The Australian Square Kilometre Array Pathfinder (ASKAP) will be the fastest dedicated cm-wave survey telescope, and will consist of 36 12-meter 3-axis antennas, each with a large chequerboard phased array feed (PAF) receiver operating between 0.7 and 1.8 GHz, and digital beamforming prior to correlation. The large raw data rates involved (~100 Tb/sec), and the need to do pipeline processing, has led to the antenna incorporating a third axis to fix the parallactic angle with respect to the entire optical system (blockages and phased array feed). It also results in innovative technical solutions to the data transport and processing issues. ASKAP is located at the Murchison Radio-astronomy Observatory (MRO), a new observatory developed for the Square Kilometre Array (SKA), 315 kilometres north-east of Geraldton, Western Australia. The MRO also hosts the SKA low frequency pathfinder instrument, the Murchison Widefield Array and will host the initial low frequency instrument of the SKA, SKA1-Low. Commissioning of ASKAP using six antennas equipped with first-generation PAFs is now complete and installation of second-generation PAFs and digital systems is underway. In this paper we review technical progress and commissioning to date, and refer the reader to relevant technical and scientific publications.
The Australian Square Kilometre Array Pathfinder (ASKAP) will be the fastest cm-wave survey radio-telescope and is
under construction on the new Murchison Radio-astronomy Observatory (MRO) in Western Australia. ASKAP consists
of 36 12-meter 3-axis antennas, each with a large chequerboard phased array feed (PAF) operating from 0.7 to 1.8 GHz,
and digital beamformer preceding the correlator. The PAF has 94 dual-polarization elements (188 receivers) and the
beamformer will provide about 36 beams (at 1.4 GHz) to produce a 30 square degree field of view, allowing rapid, deep
surveys of the entire visible sky. As well as a large field of view ASKAP has high spectral resolution across the 304
MHz of bandwidth processed at any one time generating a large data-rate (30Gb/sec in to the imaging system) that
requires real-time processing of the data. To minimise this processing and maximise the field of view for long
observations the antenna incorporates a third axis, which keeps the PAF field of view and sidelobes fixed relative to the
sky. This largely eliminates time varying artefact in the data that is processed.
The MRO is 315 kilometres north-east of Geraldton, in Western Australia’s Mid West region. The primary
infrastructure construction for ASKAP and other telescopes hosted at the Murchison Radio-astronomy Observatory has
now been completed by CSIRO, the MRO manager, including installation of the fibre connection from the MRO site to
Perth via Geraldton. The radio-quietness of the region is protected by the Mid West Radio Quiet Zone, implemented by
the Australian Federal Government, out to a radius of 260km surrounding the MRO.
ABU is a NOAO IR imaging camera designed for evaluating the performance of the 1024x1024 Aladdth InSb array. For this experiment, it was outfitted with five filters (see Figure 9) m the 3-5 micron range to exploit the low water vapor and lower air temperatures at the South Pole. At the South Pole it was integrated with the CARA SPIREX (South Pole Infrared Explorer) telescope. Figure 1 is a picture of the telescope showing the environmental box (the white box by the author). which protected ABU and its electronics from ambient environmental conditions.
The design of the Leighton telescopes and the unique techniques used in their fabrication make these telescopes particularly amenable to precise modeling and measurement of their performance. The surface is essentially a continuous membrane supported at 99 uniformly distributed nodes by a pin joint triangular grid space frame. This structure can be accurately modeled and the surface can be adjusted using low- resolution maps. Holographic measurements of the surface figure of these telescopes at the Caltech Submillimeter Observatory (CSO) and the Owens Valley Radio Observatory (OVRO) have been made over several epochs with a repeatability of 5 - 10 micrometer over the zenith angle range from 15 to 75 degrees. The measurements are consistent with the calculated gravitational distortions. Several different surface setting strategies are evaluated and the 'second order deviation from homology,' Hd, is introduced as a measure of the gravitational degradation that can be expected for an optimally adjusted surface. Hd is defined as half of the RMS difference between the deviations from homology for the telescope pointed at the extremes of its intended sky coverage range. This parameter can be used to compare the expected performance of many different types of telescopes, including off-axis reflectors and slant-axis or polar mounts as well as standard alt-az designs. Subtle asymmetries in a telescope's structure are shown to dramatically affect its performance. The RMS surface error of the Leighton telescope is improved by more than a factor of two when optimized over the positive zenith angle quadrant compared to optimization over the negative quadrant. A global surface optimization algorithm is developed to take advantage of the long term stability and understanding of the Leighton telescopes. It significantly improves the operational performance of the telescope over that obtained using a simple 'rigging angle' adjustment. The surface errors for the CSO are now less than 22 micrometer RMS over most of the zenith angle range and the aperture efficiency at 810 GHz exceeds 33%. This illustrates the usefulness of the global surface optimization procedure.