Astronomy, radio

In the science of radio astronomy continuum radiation and absorption or emission lines due to celestial phenomena are observed in the frequency region of 1-300 GHz (300mm A 1mm). The absorption bands due to the terrestial atmosphere, as illustrated in Fig. 7.23, will clearly place limitations 

In order to increase the angular resolution in radio astronomy the technique of aperture synthesis is used, in which signals from spatially separated antennae are combined, monitoring the correct relative phases. For ex- 

Molecular lines observed in the microwave region are due to pure rotational or hyperfine transitions. Hydrogen is the most abundant element in the universe. The H2 molecule does not possess a dipole moment and thus no allowed electric dipole transitions are observed. This molecule can, therefore, not be observed. 

As with any other spectral lines, radio lines exhibit Doppler shifts if the molecular gas is in motion. In Fig.7.30 velocity distributions are shown based on Doppler observations on OH, CH and HCHO lines observed in the direction of Cassiopeia A. The components around 45 km/s are due to clouds in the Perseus arm of our galaxy, while the components around 0 km/s are due to the Orion arm. While OH and HCHO appear in absorption, CH is seen in (weak maser) emission against the background source. 

Apart from studying interstellar clouds a large number of point sources have also been investigated using radio astronomy. The observation 

In the science of radio astronomy continuum radiation and absorption or emission lines due to celestial phenomena are observed in the frequency region of 1—300 GHz (300 mm A 1mm). The absorption bands due to the terrestrial atmovsphere, as illustrated in Fig. 7.24, will clearly place limitations on such measmements. The diffraction-limited resolution for a telescope is determined by the ratio A/d, where d is the diameter of the telescope. In a comparison with optical telescopes it can be noted that a radio telescope has 10 to 10 times worse angular resolution for a given value of d. For practical reasons a telescope diameter of the order of 100 m is an upper limit, especially since a surface precision of about A/20 must be maintained. Examples of radio telescopes are the 100 ni telescope near Bomi in Germany, which is used in the frequency region 1—45 GHz and the 20 m Onsala telescope in Sweden, which can be used for frequencies up to 120 GHz (A = 2.5 mm). A diagram of the latter telescope is shown in Fig. 7.28. 

To amplify the weak radio signals picked up by the telescopes, travelling-wave masers and superconducting mixers are sometimes used. An example 

Diagram of the 20 m diameter millimetre-wave telescope at Onsala, Sweden (Courtesy Onsala Speice Observatory) 

Onsala in 1973. The water molecule H2O, which lias an angle of 105° between the H atoms, has a complex rotational spectrum. Litense maser action has been observed for the 22 GHz transition, wliich is also seen in atmospheric absorption (Fig. 7.24). High-frequency radio astronomy observations have to be performed on dr y days. A large number of other molecules, some of them quite complex, have been observed in space. The (very) remote sensing of the physical conditions in the interstellar clouds has been discussed in [7.93-7.95]. 

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Barrett A H 1983 The beginnings of moleoular radio astronomy Serendipitous Discoveries in Radio Astronomy ed K Kellerman and B Sheets (Green Bank, WV NRAO)... 

Radio antennas Radio astronomy Radioautogiaphy Radiochemical analysis Radiochemical technology Radiofibrosis Radio frequency... 

The longest wavelengths of the electromagnetic spectmm are sensitive probes of molecular rotation and hyperfine stmcture. An important appHcation is radio astronomy (23—26), which uses both radio and microwaves for chemical analysis on galactic and extragalactic scales. Herein the terrestrial uses of microwave spectroscopy are emphasized (27—29). 

J. D. Kraus, Radio Astronomy, 2nd ed., Cygnus-Quasar Books, PoweU, Ohio, 1986. 

G. L. Verschuur and K. I. KeUermann, eds.. Galactic and Extragalactic Radio Astronomy, 2nd ed., Springer-Vedag, New York, 1988. 

B. E. Burke and E. Graham-Smith, An Introduction to Radio Astronomy, Cambridge University Press, New York, 1996. 

We investigate here whether coherent detection of optical waves issued from celestrial sources can be processed in a way similar to radio astronomy (Fig. 1). 

Abstract Either because observed images are blurred by the instrument and transfer medium or because the collected data (e.g. in radio astronomy) are not in the form of an image, image reconstmction is a key problem in observational astronomy. Understanding the fundamental problems underlying the deconvolution (noise amplification) and the way to solve for them (regularization) is the prototype to cope with other kind of inverse problems. 

National Radio Astronomy Obs., Green Bank dbalser0nrao.edu... 

Edge, David O., and Michael J. Mulkay. Astronomy Transformed The Emergence of Radio Astronomy in Britain. New York Wiley, 1976. 

Although the present work is concerned primarily with spectra, its applicability does not lie only in that area. The term spectra should be understood to include one-dimensional data from experiments that do not explicitly involve optical phenomena. Data from fields as diverse as radio astronomy, statistics, separation science, and communications are suitable candidates for treatment by the methods described here. Confusion arises when we discuss Fourier transforms of these quantities, which may also be called spectra. To avoid this confusion, we adopt the convention of referring to the latter spectra as Fourier spectra. When this term is used without the qualifier, the data space (nontransformed regime) is intended. 

One way of linearizing the problem is to use the method of least squares in an iterative linear differential correction technique (McCalla, 1967). This approach has been used by Taylor et al. (1980) to solve the problem of modeling two-dimensional electrophoresis gel separations of protein mixtures. One may also treat the components—in the present case spectral lines—one at a time, approximating each by a linear least-squares fit. Once fitted, a component may be subtracted from the data, the next component fitted, and so forth. To refine the overall fit, individual components may be added separately back to the data, refitted, and again removed. This approach is the basis of the CLEAN algorithm that is employed to remove antenna-pattern sidelobes in radio-astronomy imagery (Hogbom, 1974) and is also the basis of a method that may be used to deal with other two-dimensional problems (Lutin et al., 1978 Jansson et al, 1983). 

Radio-astronomy searches (Project Ozma) for communications from intelligent extraterrestrial beings have been carried out at this frequency see Interstellar Communication, A. G. W. Cameron, etL, Benjamin, New York, 1963 Communication With Extraterrestrial Inteltt-genee, C. Sagan, ed. MIT Press, 1973. 

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. 

Radio astronomy -role of optical spectroscopy [SPECTROSCOPY, OPTICAL] (Vol 22)... 

A. Thompson, J. Moran, and G. Swenson, Interferometry and Synthesis in Radio Astronomy, 2nd Edition, Wiley Interscience, New York (2001). 

JOSEPH K. ALEXANDER, senior program officer, served previously as director of the Space Studies Board (1999-2005), deputy assistant administrator for science in the Environmental Protection Agency s Office of Research and Development (1994-1998), associate director of space sciences at NASA Goddard Space Flight Center (1993-1994), and assistant associate administrator for space sciences and applications in the NASA Office of Space Science and Applications (1987-1993). Other positions have included deputy NASA chief scientist and senior policy analyst at the White House Office of Science and Technology Policy. Mr. Alexander s own research work has been in radio astronomy and space physics. He received B.S. and M.A. degrees in physics from the College of William and Mary. 

K. Rohlfs and T.L. Wilson, Tools of Radio Astronomy (Third Edition), Springer-Verlag, Berlin, 2000. 

During the past fifty years extensive effort in many laboratories has led to enhancements of the simple system, turning microwave spectroscopy into an extremely sensitive and versatile tool. We now review some of these developments. We shall also describe the essential features of a radio telescope because almost thirty diatomic molecular species, many of which would be transient species in the laboratory, have been detected in interstellar gas clouds. Molecular radio astronomy is closely linked with and complementary to laboratory microwave spectroscopy. Or, if you wish, you can reverse the emphasis of that last statement ... 

Superherodyne spectrometers are now not common in laboratory microwave experiments, but superheterodyne detection plays a major role in radio astronomy, as we shall see later. The reasons are obvious one cannot modulate the energy levels of extraterrestrial molecules, and a radio telescope collects radiant energy at all frequencies simultaneously. One does not have a primary monochromatic source of radiation, as in laboratory experiments. 

The existence of molecular species in interstellar space has been known for almost seventy years. The first observations involved the electronic spectra, seen in absorption in the near-ultraviolet, of the CN, CH [28] and CH+ [29] species. Radiofrequency lines due to hydrogen atoms in emission [30] and absorption [31], and from the recombination of H+ ions with electrons were also known. However, molecular radio astronomy started with the observation of the OH radical by Weinreb, Barrett, Meeks and Henry [32] in 1963 in due course, this was followed by the discovery of CO [33]. In the subsequent years over 110 molecules have been observed in a variety of astronomical sources, including some in galaxies other than our own. Nearly a third of these are diatomic molecules, with both closed and open shell electronic ground states, and some were observed by astronomers prior to being detected in the laboratory. 

In this section, we shall restrict ourselves to those aspects of radio astronomy which are relevant to the study of the rotational spectra of diatomic molecules. We will not deal with the study of continuum sources, with cosmology, or with the detailed structure, dynamics and chemistry of interstellar clouds. These are important parts of astrophysics, covered in many research articles, reviews and books [34, 35, 36]. We will describe the main features of the dishes which collect radiation (i.e. the telescope), the detectors and signal processing equipment, and the analysis of the spectra. Many of the microwave spectra of diatomic molecules are now used as important probes to... 

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. 

J.D. Krauss, Radio Astronomy, McGraw-Hill Book Company, 1966. 

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