2.5 Observations below 30 MHz Several phenomena of astrophysical interest manifest themselves only at observations at wavelengths of the order of 10 m or longer. Free-free absorption in ionized regions of the Galaxy, self-absorption in radio galaxies and in quasars, and low-frequency emission from tenuous plasmas in clusters of galaxies are a few which require extensive investigation. 2.6 Summary This outline of the nature of radio signals in radioastronomy shows two general facts. First, there is a wide variety of phenomena to be studied over the whole accessible range of radio frequencies. Second, the science is still growing at a rapid rate and enormously increasing our knowledge. These two facts demonstrate clearly the difficulties which face both the astronomer and the frequency allocation authorities in their search for the best solution to the problem of achieving the right degree of protection for a radioastronomy service. 3. Frequency considerations of the radioastronomy service Spectral line observations must be made at the specific frequency or frequencies set by nature for the spectral emission of the atoms or molecules of interest. Report 223-4 lists many details of spectral line observations already made by radio astronomers. Of the many spectral lines listed there, radio astronomers have identified a much smaller number which are of major importance and form part of the frequency requirements of the radioastronomy service. These are listed in Annex 1. Bandwidths required for these spectral line observations are set by the changes in line frequency resulting from the Doppler frequency shifts. For many molecules, observable only within the Galaxy, the Doppler shifts are related directly to the velocity of galactic rotation. For the hydrogen line near 1420 MHz there is an additional requirement for a frequency band below 1400 MHz. In this case the Doppler shift in frequency is much greater because observations are being made of other galaxies which are retreating from us at very high velocities. In the case of continuum observations the radio astronomer wishes to define the frequency variation of continuum emission, from sources of interest, over the entire spectrum available to them. It has been the experience of radio astronomers that observations at intervals of a factor of two in frequency are in general adequate for defining the overall spectrum, although closer spacings are needed for some specialized types of observation such as the measurement of polarization (see § 2.2). For ground based radioastronomy the lower limit of the frequency spectrum for which there is a requirement is about 1.5 MHz. At the present time the upper limit •is set by the availability of suitable technology and is about 300 GHz. It may be expected to go to higher frequencies in the future. Within these broad limits there is a general requirement for a frequency band in each octave of the spectrum. Some adjustment may be necessary at frequencies above 30 GHz since it is of importance that radioastronomy have access to the atmospheric windows of low attenuation. The attenuation curve is shown in Fig. 1, Report 223-4. The radioastronomy service has identified the need for bandwidths of the order of 1% to 2% for continuum measurements. As is pointed out later in this Report the sensitivity of radioastronomy receivers is improved when bandwidths are widened. For paraboloidal telescopes, wider bandwidths and consequent better sensitivities lead to improved efficiency in the use of these major astronomical installations. The same is true in the case of telescopes with unfilled apertures (such as the T or cross antenna). However, in this case, bandwidth can have a direct effect on the cost of the telescope. If the required sensitivity is not achieved because of an insufficiently wide bandwidth then it may be necessary to fill in more of the aperture at a very large cost. 4. Classes of observations 4.1 Radioastronomy observations can be broadly divided into two classes: 4.1.1 Class A observations are those in which the sensitivity of the equipment is not a primary factor. They are often used in the study of those cosmic emissions which are of relatively high intensity. Many of the solar, Jupiter, riometer, and scintillation observations fall into this class; continuity is a primary factor for these observations. 4.1.2 Class B observations are of such nature that they can be made only with advanced low-noise receivers using the best available techniques; long integration times and wide receiver bandwidths are usually involved. The significance of these observations is critically dependent upon the sensitivity of the equipment used in making them. 4.2 The sensitivity of receivers used for class B observations and the levels of harmful interference are discussed later in this Report. The simplest way to define the sensitivity of an observation in radioastronomy is to state the smallest power level change at the radiometer input which can, with high certainty, be detected and measured by the radiometer. In § 7 this quantity is defined, and typical values for it are derived. It is convenient to measure the smallest detectable change (ATJ in the equivalent temperature of the output terminals of the antenna connected to the radiometer.
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