Page 26 - EngineerIt June 2021

SPACE SCIENCES

As noted by the anonymous reviewer of the manuscript submitted for publication, the receiver indexer, to be rotated
“it is clear that new results like this from MeerKAT and other SKA pathfinders are set to horizontally. The yoke houses the
overhaul our understanding of extragalactic radio sources”. azimuth wrap, which guides all the
The South African MeerKAT radio telescope, situated 90 km outside the small Northern cables when the antenna is rotated and
Cape town of Carnarvon, is a precursor to the Square Kilometre Array (SKA) telescope prevents them from becoming entangled
and will be integrated into the mid-frequency component of SKA Phase 1. The telescope or damaged. The structure allows an
was originally known as the Karoo Array Telescope (KAT) that would consist of 20 observation elevation range from 15 to
receptors. When the South African government increased the budget to allow the building 88 degrees, and an azimuth range from
of 64 receptors, the team re-named it “MeerKAT” – ie “more of KAT”. The MeerKAT -185 degrees to +275 degrees, where
(scientific name Suricata suricatta) is also a much beloved small mammal that lives in the north is at zero degrees.
Karoo region. The steerable antenna positioner
The MeerKAT telescope is an array of 64 interlinked receptors (a receptor is the can point the main reflector very
complete antenna structure, with the main reflector, sub-reflector and all receivers, accurately, to within 5 arcseconds
digitisers and other electronics installed). The configuration of the receptors is determined (1,4 thousandths of a degree) under
by the science objectives of the telescope. 48 of the receptors are concentrated in the low-wind and night-time observing
core area which is approximately 1 km in diameter. The longest distance between any two conditions, and to within 25 arcseconds
receptors (the so-called maximum baseline) is 8 km. (7 thousandths of a degree) during
normal operational conditions.
Each MeerKAT receptor consists of three main components:
• The antenna positioner, which is a steerable dish on a pedestal; How does MeerKAT work?
• A set of radio receivers; Electromagnetic waves from cosmic radio
• A set of associated digitisers. sources bounce off the main reflector,
then off the sub-reflector, and are then
The antenna positioner is made up of the 13,5 m effective diameter main reflector, and focused in the feed horn, which is part of
a 3,8 m diameter sub-reflector. In this design, referred to as an ‘Offset Gregorian’ optical the receiver.
layout, there are no struts in the way to block or interrupt incoming electromagnetic Each receptor can accommodate
signals. This ensures excellent optical performance, sensitivity and imaging quality, as well up to four receivers and digitisers
as good rejection of unwanted radio frequency interference from orbiting satellites and mounted on the receiver indexer. The
terrestrial radio transmitters. It also enables the installation of multiple receiver systems indexer is a rotating support structure
in the primary and secondary focal areas, and provides a number of other operational that allows the appropriate receiver
advantages. to be automatically moved into the
The combined surface accuracy of the two reflectors is extremely high with a antenna focus position, depending on
deviation from the ideal shape being no more than 0,6 mm RMS (root mean square). the desired observation frequency.
The main reflector surface is made up of 40 aluminium panels mounted on a steel The main function of the receiver is
support framework. to capture the electromagnetic radiation
This framework is mounted on top of a yoke, which is in turn mounted on top of a and convert it to a voltage signal that is
pedestal. The combined height of the pedestal and yoke is just over 8 m. The height of the then amplified by cryogenic receivers that
total structure is 19,5 m, and it weighs 42 tons. add very little noise to the signal. The first
The pedestal houses the antenna’s pointing control system. two receivers will be the L-Band and UHF
Mounted at the top of the pedestal, beneath the yoke, are an azimuth drive and Band Receivers.
a geared azimuth bearing, which allow the main and sub-reflectors, together with 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).
The L-band digitiser samples at a rate
of 1 712 million samples every second.
(The amount of data that is generated by
the digitiser for a receiver is equivalent to
approximately 73 000 DVDs every day or
almost 1 DVD per sec.)
Once the signal is converted to
digital data, the digitiser sends this data
via fibre optic cables to the correlator,
which is situated inside the Karoo
Array Processor Building (KAPB) at the
Losberg site complex.

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