds by comparing with detailed PDR models (e.g. Tielens & Hollenbach, 1985). This is not only useful to study the ISM within our own galaxy but can also be applied to observations of extragalactic spectra (e.g. Malhotra et al., 2001).

The telescope that opened up the FIR region for observational study was the Kuiper Airborne Observatory (KAO), which operated from 1974 until 1995. It made many observations with various instruments for imaging and spectroscopy (e.g. Haas et al., 1995). However, airborne observatories must still work within the atmosphere and so KAO spectra were not completely free from atmospheric features. Also, observations could only be made for the duration of the flight and so long observations of the same object were difficult. The Infra-Red Astronomical Satellite (IRAS; Neugebauer et al., 1984) was the first space-based IR observatory and this operated for 10 months in 1983. Virtually the whole sky was surveyed in 4 channels at 12, 25, 60 and 100 $\mu$m with some slitless spectroscopy at 8-23 $\mu$m and this lead to the discovery of many new IR sources. The Infrared Space Observatory (ISO; Kessler et al., 1996) was a European Space Agency mission to follow on from IRAS, utilising advances in detector and instrument technology to obtain higher sensitivity and angular and spectral resolution. In order to achieve high sensitivity at IR wavelengths it was crucial to reduce the photon noise associated with thermal emission from the instrument and telescope optics. ISO contained four instruments operating over the range 2.5 to 240 $\mu$m. These instruments and the telescope were cooled to cryogenic temperatures with a large liquid helium cryostat. The satellite was launched in November 1995 and operated until its liquid helium supply ran out in April 1998. It was placed in a highly elliptical orbit with a period of approximately 24 hours.

Table 1.11.1: ISO instruments and observing capabilities

Instrument	Principal Investigator	Instrument Mode Outline	Wavelength covered
ISOCAM	C. Cesarsky,	32$\times$32 array	2.5-5 $\mu$m
	(CEA-Saclay, France)	32$\times$32 array	4.5-17 $\mu$m
ISOPHOT	D. Lemke	Multi-aperture,
	(MPIA, Heidelberg, Germany)	multi-band photopolarimeter	3-110 $\mu$m
		FIR camera: 3$\times$3 pixel	30-100 $\mu$m
		FIR camera: 2$\times$2 pixel	100-240 $\mu$m
		Spectrophotometer	2.5-12 $\mu$m
SWS	Th. de Graaw	Two gratings	2.38-45.2 $\mu$m
	(SRON, Groningen, NL)	Two Fabry-Pérots	11.4-44.5 $\mu$m
LWS	P. Clegg	Grating	43-196.9 $\mu$m
	(QM College, London, UK)	Two Fabry-Pérots	47-196.9 $\mu$m

The ISO telescope was a Ritchey-Chrétien type with a 60 cm primary mirror and fixed secondary (Kessler et al., 1996). The beam was split into 4 by a pyramidal mirror so that each instrument received an $f$/15 beam. The instruments carried by ISO were a camera (ISOCAM; Cesarsky et al., 1996), an imaging polarimeter (ISOPHOT; Lemke et al., 1996), a short wavelength spectrometer (SWS; de Graaw et al., 1996) and a long wavelength spectrometer (LWS; Clegg et al., 1996). The characteristics of these 4 instruments are summarised in Table 1.1. Detailed information about each instrument is provided in a series of instrument handbooks to be published by ESA and available on the web^A.1 (Blommaert et al., 2001; Gry et al., 2002; Leech et al., 2002; Laureijs et al., 2002).

The pointing of the satellite was 3-axis-stabilised using a CCD star tracker and Fine Sun Sensor. The x-axis of the ISO body co-ordinate system was defined by the bore-sight of a Quadrant Star Sensor which was aligned with the telescope optical axis. Control in the satellite y and z directions was provided by tracking a single guide star with the Star Tracker, whose alignment with the telescope beam was determined at the start of every orbit using the Quadrant Star Sensor. The Fine Sun Sensor was used to provide control about the x-axis. Movement was controlled with gyroscopes and reaction wheels. In-flight tests showed that the satellite had excellent stability and the final $2\sigma$ absolute pointing error was $<2\hbox{$^{\prime\prime}$}$ (Leech & Pollock, 2000).