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• Bolometer Array
• Data Acquisition
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• Science with LABOCA

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LABOCA is a very complex system, composed of a variety of devices related to different technological fields, like optics, high vacuum, low temperature cryogenics, digital electronics, software, etc. A general view of the infrastructure is given in the block diagram below.




The heart of the LABOCA facility instrument is a bolometric continuum receiver made of 295 semiconducting composite bolometers. Unstructured silicon nitride membranes micromachined on one single 4-inch silicon wafer, carry the composite bolometers  to form an array of detectors. The membranes are only 0.4 µm thick and are coated with a thin titanium film which absorbs the incoming radiation. Neutron-transmutation-doped (NTD) germanium chips (called thermistors), soldered to the membranes, detect the temperature rise due to the absorption of radiation.

The array is mounted into a cryostat, which uses liquid nitrogen and liquid helium and a closed cycle double-stage sorption cooler to reach an operation temperature of 285 mK. The cryostat is mounted into the Cassegrain cabin of the Picture Gallery and the optical coupling to the main telescope beam is provided by an optical system made of a series of metal mirrors and a lens placed at the cryostat entrance. A set of cold filters, mounted on the liquid nitrogen and liquid helium shields, define the spectral passband, centred  at a wavelength of 870 µm (345 GHz) and about 150 µm (60 GHz) wide. A monolithic array of conical horn antennas, placed in front of the bolometer wafer, collects the radiation onto the bolometers.

The voltages at the thermistors are channeled to the outside of the cryostat along 12 flat cables (made of manganin wires on kapton substrate) going through low-noise, unity gain JFET amplifiers heat sunk to the liquid nitrogen bath. Upon exiting the cryostat, the signals pass to room temperature low-noise amplifiers and electronics. The 295 signals are distributed to 4 identical, custom made, amplification units, providing 80 channels each for a total of 320 available channels. The extra 25 channels are used for technical purposes like noise monitoring and calibrations.

The 4 amplification units are equipped with microprocessors providing a digital interface, accessible remotely via the local network, to control some of their properties, like the amplification gain. At the beginning of each observation, the DC offsets are removed from each channel (to avoid the risk of saturation) and stored in the data file, to be used during reduction process. The 320 channels are digitized over 16 bits by 4 multifunction DAQ PCI boards (NI PCI 6225M) mounted into an industrial computer (IEI SAGP-8650EVG).

The data acquisition software provides a SCPI (Standard Commands for Programmable Instrumentation) interface to the APEX control software, used to set up the hardware, and a TCP data server, for the data output. The amplification units provide an AC current to bias the bolometers and perform real time demodulation of the 320 signals. This electronic scheme is fundamental for the stability of the post-detection signals at low frequencies. The AC bias frequency is provided by the data acquisition system as a submultiple of the sampling frequency (usually set to 1 kHz) thus synchronizing the bias to the data sampling.

Before reaching the telescope's control software, the data (about 4 MB/s) are digitally filtered and down sampled to 25-50 Hz in real time by a computer specifically equipped for bridging between data acquisition and control software.

Another computer is devoted to monitor and control most of the electronics embedded into the receiver (e.g. monitor of all the temperature stages, control of the sorption cooler, calibration unit) and also provides an interface to the APEX control software, allowing remote operation of the system.