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The Messenger LABOCA commissioned on APEX The Messenger Testing of Multi-conjugate AO Demonstrator Astroparticle Physics in Europe Weighing Ultracompact Dwarf Galaxies No. 129 – September 2007 –September 129 No. Telescopes and Instrumentation A New Era in Submillimetre Continuum Astronomy has Begun: LABOCA Starts Operation on APEX Giorgio Siringo1 Figure 1: A ‘naked’ Axel Weiss1 LABOCA silicon wafer. 1 Each small square is a Ernst Kreysa bolometer. Frederic Schuller1 Attila Kovacs1 Alexandre Beelen1, 2 Walter Esch1 Hans-Peter Gemünd1 Nikhil Jethava1 Gundula Lundershausen1 Karl M. Menten1 Rolf Güsten1 Frank Bertoldi 3 Carlos De Breuck 4 Lars-Åke Nyman 4 Eugene Haller 5 Jeff Beeman 5 1 Max-Planck-Institut für Radioastro- nomie, Bonn, Germany 2 Institut d’Astrophysique Spatiale, Université Paris-Sud, Paris, France 3 Argelander-Institut für Astronomie, Universität Bonn, Germany oped by the Bolometer Group of the Observations of astronomical objects 4 ESO MPIfR, LABOCA is the most complex from ground-based telescopes have to 5 Lawrence Berkeley National Laboratory, system ever developed by this group. The pierce that screen presented by the at- Berkeley, California, USA design of this new facility takes advan- mosphere, therefore requiring techniques tage of the experience accumulated over to minimise its effects. The most widely- several years in developing bolometers used technique is application of a switch- In May 2007, the Large APEX Bolometer for millimetric and submillimetric atmos- ing device, usually a chopping secondary Camera LABOCA was commissioned pheric windows and operating them on mirror (com monly called a ‘wobbler’), to as a facility instrument on the APEX ground-based telescopes. observe alternatively the source and an 12-m submillimetre telescope located at area of blank sky close by, at a frequen - an altitude of 5100 m in northern Chile. The main obstacle, when observing at cy higher than the variability of the sky The new 870-µm bolometer camera, in millimetre and submillimetre wavelengths, noise. Invented for observations with sin- combination with the high efficiency of is our Earth’s atmosphere, which is seen gle pixel detectors, this method is also APEX and the excellent atmospheric by a bolometer like a bright screen. It used with arrays of bolometers. However, transmission at the site, offers unprece- is as difficult as trying to do astronomical it presents some disadvantages and the dented capability in mapping submilli- observations in the optical during day- most evident are, among others, that the metre continuum emission. An overview time. This is largely due to the water va- wobbler is usually slow (1 or 2 Hz), posing of LABOCA and the prospects for sci- pour present in the atmosphere, with a lim itation to the scanning speed, and ence are presented. only small contributions from other com- that not all telescopes are equipped with ponents, like ozone. In the submillimetre a wobbler. range the only sources in the sky brighter A technological challenge than the atmosphere are the planets LABOCA has been specifically designed Venus, Mars, Jupiter and Saturn (and, of to work without a wobbler to remove the A new facility instrument has started op- course, the Sun and the Moon). All other atmospheric contribution, using a differ- eration on the APEX telescope (Atacama celestial objects have weaker fluxes, ent technique which well suits observa- Pathfinder Experiment, Güsten et al., usually orders of magnitude weaker than tions with an array of detectors. This tech- 2006) as a collaborative effort between the atmospheric emission. Besides, the nique, called ‘fast scanning’ ( Reichertz the Max-Planck-Institut für Radioastron- atmosphere is not stable and the amount et al. 2001), is based on the idea that, omie in Bonn (MPIfR), ESO and the of water vapour along the line of sight can when observing with an array, each unit Onsala Space Observatory (OSO). The change quickly, giving rise to instabilities bolometer looks at a different part of the new Large APEX BOlometer CAmera of emission and transmission, called ‘sky sky and chopping is no longer needed. A (LABOCA) is an array of bolometers de- noise’. modulation of the signal is produced by signed for fast mapping of large sky moving the telescope across the source areas at high angular resolution and with field of interest. The atmospheric contri- high sensitivity: a challenging task. Devel- bution (as well as part of the instrumental 2 The Messenger 129 – September 2007 noise) will be strongly correlated in all see Figure 1), making the correlation re- Technical overview bolometers and a post-detection analysis moval extremely efficient. Another point of the correlation across the array will al- is the large post-detection bandwidth. The detector array of LABOCA is micro- low extraction of the signals of astronomi- SIMBA was built following the same de- machined on a 4-inch (102-mm) silicon cal interest from the atmospheric fore- sign scheme as MAMBO: that is both wafer where unstructured silicon nitride grounds. The post-detection bandwidth is receivers are optimised for the differential membranes carry the composite bolom- defined by the beam size and by the technique with a wobbler, and a high- eters. The membranes are only 0.4 μm scanning speed; relatively high scanning pass filter is used to cut off frequencies thick and are coated with a thin titanium speeds are ideal. This technique was first below the chopping frequency. LABOCA, film which absorbs the incoming radia- tested by the MPIfR bolometer group in instead, is a true total power system tion. Neutron-transmutation-doped (NTD) 2000 with the MAMBO (Max-Planck Milli- (without high-pass filtering) with a large germanium chips (called thermistors), sol- metre Bolometer, Kreysa et al. 1999) ar - stable post-detection bandwidth, extend- dered to the membranes, detect the tem- ray of 37 bolometers, installed on the ing down to 0.1 Hz. Moreover the reduc- perature rise due to the absorption of IRAM 30-m telescope (Instituto de Radio- tion of the data acquired in fast scanning the radiation. The array is mounted inside astronomía Milimétrica, Pico Veleta, requires the use of special algorithms a cryostat, which uses liquid nitrogen and Spain; Baars et al. 1987). The same tech- (Weferling et al. 2002) and the lack of a liquid helium for thermal shielding and nique was extensively used in the follow- software package ready to reduce the pre-cooling of the array. A closed-cycle ing years for observations with the SIMBA data was the major drawback of the fast double-stage sorption cooler is then used (SEST Imaging Bolometer Array, Nyman scanning technique applied to MAMBO to reach a stable operation temperature et al. 2001) bolometer array on the and SIMBA. For this reason, in paral - of 0.285 K. The cryostat is mounted in the SEST (Swedish-ESO Submillimetre Tele- lel with the hardware development of Cassegrain cabin of the telescope (see scope, La Silla, Chile; Booth et al. 1989) LABOCA, completely new software was Figure 2) and the optical coupling to the telescope, which is not equipped with a developed, the Bolometer Data Analysis main telescope beam is provided by a chopping secondary mirror. package (BoA, Schuller et al., in prep.), series of metal mirrors and a lens placed which is able to reduce data acquired at the cryostat entrance. A set of cold The experience with MAMBO and SIMBA with LABOCA in any of the possible ob- filters, mounted on the liquid nitrogen and has been essential for the design of serving modes. liquid helium shields, define the spec­­­- LABOCA, which represents the evolution tral passband, centred at a wavelength of to a receiver specifically optimised for APEX is the ideal telescope for using the 870 μm (345 GHz) and about 150 μm the fast scanning technique. Challenging fast scanning technique as it can move (60 GHz) wide (see Figure 3). A monolithic technological choices have been imple- extremely fast and its control software array of conical horn antennas, placed mented in its design. The most evident is allows new observing patterns which fit in front of the bolometer wafer, collects the large number of pixels (nominally 295, well to the fast scanning technique. The the radiation onto the bolometers. One 5100 metre high site on Llano de Chaj- LABOCA beam is 18.6 arcseconds wide Figure 2: LABOCA in the Cassegrain cabin of the nantor, where APEX is located, on the (full width at half maximum, FWHM) APEX telescope. The receiver is in the centre of the one hand can make the maintenance and the field of view (FoV) of the complete picture. Four of the five mirrors used for the optical coupling are visible. of the system uncomfortable, but on the array covers 11.4 arcminutes. The array oth er hand provides excellent atmos- undersamples the sky, with a distance of pheric conditions for most of the year. two beams between adjacent pixels 1.0 Figure 3: Spectral re- sponse of LABOCA. The central frequency is 345 GHz and the 50 % 0.8 transmission is between 313 and 372 GHz. 0.6 0.4 Transmission 0.2 313 372 0.0 250 300 350 400 450 Frequency (GHz) The Messenger 129 – September 2007 3 Telescopes and Instrumentation Siringo G. et al., LABOCA Starts Operation on APEX (see Figure 4). The voltages at the edges Raster of Spirals Figure 4: The coloured lines show the 400 scanning pattern of a single bolometer of the thermistors are channelled to the 146 159 148 for a four-point raster of spirals. The 151 outside of the cryostat along 12 flat ca- 157 171 circles show the measured positions 80 bles (made of manganin wires on kapton 180 153 149 and sizes on sky of all the functional 175 152 170 169 74 163 73 165 176 75 LABOCA detectors.
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