RSAA News of the Month: April 2005
Looking Back to the Dark Age
"First Light" for prototype of radical new radio telescope
The prototype of a new type of radio telescope was tested in a remote
area of Western Australia during March. The telescope, the Mileura Widefield
Array - Low Frequency Demonstrator (MWA-LFD) is the first
element (tile) of what will grow to a large array of many such
elements. It operates at frequencies around 100 MHz, much lower
than other radio telescopes. The prototype element was assembled
and tested by Drs Frank Briggs (ANU and CSIRO's ATNF),
David Barnes and Jamie Stevens (Melbourne University), Brian
Corey (Massachusetts Institute of
Technology), and Merv Lynch and David Herne (Curtin University of
Technology).
The telescope is located in one of the quietest areas on
Earth as far as stray radio frequency noise is concerned. This is
vital for the principal aim of the experiment, to observe
conditions in the very early Universe, at the end of the so-called
Dark Age and the start of the "Epoch of Reionization". This was
the time that the first stars formed and their radiation began to
ionize the neutral hydrogen gas that filled the early Universe.
Three images of the region around the MWA site.
The radiation from the ionized hydrogen, now at huge distances, has
been Redshifted
to very low frequencies, into the FM broadcast band. Telescopes like
the MWA sited well away from radio stations are the best instruments
to probe this unknown stage of the evolution of the universe. Mapping
the sky at these frequencies will tell us much about how the earliest
stars formed, the sizes of the first star clusters and galaxies, and
how they evolved.
Note that the MWA will not see stars, but the hydrogen from which they
are forming. The star-forming regions will show as empty voids in the
overall clouds of hydrogen. As Frank Briggs puts it "It's
like looking at Swiss cheese; the cheese is the hydrogen, the holes
are where stars have formed".
The first MWA-LFD antenna tile was successfully installed in the first
weeks of March. Each tile is six meters square and has 16 antennae
mounted on it. Sophisticated electronics allow the beam of the
antennae to be pointed to any part of the sky. Power for the
antennae comes from a solar-power unit with battery backup. The
signals from the antenna were taken by coax cable to the caravan,
digitized and stored on a PC for later full analysis.
The convoy arrives from Perth,
the caravan laboratory is set up and the ground plane is erected.
Construction of the test array is an international collaborative
effort. Most of the radio frequency components and array control
electronics were built at MIT's Haystack Observatory, near
Boston, with the support of the US
National Science Foundation. Curtin University and the the
University of Melbourne handled the logistics, bringing the
caravan, generator, and the vital air-conditioner to the site. They
also constructed the metal "ground plane", the mesh grid
that acts as a mirror behind the 16 dipole antennae.
The 16 dipole antennae are attached to the ground plane,
solar power is connected and the tile is ready for testing to
commence.
The Canberra group was led by Frank Briggs, who holds a joint
ANU/CSIRO appointment. With support from the Australian Research Council
(ARC), they built the digital receiver, which converts the radio
signals to numbers that are streamed to computer disk where ANU
software analyses the frequency and time variability of the
signals. This highly-portable system, affectionately known as
"the Stromlo Streamer", was designed for just this type
of radio astronomy experiment, specifically, the study of the Epoch
of Reionization from remote sites. The Canberra Data Acquisition
Working Group (DAWG) includes researchers from the ANU, ADFA and the University of Sydney's Molonglo Radio
Observatory, where the Streamer has been making studies of
radio interference for over a year.
The Stromlo Streamer set up in the caravan laboratory,
and an aerial view of the site.
Preliminary analysis of the new data from the Western Australian site
shows that the system worked exactly
as expected. "First Light" was achieved on March 11. The
results show that the Mileura site is almost completely
radio-quiet. The main sources of interference seen during testing
were from satellites, which pass overhead at all sites on
Earth.
The first spectrum from the MWA.
For comparison, if the spectrum shown here were taken near a city,
there would be a mess of large spikes from FM radio stations
reaching to more than +50dB on the Relative Power scale.
A drift scan at five frequencies showing the plane of the
Milky Way passing through the telescope beam.
Over the next few months two more tiles will be added to the
array. The tiles will be linked together to operate as an
interferometer. Extra tiles will be added over time to build up an
array of 500 tiles covering an area about a kilometer in
diameter. The completed array should begin operation in
2007.
Showers in the semi-desert.
For astronomers investigating the Epoch of Reionization,
Mileura may well hold some of the gold at the rainbow's end.
Images: Frank Briggs (ANU/CSIRO) and P. Walsh (Mileura Cattle Co.)
For previous Monthly News items, click here.
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