Radio Astronomy

The Radio Sky

If our eyes were sensitive for radio waves, we would see this: the ground, buildings, trees, and human beings would appear bright, because of their emission of thermal radiation and because the radio region is the long-wave continuation of the infrared. The sky would be black, as from it comes only the radiation of the Cosmic Microwave Background (CM) with a temperature of 2.7 K, with the addition of a similarly strong contribution of thermal radiation of the Earth atmosphere. Day and night would have the same appearance. Our Sun would be a very bright small disk, the Moon and the planets would appear as faint and very small spots, similar to the visual region. There would be no stars visible, but instead a number of single sources such as gaseous nebulae and galaxies. At a wavelength of 21 cm the Milky Way would shine as a bright band across the sky, with a brighter part in direction of the Galactic Centre.

Electromagnetic radiation from celestial sources can be observed from the ground between frequencies of 30 MHz to about 30 GHz, corresponding to wavelength of 10 m to 1 cm. The lower limit is due to the presence of the ionosphere, the upper limit due to absorption by molecules in the air.

In this radio window we can observe a number of sources: The Sun is by far the brightest object. Apart from a steady, quiet component of thermal radiation from the photosphere, chromosphere and corona it may emit a substantially enhanced radiation during active phases. The Moon and planets also send out thermal radiation, in a blackbody spectrum corresponding to their surface temperature.

Spectral energy distribution of sky objects in the radio region (adapted from J.D.Kraus). The orange horizontal lines indicate the sensitivity limits of telescopes with 1 and 10 m diameter for a detection limit of 100 K in antenna temperature. The red squares show for our antennas the minimum fluxes necessary to notice a source already by simple registration of the data. With more sophisticated methods substantially weaker sources can be measured. The thick green vertical line marks the 21 cm spectral line of neutral hydrogen at 1420 MHz, which sits on top of the continuum spectrum of e.g. the Milky Way.}

In the Galactic Plane and external galaxies (M31), supernova remnants (CasA), quasars (3C273) and radio galaxies (Cygnus) the radio flux decreases towards higher frequencies. The responsible mechanism is synchrotron emission, which is produced by electrons which the interstellar magnetic field forces on circular trajectories.

The plasma in H~II regions (Orion) is transparent at high frequencies and the continuum emission is rather flat. At lower frequencies the region becomes optically thick and shows the blackbody spectrum, i.e. the flux rises with frequency,

The underlying processes

Electromagnetic radiation - from the radio to the X-ray region - is produced by charged particles (elektrons, protons). It appears in the form of a broad-band continuous component and also as very narrow-band radiation of spectral lines:

Continuum
- Thermal radiation: depending on its temperature every material body emits electromagnetic radiation, mostly in the infrared and optical, and in the radio region. With increasing temperature the maximum of the spectrums (blackbody spectrum) is shifted to higher frequencies (Sun, Moon, gaseous nebulae, Milky Way).
- Synchroton emission: electron are deflected by a magnetic field onto circular or helical trajectories. This acceleration causes them to emit electromagnetic waves (Milky Way, supernova remnants, active galaxies, quasars)
Lines
- 21 cm line: the hydrogen atom consists of a proton which is orbited by an electron. Both particles have an instrinsic 'spin'. If a collision with another particle causes to bring the tso spins in a parallel orientation, the electron will re-align its spin to the anti-parallel orientation by radiating away the excess energy in a radio wave at a single, well-defined frequency of 1420.405751 MHz. This makes it possible to observe neutral hydrogen in the Milky Way and other galaxies. By measuring the frequency shift with respect to the theoretical frequency one can determine via the Doppler effect velocities of gas clouds and galaxies.
- Lines of atoms and molecules: The space between the stars is not empty, but filled with gas clouds of different temperatures. In cold and dense regions one finds a great variety of molecules, from simple one (carbon monoxide, water, ammonia) to more complex oragnic compounds, which are identifiable due to their characteristic spectral lines.

Radio Telescopes

Such an instrument consists of an antenna to intercept the radio waves and convert them into an electronic signal, the receiver to amplify and filter the signals, and a computer to deal with any subsequent processing and recording, as well as the control or the telescope.

For antennas we use parabolic mirrors which concentrate the radio waves onto a dipole in the focus. The oscillating field of the electromagnetic wave induces in the dipole an alternating voltage which is boosted by a low-noise preamplifier and is fed to the receiver. As with an optical telescope, the mirror has two functions: to capture as much energy from the wave, and to concentrate the sensitivity to a suitably small region in the sky.

More details may be found here.

Observational Methods

The principal issue in radio astronomy is noise: The signals from celestial bodies are nothing but unmodulated noise, which is often just above the noise of the receiver electronics. In addition, there is theremal radiation from the Earth atmosphere, fluctuations of the signals due to temperature and weather effects, and radio interference. Thus, it is a necessity to compare the signal of a source with other (known) signals, before conclusions can be drawn.

At DL0SHF these methods are in use:

Averaging over as many data as possible (or suitable)
pointed measurements at one position and integrating over all single measurements
pointed measurements of a celestial source which is tracked on its apparent path across the sky
alternating between measuring the object and the 'empty' sky nearby. In this way also weak sources can be detected which may be only just above the sky background
drift scans of single objects: The antenna is held fixed at a position where the object will be in a few minutes. During the passage of the object through the antenna lobe data are continuously recorded
mapping of the sky by drift scans: Leaving the antenna pointed at a position, the Earth rotation lets a strip of the sky pass through the antenna beam, whose signals are continuously recorded. Since the rotation is very precise, one can derive from the time of observation the instantaneous position.