APEXOnsala Space Observatory at Chalmers University of Technology (OSO) is the Swedish National Facility for Radio Astronomy. It operates two telescopes at Onsala, a 25 m cm-wave telescope and a 20 m mm-wave telescope, and it is one of three partners in the Atacama Pathfinder Experiment (APEX) project, a 12 m sub-mm telescope in Chile. OSO also provides the channel through which Sweden is involved in large international radio astronomy projects, such as the EVN, JIVE, LOFAR, ALMA, and SKA, and it is a partner in RadioNet.

In RadioNet3, a total of 279 hours (about 70 hours/year) of observing time on APEX is offered to the TNA programme. APEX is a 12-m sub-mm radio telescope located at 5100 m altitude on Llano Chajnantor, Chile (see http://www.apex‐telescope.org/). The telescope is of excellent quality (15 μm rms surface accuracy; Güsten et al. A&A 454, L13) and the site is also excellent as proven by the successful operation at 1.5 THz (Wiedner et al. A&A 454, L33). Observations are carried out from early April to late December (excluding the Bolivian winter). In 2011, it started its fifth year of regular, scheduled observations.

OSO is one of three partners that operate APEX, and its share of the total costs is 23%. This is also the Swedish share of the observing time, but, as the host country, Chile gets 10% of the Swedish time. Consequently, OSO distributes 21% of the observing time to the community. The partners have signed a contract to operate APEX until the end of 2015, with a possible extension to 2017.

APEX is equipped with a suite of bolometer cameras and single-pixel heterodyne receivers as common-user instruments, covering the range 1.3 mm to 0.2 mm. Currently, the LABOCA 295- channel 870 μm bolometer array, the SABOCA 37‐channel 350 μm bolometer array, and the 4‐ channel heterodyne receiver (230, 350, and 500 GHz, an 1.3 THz) are installed as common-user instruments. Additional instruments, so called PI-instruments, are available through collaborations with the groups responsible for them. Among the PI instruments, CHAMP+ (two 7-channel heterodyne arrays at 690 and 810 GHz) can be mentioned. The spectrometer is of the FFT-type and covers 2 GHz with a total of about 32000 channels.

The telescope and its instruments provide a unique opportunity for European astronomers to observe southern sky objects in continuum and spectroscopic mode at sub-mm wavelengths. Astrophysical questions such as the origin of large-scale structure in the universe and the origin of stars and planetary systems are addressed.

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Dr. M. Thomasson
e-mail: magnus [dot] thomasson [at] chalmers [dot] se
Onsala Space Observatory
SE- 439 92 Onsala



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