Wide-field radio interferometric telescopes such as the Square Kilometre Array now being designed are subject to a number of aberrations. One particularly pernicious aberration is that due to non-coplanar baselines whereby long baselines incur a quadratic image-plane phase error. There are numerous algorithms for dealing with the non-coplanar baselines effect. As a result of our experience with developing processing software for the Australian Square Kilometre Array Pathinder, we advocate the use of a hybrid algorithm, called w snapshots, based on a combination of w projection and snapshot imaging. This hybrid overcomes some of the deficiencies of each and has advantages from both. Compared to pure w projection, w snapshots uses less memory and execution time, and compared to pure snapshot imaging, w snapshots uses less memory and is more accurate. At the asymptotes, w snapshots devolves to w projection and to snapshots.
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.
We describe a scale sensitive deconvolution algorithm for interferometric images. An ideal sky model should be able to pick up correlated emission on all scales and of all shapes. Though the problem can be well formulated mathematically, there are two issues: (1) it is computationally prohibitive, and (2) the choice of an appropriate basis to represent the image is not clear. The work presented here is an interim step towards developing a fully scale sensitive deconvolution algorithm which is computationally efficient as well. This approach, though restrictive as compared to the most general model for the sky, is an enormous improvement over other scale insensitive algorithms and forms the logical limit of multi-resolution CLEAN approach. The sky image is represented using parameterized basis functions with finite support and the algorithm solves for these parameters. The computational load in this approach is reduced by working with an analytical approximation of the Point Spread Function.
KEYWORDS: Data archive systems, Telescopes, Observatories, Data conversion, Radio telescopes, Prototyping, Data storage, C++, Astronomy, Radio astronomy
We have constructed an archive system for NRAO telescopes using mainly tools available in the Astronomical Information Processing System (AIPS++). Since the tools are available to anyone using AIPS++, this amounts to a generic archive capability for any telescope for which the AIPS++ data conversion program exists. The rich tool set available in AIPS++ has enabled very rapid development: our entire effort took no more than about 1 FTE-year. Additional capabilities were required to connect AIPS++ to the web. The system is now being deployed at the NRAO as a prototype archive system for the Very Large Array with deployment for the Green Bank Telescope and Very Long Baseline Array planned for 2003.
Non-coplanar sampling of the visibility function measured by interferometric arrays leads to difficulties in imaging wide-fields. Unlike the case for co-planar sampling or small fields of view, the relationship between sky brightness and the visibility is not a simple two-dimensional Fourier transform, and so the usual methods of image reconstruction cannot be applied. We describe and analyze some of the many schemes which have been advocated to overcome this problem. The most promising is based upon an observation by Clark that if the sky brightness is thought of as lying on a surface embedded in a three dimensional space, a Fourier relationship does hold.
Conference Committee Involvement (3)
Image Reconstruction from Incomplete Data VIII
11 August 2015 | San Diego, California, United States
Image Reconstruction from Incomplete Data VII
14 August 2012 | San Diego, California, United States
Image Reconstruction from Incomplete Data VI
2 August 2010 | San Diego, California, United States
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