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February 2011
In this issue
First HartRAO-KAT-7 VLBI fringes signal new capability
MeerKAT engineers launch new ROACH board
MeerKAT Science - the Large Survey Projects
More power and connectivity milestones for Karoo astronomy reserve
SKA South Africa and NRAO to deepen collaboration in 2011
MeerKAT Karoo Express - flying to the MeerKAT site
Postdoctoral opportunities at SKA South Africa
MeerKAT advances cutting-edge expertise
Students star at SA SKA postgraduate conference
Technician team deployed at KAT-7 in Karoo
Review of Karoo reserve infrastructure
Virtual sky display in MeerKAT control room
SKA online - useful websites

First HartRAO-KAT-7 VLBI fringes signal new capability

Jasper Horrel, MeerKAT project office, Cape Town



Correlated signal from the 3C273 radio source - view larger version


Desktop PC data recorder used in the VLBI experiment


The VLBI baseline between KAT-7 and HartRAO - view larger version


Simon Ratcliffe and Jasper Horrell worked on the VLBI experiment from the MeerKAT office in Cape Town


Jonathan Quick coordinated the VLBI experiment from HartRAO
A milestone has been achieved in South Africa with the successful detection of "fringes" in a joint very long baseline interferometry (VLBI) observation performed using one of the seven 12 m dishes of the KAT-7 radio telescope, near Carnarvon in the Northern Cape, together with the 26 m dish of the Hartebeesthoek Radio Astronomy Observatory (HartRAO) near Pretoria.

VLBI is a well established technique, where the signals recorded by widely separated radio telescopes, simultaneously observing the same part of the sky, are brought together to produce a very high resolution radio picture of that region of the sky. While HartRAO has been involved in VLBI observations for many years with telescopes around the world, this is the first time that a KAT-7 antenna has been used and the first time that all the data processing has been done in South Africa.

Engineers and scientists at the MeerKAT site and HartRAO jointly observed a distant radio source known as 3C273, recorded the data and then correlated the signals in Cape Town to produce the successful fringe detection for the first time and at the first attempt. Fringe detection is a prerequisite for full VLBI operations, where the high resolution images are made from simultaneous observations using many telescopes.

In order to achieve the fringe detection, a number of significant technical challenges had to be overcome. For example, apart from being separated by 900 km and needing to be operated in tandem, the two telescopes used for the observations are completely different in design and construction, both in the mechanics and electronics. The HartRAO antenna recorded the data to a Mark5A VLBI recorder, while the KAT engineers constructed a new and flexible data recorder system (using ROACH-1; graphics processing units and other high-end PC components) to perform the job. The HartRAO antenna makes use of a hydrogen maser as the master clock, while a GPS-disciplined rubidium oscillator was used in the Karoo.

To produce the detection, the signals were jointly recorded, the data were transported to Cape Town and converted to the same format, a so-called "fringe stopping" correction was applied to the data (a correction for the earth's rotation, using the accurately known antenna and radio source positions) and the data were correlated to produce a "lag plot". A significant spike in the lag plot shows the point at which the two recorded signals are perfectly time-aligned and that the detected signal comes from the radio source on the sky (not from the inherent background noise of either of the two systems).

In the attached correlator lag plot, the big spike near the right, towering above the noise floor of the observations, is the detection of a correlated signal from the 3C273 radio source using a 16 MHz frequency band, centered at 1704.49 MHz. The x-axis represents a correlation interval of 0.524 secs (167 77 216 samples at 32 Msps). The very narrow peak (approx 1 sample) shows where the two signals match (correlate) or, in other words, indicates the common signal from the radio source simultaneously detected by the two widely separated antennas.

This result is a signal of things to come not only for South Africa, but for Africa in general. A proposed new initiative for a VLBI network composed of dishes stationed around Africa, known as the African Telescope Array, is gaining momentum. Such cutting-edge science collaboration brings together countries in ways which not only foster closer collaborations and spread new expertise and technology, but also remind us in a concrete and practical way that together we can do more.