Infrared telescopes

Fig. 7.1. The British infrared telescope at Mauna Kea, Hawaii. The picture shows the housing for the 3.8 m diameter mirror, the supporting frame, and the interior of the aluminium dome with its sliding window ( 1979 by Photolabs, Royal Observatory, Edinburgh.)...

Studies carried out on Earth, for example, by the NASA infrared telescope on Mauna Kea (Hawaii), showed albedo variations which indicated the presence of holes in the Titanian cloud formations (Griffith, 1993). It is, however, still unclear as to whether these inhomogeneities result from differences in the surface composition. Lorenz et al. (1997) reported large variations in Titan s atmosphere due to photochemical processes. The methane contained in the dense nitrogen atmosphere is decomposed by solar and thermal radiation, and its content may be replenished from methane lakes or from clathrates. 

Rotational spectroscopy and microwave astronomy are the most accurate way to identify a molecule in space but there are two atmospheric windows for infrared astronomy in the region 1-5 im between the H2O and CO2 absorptions in the atmosphere and in the region 8-20 xrn. Identification of small molecules is possible by IR but this places some requirements on the resolution of the telescope and the spacing of rotational and vibrational levels within the molecule. The best IR telescopes, such as the UK Infrared Telescope on Mauna Kea in Hawaii (Figure 3.13), are dedicated to the 1-30 xm region of the spectrum and have a spatial resolution very close to the diffraction limit at these wavelengths. 

Figure 3.13 UK Infrared Telescope situated close to the summit of Mauna Kea, Hawaii, at an altitude of 4092 m. (A colour reproduction of this figure can be seen in the colour section). (Reproduced with permission by UKIRT)...

Identification of molecules in space, even small molecules, by IR astronomy requires a rotational progression in the spectrum to be measured and resolved by the telescopes. For the transitions in the simpler molecules such as CO the telescope must be capable of aresolution of 2150/1.93 1114, which is within the resolution limit of the UK Infrared Telescope (3000-5000). However, the rotational constant for CO is rather large and many molecules, especially polyatomic species, will have a rotation constant ten times smaller than this, placing the observation of a resolved rotational progression beyond the resolution of the telescopes. Confidence in the identification of the molecule is then severely dented. The problem is worse for visible astronomy. 

The choice of visible or invisible colours, i.e. the range of wavelengths, in which an object or class of objects will be observed, is carefully premeditated. Pointing an infrared telescope towards an interstellar cloud, seeking out this gentle radiation, so red that it cannot be seen, the astronomer becomes sensitive to star birth, or emissions from newborn stars letting out their first cry of light from a dusty and cloudy placenta. 

United Kingdom Infrared Telescope Unit, Hawaii. 

Pinet, P. Chevrel, S. (1990) Spectral identification of geological units on the surface of Mars related to the presence of silicates from Earth-based near-infrared telescopic charge-coupled device imaging. J. Geophys. Res., 95,14435—46. 

Infrared telescopes (on Earth) Solar limb and sunspots... 

It should not be thought that OH always exhibits the unusual behaviour described above the recent developments in tunable far-infrared sources have had an important impact in astronomy, so that interstellar rotational transitions can now be observed. We described an airborne far-infrared telescope in the first part of this chapter, and figure 10.60 shows two examples of interstellar OH rotational transitions, observed by Watson, Genzel, Townes and Storey [170],... 

Earth-based observations made in 1991 and 1992 made in near-infrared wavelengths with the United Kingdom Infrared Telescope Facility on Mauna Kea, Hawaii showed evidence of carbon monoxide and carbon dioxide ices on Triton s surface as well as those of nitrogen and methane. They confirmed the 391°F temperature of Triton s surface and found that there is less than 10% carbon monoxide ice dissolved in the nitrogen ice on Triton s surface. 

By placing an infrared telescope in space, atmospheric emission and absorption can be eliminated and, in addition, the telescope can be cooled to eliminate... 

It should be noted that the rotational spectroscopy of CO confined to a single vibrational level, usually the ground v = 0 level, provides only a limited amount of information about molecular structure. In the field of vibration-rotation spectroscopy, however, CO has been studied extensively and particular attention paid to the variation of the rotational and centrifugal distortion constants with vibrational quantum number. Vibrational transitions involving d up to 37 have been studied with high accuracy [78, 79, 80], and the measurements extended to other isotopic species [81] to test the conventional isotopic relationships. CO is, however, an extremely important and widespread molecule in the interstellar medium. CO distribution maps are now commonplace and with the advent of far-infrared telescopes, it is also an important... 

In general, for thermally emitted radiation from a disk, far-infrared systems can probe the outer ring of a disk (from 10 to a few 100 AU), while mid-infrared telescopes and near-infrared telescopes can observe intermediate radii (about a few AU) and inner rims, respectively. Optical and UV systems can observe regions very close to the stellar surface. 

Aerospace Industry AppUcations. NASA s space program utilizes cryogenic liquids to propel rockets. Rockets carry liquid hydrogen for fuel and liquid oxygen for combustion. Cryogenic hydrogen fuel is what enables NASA s workhorse space shuttle to get into orbit. Another application is using liquid helium to cool the infrared telescopes on rockets. 

Shuttle Infrared Telescope Facility (SIRTF) Preliminary... 

The European Space Agency is currently studying the feasibility of an infrared telescope of about 3 m diameter for Spacelab. Provision of such a facility would offer enormous advantages over existing aircraft and balloon-borne systems for detailed studies of astronomical sources in the far infrared. 

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