The technique of very long base-line (VLB) interferometry began at about this time. In these experiments, two very distant radio telescopes observe the same source, and work as an interferometer because the local oscillator or phase reference at each telescope is derived from a highly stable rubidium maser. The resulting records for each telescope are subsequently carried to a computer and cross-correlated to develop the interference fringes. This technique was shown to work first for continuum observations [Broten et al., 1967; Bare et al., 1967]; but it is now extensively applied to OH line studies. As examples of base lines used, the radio telescopes at Hat Creek (University of California), Lincoln Laboratory (Massachusetts Institute of Technology, near Boston), National Radio Astronomy Observatory (Green Bank, West Virginia) and Onsala (near Gothenburg, Sweden) have all experimented on OH line work in pairs. The results show that OH emission sources can be as small as 0.005 seconds of arc in size. This limit may be still further reduced as experiments continue. If the frequency and phase stability of the VLB experiments can be improved (by the use of hydrogen masers, for example), the technique may be usable for measuring the absolute positions of sources in the sky to better than 10-3 seconds of arc. This in turn corresponds to position measures on Earth of 3 cm and the possibility of using such measures for a variety of geodetic and geophysical measurements is very attractive [Gold, 1967; MacDonald, 1967], 7. The emission mechanism The emission line intensities and the very small angular sizes of the sources raise difficult problems in the understanding of the mechanism by which the sources emit. Most attempts to explain the OH observational results have centred around some kind of population inversion of the energy levels, whereby the OH medium would be converted into a medium which amplifies rather than absorbs. For amplification to occur, the population of the higher energy levels (those defining the radio lines) must exceed those of the lower-energy levels, so that the net effect of the passage of a radio wave through the OH is to induce more transitions from high to low energy levels than from low to high levels. As transitions from high to low energy levels add energy to the radio wave, and those from low to high levels subtract energy, a medium with population inversion will add energy to the radio wave — that is, will amplify the radio wave. The most obvious reason for attempting to invoke maser action to explain the OH observations is to meet the requirement for explaining the extremely intense lines. The intrinsic temperatures of the lines correspond to temperatures in the range 1012 to 1014 K, a circumstance which is highly suggestive of amplification processes, because such temperatures exceed, by several orders of magnitude, known physical temperatures. But the intensity of the lines is not the only reason for suspecting maser processes. Extremely narrow lines have been observed, and amplifying processes can lead to a line narrowing, line shapes as well as widths being altered by the amplification. Thus line widths of 600 Hz, corresponding to kinetic temperatures of 5 K, could exist in situations where the line intensities correspond to temperatures between 1012 K. and 1014 K. Furthermore, if the maser mechanism were sensitive to polarization, then the observed radiation might exhibit a large degree of polarization, such as is observed. Finally, another property of maser processes should not be overlooked. If an OH medium can be brought to a state of amplification, then sources of radiation lying behind it, as viewed from the Earth, or emission of the medium itself, may appear as sources of extremely small angular size. This may be true because of the coherent nature of the amplifying processes, which produces a high degree of directionality in the amplified radiation. If this is indeed happening, then the small angular sizes observed in the interferometric observations may actually be “apparent” sizes rather than true angular sizes. For this reason, the linear dimensions of the OH emitting regions may be considerably larger than those inferences obtained from observations. 8. Techniques The continuation of this research, and particularly the investigation of the mechanism by which the OH molecule is formed in interstellar space; will involve further detailed observations using the greatest attainable sensitivity and freedom from harmful interference. Receivers used to study the OH lines need to have narrower resolutions than those used in the observation of the 1420 MHz line of hydrogen. The OH lines are usually narrower than the hydrogen lines because the OH molecule exists only in regions which are both more quiescent and of higher density. Linewidths of 1 to 10 kHz are typical. The receivers need, however, to be tunable over an appropriate range, since the lines observed are broadened and displaced in frequency up to several megahertz, as a result of Doppler effects due to relative motions in the line-of-sight. Furthermore, accurate measurements of the shape of the spectral lines require comparison measurements at adjacent frequencies which are free from the effects of OH absorption or emission. The overall frequency band technically necessary for detailed study of the two principal lines at 1665.4 and 1667.4 MHz, taking into account the requirements for comparison observations and Doppler shifts, is at least 5 MHz, and preferably about 10 MHz. In the foregoing, the OH molecule referred to is the OH molecule within our galaxy; however, OH has also been detected in external galaxies [Welizchew, 1971]. The studies in this area require downward extensions in frequency since the Doppler shift of the lines is much larger for gas residing in external galaxies than in our own galaxy.
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