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Arcminute Microkelvin Imager (AMI)

The Arcminute Microkelvin Imager (AMI) telescope is a dual array synthesis telescope consisting of two separately correlated arrays of receivers operating in the 12 to 18 GHz band. The first of these is the Small Array (SA), which is formed out of ten 3.7m shaped paraboloid dishes in a compact configuration suitable for observing on angular scales of approximately 2-16 arcmin; the second is the Large Array (LA), which was created by reconfiguring eight of the 12.7m dishes of the Ryle Telescope into a more compact configuration, including three offset dishes to create north-south baselines, to recover angular scales of approximately 0.5-5 arcmin.

The two arrays were designed to be operated simultaneously, as a survey instrument for detecting a large sample of galaxy clusters with the SA through their Sunyaev-Zel’dovich (SZ) effect, with subtraction of contaminating radio sources performed using LA data. As such the receiver system was originally designed to have a wide overall bandwidth (6 GHz) divided into eight broad-band (750 MHz) channels for high sensitivity to the continuum SZ signal. Consequently the original correlator was installed as a lag based system, which suffers from a number of systematics causing large errors in correlator lag spacing and is prone to persistent and intermittent RFI which substantially decreases the sensitivity of the instrument, particularly at low declinations where due to geostationary satellites, data flagging up to the 90% level is possible.

The primary goal of the AMI Digital Correlator (AMIDC) project is to equip the telescope with a highly channelized digital correlator system giving more flexibility in the location of this band and a much more uniform response across it, which would provide the potential to avoid or mitigate to a large extent many of the problems with the current system.

With these improvements to the system the re-commissioned instrument will be able to be used to provide complementary data for both the LOFAR and MeerKAT cluster science cases, providing prior information on electron density distributions in galaxy clusters, which can then be combined with Faraday rotation measurements at low radio frequencies to determine the morphology and strength of the magnetic field distribution in these clusters. For the MeerKAT programs specifically, this will require the ability to observe equatorial fields at high sensitivity.