Astronomy Principles and Practice Fourth Edition.pdf

Millimetre astronomy 393
infrared relate to cool bodies, for example, solar system objects and cool stars. The same calculation
also gives an immediate hint to some of the problems and difficulties of making infrared observations.
The telescope itself will radiate strongly in the infrared and special care must be given to its design and
structure. For optical telescopes, it is common practice to surround the secondary mirror by a baffle
tube to prevent stray light from the sky entering the system. In the infrared, such an arrangement is an
embarrassing source of radiation and is generally removed, particularly for observations at 5 µm and
beyond. The optical components within the subsidiary analysing equipment are invariably refrigerated
and the detector itself cooled to liquid helium temperatures. Again, by virtue of its temperature, the
atmosphere between the incoming radiation and the telescope also radiates strongly and the infrared
brightness is continually changing. This problem is dealt with by chopping frequently between the
source and a nearby sky position and integrating the difference signal. The chopping may be performed
by having a secondary mirror which ‘wobbles’ between two positions. A specially designed infrared
telescope known as UKIRT (United Kingdom Infrared Telescope) is depicted in figure 23.11.
Modern infrared detectors include ‘thermal’ devices and solid state photo-conductive and
photovoltaic cells. Perhaps the most widely used detector is the germanium bolometer, operated
at liquid helium temperatures. Essentially, it is a gallium-doped germanium chip with a blackened
(for absorption and increased efficiency) sensing area of less than 0·1 mm 2 . The sensing depends
on the change in electrical resistance of the chip as it absorbs the infrared radiation and increases its
temperature.
The sensitivity of the infrared detector is normally characterized by its noise equivalent power
(NEP), this being defined as the power required of any incident radiation such that it would produce
an rms (root mean square) value of the noise. In other words, the NEP is the incident power required
to obtain a signal-to-noise value of unity. A good value for the NEP of a germanium bolometer is of
the order of 7 × 10 −15 WHz −1/2 .
A well-known success has been the IRAS (Infrared Astronomical Satellite—see figure 23.12)
which, in 1983, carried a refrigerated 57 cm telescope into orbit, surveying virtually the whole of
the sky at four wavelength bands centred at 12, 25, 60 and 100 µm. The telescope optics and the
main system were maintained below 5 K for the 11 months of the survey. The focal plane of the
telescope held a template with a series of 62 slots providing different fields of view and followed by
detectors operating at 1·8 K. The detector pattern was designed so that any source crossing the field
was monitored by at least two of the detectors in the same wavelength band. Analysis of the thousands
of recorded infrared sources has provided a new reference catalogue and has opened up new areas of
astrophysical research.
23.6 Millimetre astronomy
At high sites where the amount of precipitable water vapour is particularly low, several atmospheric
windows open in the sub-mm range of the electromagnetic spectrum. Radiations from space at
these wavelengths are related to the spectral emissions of molecules within interstellar dust clouds
(see section 15.7.4). For this spectral domain, it is more usual to express the radiation in terms of
frequency rather than wavelength. The frequency range for this newly open window is from about 25
to 1000 GHz. One particular spectral line of interest in the mm range is that emitted by the CO molecule
at 230 GHz. Figure 23.13 indicates the various windows with the frequencies of some molecular lines
clearly available for observation. Special telescopes and techniques are required and, recently, several
research groups have developed the necessary facilities.
The problem of telescope design is classical but new concepts have been incorporated because
of the working wavelengths. A large aperture is required to collect the low fluxes. To obtain a good
angular resolution, the mirror needs to be smooth to a fraction of a wavelength, that is a fraction of
a mm. The James Clerk Maxwell Telescope (JMCT) (see figure 23.14) commissioned in 1987 and