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April/May 2007
Number 6
MeerKAT prototype antenna installed at HartRAO
Feeding the meerKAT prototype
SKA champions in South Africa
CONRAD reaches second milestone
Major European award for Southern African astronomy project
PED dish first light
Exciting C-BASS science now at HartRAO
MeerKAT meeting in Amsterdam
Sharing science with South Africans
Exciting C-BASS science now at HartRAO

Delivery of the 7.2 m C-BASS dish at HartRAO on 14 February 2007. The C-Band All Sky Survey (C-BASS), will map the whole sky in temperature and polarization at 5GHz.

By Justin Jonas

Radio astronomy dishes are mushrooming all over the Hartebeesthoek Radio Astronomy Observatory (HartRAO) facility. Not long ago, the only radio telescope on the site was the 47-year old 26-metre dish that has been making first-rate science observations since NASA left it in the 1970s. HartRAO is now also the site for the KAT prototype, constructed there with new-generation composite fabrication techniques.

Another recent arrival is a new 7.2-metre dish, delivered at the site with the help of a massive flatbed truck and telescopic crane. This dish will be used to develop the South African component of the C-BASS project, an international collaboration that will map the polarization of the radio emission from our own galaxy, the Milky Way, with exquisite accuracy.

This dish is one of two identical units purchased by the National Research Foundation from Telkom SA Ltd. The dishes were part of a 5-dish facility at the Telkom Hartebeesthoek ground station. Dr Khotso Mokhele, then President of the NRF, negotiated with Telkom senior management for two of the dishes for scientific use. The C-BASS partners and the South African radio astronomy community are extremely grateful to Telkom for their generosity in providing the dishes at a nominal cost, which has allowed South African participation in this important experiment, and has improved the scientific prospects of the experiment by providing a southern hemisphere component.

The acronym C-BASS stands for "C-Band All Sky Survey", which in turn means that the radio frequency to be used is 5 GHz (C-band) and that the entire sky will be mapped.

The international partners in this global project are Caltech in the USA, the Universities of Oxford and Manchester in the UK, and Rhodes University and HartRAO in South Africa. Although the polarization of the Galactic radio emission is interesting in its own right (because it traces the Galactic magnetic field), the primary goal of the C-BASS project is to apply corrections to current and planned measurements of the polarization of the Cosmic Microwave Background Radiation (CMBR).

The CMBR is the relic emission from the early universe that was generated some 300 000 years after the Big Bang. The universe is now some 13 billion years old, and the CMBR gives us the earliest "picture" of the state of the universe, at an epoch some billion years before the first stars and galaxies were formed. The first detection of the CMBR in the 1960s earned Penzias and Wilkinson a Nobel Prize, and more recently George Smoot and John Mather were also awarded a Nobel prize for detecting "ripples" in the CMBR and measuring its temperature using the COBE satellite. The ripples are caused by very small density fluctuations in the primordial gas which were the seeds for later galaxy formation.

Although the CMBR gives us our earliest picture of the universe, scientists wish to determine what happened in the universe in the period from the Big Bang till the CMBR epoch. In other words, they want to understand how the primordial density ripples formed, and in doing so get a better understanding of the structure of time and space. Einstein's Theory of Relativity and competing theories provide a number of different interpretations of how the universe expanded in its infancy. The popular "inflation" theory predicts that the universe experienced a short period of explosive expansion shortly after the Big Bang. This expansion would have left a distinctive imprint in the polarization of the CMBR, and hence the measurement of the CMBR polarization has become an important scientific pursuit.

These measurements are extremely difficult to do and require very stable high-frequency receivers. The measurements are affected by many interfering effects, such as the earth's atmosphere and the radio emission from the Milky Way. The effects of the atmosphere are reduced by placing the CMBR telescopes on high mountains, high-altitude balloons and satellites. The Planck Surveyor satellite, due for launch next year, has as its primary goal the measurement of the CMBR polarization, and there are a number of terrestrial and balloon-borne experiments currently underway.

One can escape the effects of the atmosphere, but we cannot escape the radio emission from our own galaxy, so CMBR scientists need to have extremely accurate models of the Galactic emission so that they can remove its effect from the measured CMBR images. This is where C-BASS comes in: to provide the CMBR scientists with the required model of the "Galactic foreground" polarization.

The need to map the entire sky implies that two telescopes are required, one in each hemisphere. The northern hemisphere telescope will be located at the Owens Valley Radio Observatory in the USA. Ultimately the southern hemisphere telescope will be located in the Radio Quiet Reserve in the Karoo, close to the MeerKAT and proposed SKA sites. The dish at HartRAO will be used to develop and test the receiver and other telescope systems. A second dish will be transported to the Karoo site for the actual measurements. The radio frequency interference (RFI) environment is far better at the Karoo site, and the atmosphere is much more stable and dry. The HartRAO dish will be used for education and public outreach programmes once C-BASS operations move to the Karoo.