Far Infrared Astronomy

In this Thesis an instrument simulator for a Far Infrared space interferometer is presented, as well as a test bed implementation of the technique intended to be used to achieve high spectral and spatial resolutions from space. In this Introduction the motivation for this system is given from a general view of the Far Infrared astronomy and the possible science cases, through the past and present Far Infrared instruments, to FIRI, the concept of a space based Far Infrared Interferometer. 

The Far Infrared and submillimeter wavelength range, which spans from 25 (xm up to 1mm, is of significant importance to astronomy. Its potential is most clearly illustrated by considering the three main components that dominate the electromagnetic energy content of the Universe shown in Fig. 1.1. 

The dominant component is the microwave background produced by the primordial Universe at recombination (z 1089). The second most important is the FIR background, produced by galaxies in the young Universe. The third is the optical background dominated by evolved stars/galaxies and AGN (Dole et al. 2006). The first and third of these components have now been mapped in detail over the entire sky, while virtually no sky has been imaged in the FIR to any reasonable depth. 

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Bolometers have been produced in small arrays by simply mechanically placing them at the foci of an array of feed horns designed to channel far infrared radiation to them. Since bolometers are not as sensitive as other detectors and they are difficult to produce in large arrays, they are generally only used for far infrared astronomy where other detectors are not sensitive. They are operated at temperatures of 1 to 2 K or below in dewars containing liquid helium. 

The revelatory power of the new astronomy, especially astronomy associated with the extreme forms of radiation, resides in its capacity to expose previously unknown processes to reason and understanding gamma astronomy, the most violent phenomena in the Universe, such as the rupture and destruction of stars, and infrared astronomy, the gentle events, such as the birth of stars. Optical astronomy fills the relatively calm gap between stellar birth and death, whilst millimetre radioastronomy opens our minds to the formation of molecular structure in great clouds of cold gases and opaque dusts, far from any devastating light. 

Fourier transform methods have revolutionized many fields in physics and chemistry, and applications of the technique are to be found in such diverse areas as radio astronomy [52], nuclear magnetic resonance spectroscopy [53], mass spectroscopy [54], and optical absorption/emission spectroscopy from the far-infrared to the ultraviolet [55-57]. These applications are reviewed in several excellent sources [1, 54,58], and this section simply aims to describe the fundamental principles of FTIR spectroscopy. A more theoretical development of Fourier transform techniques is given in several texts [59-61], and the interested reader is referred to these for details. 

It will be clear from this subsection that much skillful and imaginative instrument design, by a number of different groups, has been directed towards the development of far-infrared spectroscopy. Quite apart from the developments in laboratory spectroscopy, the impact on astronomy in this region of the spectrum is of major importance. A high power tunable far-infrared source can serve as the local oscillator for the detection of far-infrared interstellar radiation. We can anticipate exciting developments in this field. 

We have already discussed the high-resolution spectroscopy of the OH radical at some length. It occupies a special place in the history of the subject, being the first short-lived free radical to be detected and studied in the laboratory by microwave spectroscopy. The details of the experiment by Dousmanis, Sanders and Townes [4] were described in section 10.1. It was also the first interstellar molecule to be detected by radio-astronomy. In chapter 8 we described the molecular beam electric resonance studies of yl-doubling transitions in the lowest rotational levels, and in chapter 9 we gave a comprehensive discussion of the microwave and far-infrared magnetic resonance spectra of OH. Our quantitative analysis of the magnetic resonance spectra made use of the results of pure field-free microwave studies of the rotational transitions, which we now describe. 

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],... 

In all ranges of the infrared, the near, middle, and far infrared (NIR, MIR, and FIR) mainly the absorption of radiation by a sample is evaluated. Emission spectra are only rarely recorded, even though they are a powerful tool for solving problems which cannot be investigated by other methods investigations of remote samples (in astronomy and for environmental analysis), of reactions of substances on catalysts, and of layers on surfaces (Mink and Keresztury, 1988) (Sec. 3.3.1). Other techniques of investigating surface layers, e.g., ATR technique and ellipsometry in the infrared range, are described in Sec. 6.4. 

S. Rinehart, C. AUen, R. Barry, D. Benford, W. Danchi, D. Fixsen, D. Leisawitz, L. Mundy, R. Silver-berg, J. Staguhn, The Balloon Experimental Twin Telescope for Infiared Interferometry (BE lTlI) High Angular Resolution Astronomy at Far-Infrared Wavelengths, in American Astronomical Society Meeting Abstracts 213, volume 41 of Bulletin of the American Astronomical Society, p. 475.19, Jan 2009... 

The FIRI laboratory testbed is the the result of an effort by Cardiff University, the Rutherford Appleton Laboratory (RAL) and UCL to develop an instrument to demonstrate the feasibility of the Double-Fourier technique at Far Infrared (FIR) wavelengths, which in a long term basis is expected to be the precursor of the space-based Far Infrared Interferometer (Helmich and Ivison 2009). It is currently located at the Physics and Astronomy Department of Cardiff University. This system is in constant development, and here the current design and issues, the latest results and the future planned improvements are presented. 

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