5 August, 2007
The world's largest bolometer camera for submillimetre astronomy is now in service at the 12-m APEX telescope, located on the 5,100m high Chajnantor plateau in the Chilean Andes. APEX is a collaboration between the Max Planck Institute for Radioastronomy, Onsala Space Observatory and the European Organisation for Astronomical Research in the Southern Hemisphere (ESO).
LABOCA was built by Max Planck Institute for Radioastronomy (MPIfR) in Bonn, Germany, specifically designed for the study of extremely cold astronomical objects. With its large field of view and very high sensitivity, it will open new vistas in the knowledge of how stars form and how the first galaxies emerged after the Big Bang.
"A large fraction of all the gas in the Universe has extremely cold temperatures of around minus 250 degrees Celsius, a mere 20 degrees above absolute zero," says Karl Menten, director at MPIfR "Studying these cold clouds requires looking at the light they radiate in the submillimetre range, with very sophisticated detectors."
Astronomers use bolometers to detect incoming radiation by registering the resulting rise in temperature. More specifically, a bolometer detector consists of an extremely thin foil that absorbs the incoming light. Any change of the radiation's intensity results in a slight change in temperature of the foil, which can then be registered by sensitive electronic thermometers.
To be able to measure such minute temperature fluctuations requires bolometers to be cooled down to less than 0.3 degrees above absolute zero, that is below minus 272.85 degrees Celsius. "Cooling to such low temperatures requires using liquid helium, no simple feat for an observatory located at 5100m altitude," says Carlos De Breuck, the APEX instrument scientist at ESO.
Nor is it simple to measure the weak temperature radiation of astronomical objects. Millimetre and submillimetre radiation may open a window into the enigmatic cold Universe, but signals from space are heavily absorbed by water vapour in the Earth's atmosphere. "As if you were trying to see stars during the day," explains Axel Weiss of the MPIfR and leader of the team that installed LABOCA on APEX.
This is why telescopes for this astronomy must be built on high, dry sites, and why the 5107m high plateau at Liano deChajnantor in the extremely dry Atacama Desert in northern Chile was chosen. Even under optimal conditions, the heat from Earth's atmosphere is still a hundred thousand times more intense than the tiny astronomical signals from distant galaxies requiring specialist software to filter such weak signals from the overwhelming disturbances.
LABOCA and its essential and associated software were developed by MPIfR. "Since so far there are no commercial applications for such instruments, we have to develop them ourselves," explains Ernst Kreysa, from MPIfR and head of the group that built the new instrument.
"The software was developed mainly by members of my group at the MPIfR " says Professor Karl Menten
director of mm and submm astornomy, "with Frederic Schuller as the leading contributor." he notes that all the control and data acquisition software for APEX, the telescope on with LABOCA has been installed, was also developed, written and tested by this group.
APECS is the distributed control system of the new Atacama Pathfinder EXperiment (APEX) telescope. APECS is based on Atacama Large Millimeter Array (ALMA) software and employs a modern, object-oriented design using the Common Object Request Broker Architecture (CORBA) as the middleware.
New generic device interfaces simplify adding instruments to the control system. The Python based observer command scripting language allows the use of many existing software libraries and facilitates creating more complex observing modes.
A new self-descriptive raw data format (Multi-Beam FITS or MBFITS) has been defined to store the multi-beam, multi-frequency data. APECS provides an online pipeline for initial calibration, observer feedback and a quick-look display.
APECS is being used for regular science observations in local and remote mode since August 2005 and LABOCA data will be stored in MBFITS format (Multi-Beam FITS) and it will be possible to reduce the data using existing software like NIC and MOPSI, as well as Boa, and a new software package specifically devoped for LABOCA.
A bolometer camera combines many tiny bolometer units into a matrix, much like the pixels are combined in a digital camera. LABOCA observes at the submillimetric wavelength of 0.87mm, and consists of 295 channels, which are arranged in 9 concentric hexagons around a central channel. The angular resolution is 18.6 arcsec, and the total field of view is 11.4 arcmin, an amazing size for instruments of this kind.
"The first astronomical observations with LABOCA reveal its great potential. In particular, the large number of LABOCA's detectors is an enormous improvement over earlier instruments," says Giorgio Siringo from MPIfR and member of the LABOCA team. "LABOCA will allow us to map large areas on the sky with high sensitivity."
The Atacama Pathfinder Experiment (APEX) where LABOCA is installed is a new-technology 12-m telescope, based on an ALMA prototype antenna, and operating at the ALMA site. It has modified optics and an improved antenna surface accuracy, and is designed to take advantage of the excellent sky transparency working with wavelengths in the 0.2 to 1.4 mm range.
"APEX is located a mere 2 km from the centre of the future ALMA array. The new LABOCA camera will be very complementary to ALMA, as its very wide view will find thousands of galaxies which will be observed in great detail with ALMA," says De Breuck.