First Light for Worlds Largest "Thermometer Camera"

LABOCA in Service at APEX

August 10, 2007
The worlds largest bolometer camera for submillimeter astronomy is now in service at the 12-m APEX telescope, located on the 5100-m high Chajnantor plateau in the Chilean Andes. LABOCA (LArge BOlometer CAmera) was built by the bolometer group of the Max-Planck-Institut für Radioastronomie in Bonn and was specifically designed for the study of extremely cold astronomical objects and, with its large field of view and very high sensitivity, will open new vistas in our knowledge of how stars form and how the first galaxies emerged from the Big Bang.
LABOCA, a bolometer camera with 295 pixels. Since May 2007 it is operational at the APEX submillimeter telescope.
APEX submillimeter telescope at 5100 m altitude at the Chajnantor plateau in Northern Chile.

"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, Principal Investigator of APEX and a director at the Max Planck Institute for Radioastronomy (MPIfR) in Bonn, Germany, that built LABOCA. "Studying these cold clouds requires looking at the light they radiate in the submillimetre range, with very sophisticated detectors."

Astronomers use bolometers for this task, which are, in essence, thermometers. They 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 the 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, which is no simple feat for an observatory located at 5100-m altitude", says Carlos De Breuck, the APEX instrument scientist at ESO.

Nor is it simple to measure the weak temperature radiation of astronomical objects. Millimeter and submillimeter radiation opens a window into the enigmatic cold Universe, but the signals from space are heavily absorbed by water vapour in the Earth’s atmosphere. "It is a bit as if you were trying to see stars during the day", explains Axel Weiss of the MPIfR, a member of the team that installed LABOCA on APEX.

This is why telescopes for this kind of astronomy must be built on high, dry sites, and why the 5100 m high plateau at Chajnantor in the extremely dry Atacama Desert was chosen. Even under such optimal conditions the heat from Earth's atmosphere is still a hundred thousand times more intense than the tiny astronomical signals from distant galaxies. Very special software is required to filter such weak signals from the overwhelming disturbances.

LABOCA and its associated software were developed by the 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.

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 submillimeter wavelength of 0.87 mm, and consists of 295 channels, which are arranged in a hexagonal pattern. The angular resolution is 18.6 arcsec, and the total field of view is 11.4 arcmin, a remarkable number for instruments of this kind.

"The first astronomical observations with LABOCA have revealed its great potential. In particular, the large number of LABOCA's detectors is an enormous improvement over earlier instruments" says Giorgio Siringo from MPIfR, a leading member of the LABOCA team. "LABOCA is the first camera that will allow us to map large areas on the sky with high sensitivity and angular resolution." Figs. 2 and 3 show two LABOCA observations of a galaxy, 10 million light years away, and a star forming region in our Milky way.

© CFHT (J.-C. Cuillandre)
© MPIfR

The galaxy NGC 253 as observed in the optical (left) and the cold dust distribution observed with LABOCA in May 2007 (right) roughly shown on the same scale. The distribution of the cold dust closely follows the dust absorption features seen in the optical.

The Atacama Pathfinder Experiment (APEX) where LABOCA is installed is a new-technology 12-m telescope, based on a prototype antenna for the Atacama Large Millimeter Array (ALMA), which will consist of 50 antennas. APEX operates at the ALMA site. It has additional tertiary optics and an excellent antenna surface accuracy, and is designed to take advantage of the extraordinary sky transparency working with wavelengths in the 0.2 to 1.4 mm range. As its name indicates, APEX is a pathfinder for ALMA. It will find a number of interesting objects in the Universe, which ALMA with its very high angular resolution, will later study in exquisite detail.

The Galactic HII region RCW120. LABOCA image showing the Galactic HII region RCW120 after an exposure of only slightly more than 3 hours. The expanding shell causes the surrounding gas to collapse into clumps, which are the cradles of massive stars. As the gas in these clumps is still very cold, around -250 degrees Celsius, they can only be seen at submillimetre wavelengths. Thanks to the high sensitivity and large field of view of LABOCA, astronomers could detect clumps that are four times fainter than was possible before. As the brightness is also a measure of the mass of these stellar embryos, this will allow scientists to study the formation of more representative, less massive stars.

APEX is a collaboration between the Max Planck Institute for Radioastronomy, Onsala Space Observatory and ESO.

Web-View
Print Page
Open in new window
Estimated DIN-A4 page-width
Go to Editor View