Very Small Array

The Very Small Array (VSA) was a 14-element interferometric radio telescope operating between 26 and 36 GHz that is used to study the cosmic microwave background radiation. It was a collaboration between the University of Cambridge, University of Manchester and the Instituto de Astrofisica de Canarias (Tenerife), and was located at the Observatorio del Teide on Tenerife. The array was built at the Mullard Radio Astronomy Observatory by the Cavendish Astrophysics Group and Jodrell Bank Observatory, and was funded by PPARC (now STFC). The design was strongly based on the Cosmic Anisotropy Telescope. The Very Small Array (VSA) was a 14-element interferometric radio telescope operating between 26 and 36 GHz that is used to study the cosmic microwave background radiation. It was a collaboration between the University of Cambridge, University of Manchester and the Instituto de Astrofisica de Canarias (Tenerife), and was located at the Observatorio del Teide on Tenerife. The array was built at the Mullard Radio Astronomy Observatory by the Cavendish Astrophysics Group and Jodrell Bank Observatory, and was funded by PPARC (now STFC). The design was strongly based on the Cosmic Anisotropy Telescope. The telescope was comparable in terms of capabilities to several other CMB experiments, including the balloon-based BOOMERanG and MAXIMA, and the ground-based DASI and CBI. The telescope consists of 14 elements (yielding 91 baselines), each of which have a horn reflector antenna focusing astrophysical signals into individual receivers (pseudomorphic HFET amplifiers, with a system temperature around 25 K and a physical temperature of 12 K, based on an NRAO design). The separate elements are combined using a correlator to form an aperture synthesis array. The elements are mounted on a tip-table, which is capable of tracking the sky and can tilt up to 35 degrees from the zenith. The telescope has been used in three different configurations – 'compact', 'extended' and 'super-extended', each of which differ in the separation distance between the elements (the difference between compact and extended is a factor of 2.25), and the size of the antennas. While the compact array has antennas 143mm in diameter, the extended array uses 322mm diameter antennas. This means that the compact array has a primary beam of 4.5 degrees, and a resolution of 30 arcminutes (multipoles between 100 and 800), while the extended array has a primary beam of 2 degrees, a resolution of 12 arcminutes and can hence observe multipoles between 250 and 1500. The extended array is also a factor of 5 more sensitive than the compact array. The super-extended array will be able to measure multipoles up to 3000, and has 550mm antenna mirrors. The front-end amplifiers were also upgraded. The telescope can be tuned to frequencies between 26 and 36 GHz, with 1.5 GHz bandwidth, meaning that the telescope can carry out observations at different frequencies. It also includes two 3.7m radio telescopes, also working at 30 GHz, which are dedicated to monitoring foreground sources. These source subtraction dishes were upgraded to more accurate ones following the first series of observations, to allow the monitoring of much weaker sources than previously. Both the source subtractor dishes, and the VSA itself, are surrounded by large metal ground shields. As the VSA is an interferometer, it directly measures the angular power spectrum of the CMB, rather than having to construct a map of the sky first. The fields observed with the VSA were chosen to minimize the amount of bright radio sources and large clusters in the field (the latter to avoid the Sunyaev-Zel'dovich effect), as well as to avoid contamination by emission from our galaxy. The radio point sources present in the VSA fields were observed with the Ryle Telescope at 15 GHz, then monitored by the VSA source subtracters during the VSA observations.