HartRAO Home

HartRAO Home > Continuum Observing with the Hart 26m Telescope

Continuum Observing with the Hart 26m Telescope

Radio continuum emission is the broadband radiation emitted in the radio part of the spectrum by celestial objects. Its intensity (brightness temperature) typically varies relatively slowly as a function of wavelength (or frequency). This is in contrast to the narrow emission lines produced at characteristic frequencies by atoms and molecules. Continuum observations can be made with all installed receivers at HartRAO.

beam sizes
This picture shows how much of the sky is "seen" by the radio telescope in four of the wavelength bands in which it operates. The angular size of the moon is shown for comparison. The circular patch of sky seen at each wavelength is called the "beam" of the telescope. The size of the beam is proportional to the wavelength.

At wavelengths of 6 and 3.5 cm, dual feed systems are installed so that the telescope "sees" two patches in the sky next to each other when operating in the beam-switched mode. This is used to cancel out atmospheric effects that are common to both beams. The receiver output is proportional to the difference in the signal in the two beams.

Technically, the pictures of what is seen at each wavelength show the main lobe of each antenna beam pattern. Physicists will recognise this as the central part of the diffraction pattern of a circular aperture. Starting at the centre of the beam, each black contour line is at half the level of the previous one. Weak ring-like sidelobes lie beyond the main lobe, but are not shown here.

Image by Mike Gaylard, HartRAO

Point source flux measurements

For flux measurements of unresolved ("point") sources, several observing methods are available: These enable pointing corrections to be determined where necessary (shorter wavelengths), so that the the on-source flux density can be corrected for pointing error.

An example of flux measurements made with the 6 cm / 5 GHz and 3.5 cm / 8.5 GHz dual-beam receivers is shown below.

The radio quasar PKS 0405-385 was recently found to show extreme variability on a timescale of less than an hour, which is unprecedented among extragalactic radio sources. First detected in observations with the Australia Telescope Compact Array, their monitoring was supplemented by observations at Hartbeesthoek when the object was below the horizon at the ATCA. In the image above, the dots represent ATCA data and the triangles HartRAO data. From top to bottom, the observing frequencies are 8.6, 4.8, 2.4 and 1.4 GHz. These data were taken on 1996 June 8 and 9. For more information see Kedziora-Chudczer et al. (1997) Astrophysical Journal, 490, L9-L12.

Image by George Nicolson, HartRAO

Mapping extended radio sources

Starting in the 1980's the radio emission of almost the entire southern sky was mapped using the 26m telescope in a collaboration between astronomers at Rhodes University in Grahamstown and at HartRAO. The completed radio map was published by Jonas, Baart and Nicolson (1998, MNRAS, 297, 977-989). The map is shown below in galactic coordinates.

The control system for the 26m telescope has the ability to make scans for creating maps using any installed receiver, under the queue scheduler.

southern sky map
This map shows the radio emission along the plane of the Milky Way at a frequency of 2326 MHz (13 cm wavelength). The map is in Galactic coordinates so that the direction towards the centre of the Milky Way is in the middle of the map. Radio emission from the inner spiral arms, seen edge on, produces the white horizontal band across the centre. The radio galaxy Centaurus A is the yellow "worm" above and to the right of the centre of the picture. Its optical counterpart is called NGC5128. The Gum nebula is the big bubble above and below the galactic plane to the right of centre, the Vela supernova remnant being the yellow area within it. The nebulae in Orion, surrounded by Barnard's Loop, are visible in the lower right corner. Bright spots away from the galactic plane are not stars, but mainly radio-emitting galaxies and quasars.

Image by Justin Jonas, Rhodes University / HartRAO