The South African MeerKAT radio telescope, currently being built 90 km outside the Northern Cape town of Carnarvon, is a precursor to the Square Kilometre Array (SKA) telescope and will be integrated into the mid-frequency component of SKA Phase 1.
The MeerKAT telescope will be an array of 64 interlinked receptors (a receptor is the complete antenna structure, with the main reflector, sub-reflector and all receivers, digitisers and other electronics installed).
The configuration (placement) of the receptors is determined by the science objectives of the telescope.
48 of the receptors are concentrated in the core area which is approximately 1 km in diameter.
The longest distance between any two receptors (the so-called maximum baseline) is 8 km.
Each MeerKAT receptor consists of three main components:
- The antenna positioner, which is a steerable dish on a pedestal;
- A set of radio receivers;
A set of associated digitisers.
- The antenna positioner is made up of the 13.5 m effective diameter main reflector, and a 3.8 m diameter sub-reflector. In this design, referred to as an ‘Offset Gregorian’ optical layout, there are no struts in the way to block or interrupt incoming electromagnetic signals.
This ensures excellent optical performance, sensitivity and imaging quality, as well as good rejection of unwanted radio frequency interference from orbiting satellites and terrestrial radio transmitters.
It also enables the installation of multiple receiver systems in the primary and secondary focal areas, and provides a number of other operational advantages.
The main reflector surface is made up of 40 aluminium panels mounted on a steel support framework.
This framework is mounted on top of a yoke, which is in turn mounted on top of a pedestal. The combined height of the pedestal and yoke is just over 8 m. The height of the total structure is 19.5 m, and it weighs 42 tons.
The pedestal houses the antenna’s pointing control system.
How does MeerKAT work?
Electromagnetic waves from cosmic radio sources bounce off the main reflector, then off the sub-reflector, and are then focused in the feed horn, which is part of the receiver.
Each receptor can accommodate up to four receivers and digitisers mounted on the receiver indexer.
The indexer is a rotating support structure that allows the appropriate receiver to be automatically moved into the antenna focus position, depending on the desired observation frequency.
The main function of the receiver is to capture the electromagnetic radiation and convert it to an voltage signal that is then amplified by cryogenic receivers that add very little noise to the signal. The first two receivers will be the L-Band and UHF Band Receivers.
Four digitisers will be mounted on the receiver indexer, close to the associated receivers.
The function of the four digitisers is to convert the radio frequency (RF) voltage signal from the receiver into digital signals. This conversion is done by using an electronic component called an analogue to digital converter (ADC).
Once the signal is converted to digital data, the digitiser sends this data via buried fibre optic cables to the correlator, which is situated inside the Karoo Array Processor Building (KAPB) at the Losberg site complex.
A total of 170 km of buried fibre cables connect the receptors to the KAPB, with the maximum length between the KAPB and a single antenna being 12 km.
The fibre cables run inside conduits buried 1 m below the ground for thermal stability.
At the KAPB, the signals undergo various stages of digital processing, such as correlation – which combines all the signals from all the receptors to form an image of the area of the sky to which the antennas are pointing – and beam-forming, which coherently adds the signals from all the receptors to form a number of narrow, high sensitivity beams used for pulsar science.
Time and frequency reference signals are distributed, via buried optical fibres, to every digitiser on every receptor, so that they are all synchronised to the same clock. This is important to properly align the signals from all receptors.
The telescope was originally known as the Karoo Array Telescope (KAT) that would consist of 20 receptors. When the South African government increased the budget to allow the building of 64 receptors, the team re-named it “MeerKAT” – ie “more of KAT”.
The MeerKAT (scientific name Suricata suricatta) is also a much beloved small mammal that lives in the Karoo region.
Photo: Clive Wright