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The bolometer array of LABOCA is nominally made of 295 composite bolometers, of these about 260 are functional. The bolometers are arranged in a hexagonal layout consisting of a center channel and 9 concentric hexagons.

A composite bolometer can be described as the combination of two elements: an extremely sensitive heat detector, called thermistor, and a radiation absorber. The radiation collected by the telescope is fed to the bolometers through a series of mirrors and a lens, some filters defining the passband, and conical horn antennas, which embed circular wave guides, used as high-pass filters, at their output.

Left: the CAD drawings show the assembling at the focal plane. The bolometer array is mounted into a copper ring, under a monolitic array of conical horn anntennas and above a plane subreflector, fixed at a quarter wavelength distance. Right, top: a picture of the "naked" LABOCA array. Right, bottom: design scheme of a single bolometer. The left drawing shows the section of a conical horn antenna ending in a circular wave guide. The green parts show the a section of the silicon wafer, the membrane is represented by a thin line.The small red square represent a NTD thermistor. The right drawing is a front view of a bolometer.

 

The array is manufactured on one single 4-inch silicon wafer coated by a silicon-nitride film on both sides, using plasma enhanced chemical vapour deposition. On one side of the wafer the wiring is made by deposition of niobium and gold thin metal layers. On the other side, 295 square cavities are machined by alkaline etching of the silicon, producing unstructured silicon-nitride membranes hosting the bolometers, only 0.4 micron thick.

In the bolometers of LABOCA, the absorbing element is made of a thin film of titanium. This version of LABOCA uses neutron-transmutation-doped (NTD) germanium semiconducting chips as thermistors (Haller & Beeman, LBNL Berkeley) to measure the temperature of the absorber. The thermistors are soldered to the membranes and electrically connected via niobium and gold wires.
The array of LABOCA mounted in the copper ring. Some bonding wires are visible.		The thermistors are visible as small cubes on the membranes. One broken membrane is visible on the top left corner.

To work properly, the array of LABOCA needs to be cooled down to below 0.3 K (i.e. to -273 degree Celsius or 0.3 degrees above absolute zero). Any change in the radiation intensity causes a corresponding change in the temperature of the absorber, by a very small amount.

A complex electronic system records the electric signals generated by the thermistors, from which the incoming radiation intensity may be deduced.

Bolometers have a very broad spectral sensitivity to the total intensity of the incoming radiation, requiring a system of spectral filters to define the passband. LABOCA has a central frequency of about 345 GHz with a bandwitdth of about 60 GHz. The APEX beam size at this wavelength is 18".6 and the total field of view for LABOCA is 11'.4. The array is undersampled on the sky. The separation between channels in one row is twice the beam size (37"). Therefore it is necessary to use special observing techniques such as scanning or jiggling to produce fully sampled maps.

Footprint of LABOCA at APEX (red) compared to MAMBO-I (37 bolometers, green) and MAMBO-II (117 bolometers, blue) at Pico Veleta. Click to download a EPS version (1.7 MB)

The picture above shows the position on the sky of the LABOCA beams (red), working at 0.87 mm wavelength on APEX, compared to the beams on the sky of MAMBO-I (green, 37 bolometers) and MAMBO-II (blue, 117 bolometers) working at 1.2 mm wavelength on the IRAM 30 meter telescope of Pico Veleta (see http://www.mpifr-bonn.mpg.de/div/bolometer/#mambo ).