With the establishment of the AUT University 12m radio telescope at Warkworth, New Zealand has now become a part of the international Very Long Baseline Interferometry (VLBI) community. A major product of VLBI observations are images in the radio domain of astronomical objects such as Active Galactic Nuclei (AGN). Using large geographical separations between radio antennas, very high angular resolution can be achieved. Detailed images can be created using the technique of VLBI Earth Rotation Aperture Synthesis. We review the current process of VLBI radio imaging. In addition we model VLBI configurations using the Warkworth telescope, AuScope (a new array of three 12m antennas in Australia) and the Australian Square Kilometre Array Pathfinder (ASKAP) array currently under construction in Western Australia, and discuss how the configuration of these arrays affects the quality of images. Recent imaging results that demonstrate the modeled improvements from inclusion of the AUT and first ASKAP telescope in the Australian Long Baseline Array (LBA) are presented.
The Warkworth Radio Astronomical Observatory (WRAO) is located some 60 km north of the city of Auckland, near the township of Warkworth. The observatory is operated by the Institute for Radio Astronomy and Space Research (IRASR) of AUT University. The observatory's 12-m radio telescope operates in three frequency bands centred on 1.4, 2.3 and 8.6 GHz. In addition to astrophysical observations this fast-slewing ( 5 • per second in Azimuth) antenna is well suited to the purposes of geodetic VLBI [1] and spacecraft navigation and tracking [2].
In February 2011 the AUT University 12m radio antenna officially joined the Australian Long Baseline Array (LBA) and now regularly participates in its VLBI sessions. The primary product of this VLBI work is high resolution radio domain images from which the physical properties of radio astronomical sources are studied.
The AUT University 12m radio antenna has expanded the maximum east-west baselines of the LBA [3] by almost a factor of two, from ≈ 1300km to ≈ 2400km. With the further addition of the ASKAP antennas in Western Australia east-west baselines of ≈ 5000 km are achieved, providing corresponding increases in resolution. The goal of this paper is to investigate the way for further improvement of the quality of the array, first of all in terms of extension of the north-south baselines to yet further improve the resultant image quality.
In Section 2, we briefly present the theory behind radio interferometry and review the current methods used to obtain radio images of astronomical sources. Section 3 specifically outlines current image recovery methods. In Section 4, we model the effects of adding the AUT University 12m radio antenna which will be referred too as Warkworth, the new ASKAP [4] and AuScope [5] antennas on the imaging performance of the LBA. Section 5 discusses current activities and presents actual images achieved with the AUT 12m antenna as part of the LBA.
A radio interferometer is a pair (or more) of antennas used to measure the visibility function due to the sky brightness within the field of view of the antennas [6]. From sampling this visibility function it is possible to recover an image of the observed field of view [6]. arXiv:1110.5360v1 [astro-ph.IM] 24 Oct 2011
Imaging of a radio astronomical source by the technique of aperture synthesis was first demonstrated by Prof. Ryle [7] using the Cambridge Radio Telescope [8]. Antennas able to track a source for an extended period as the Earth rotates will trace out elliptical paths in the u, v plane (orthogonal plane to the direction of the astronomical source). The components u, v, w may be determined from the expression [6]:
Here H, δ 0 are the hour angle and declination of the source, λ is the wavelength of the radio frequency being observed. It is customary in VLBI to eliminate the H 0 (hour angle) term by setting the x axis of the coordinate system to the Greenwich meridian H 0 = 0, resulting in:
As L x ,L y and L z are constants for a given pair of antennas, this is the equation of an ellipse in the u,v plane (it becomes the equation of a circle u 2 +v 2 when δ 0 = 90 • ). For an array of N antennas we will have N (N -1)/2 pairs of elliptical loci.
Plots of these loci (tracks) demonstrate the progressive improvement in filling of the u, v plane as additional antennas are added to the LBA array: Warkworth plus LBA in Figure 1, Warkworth, LBA and ASKAP in Figure 2, Warkworth, LBA, ASKAP and AuScope in Figure 3. All plots are for a common source declination of -37 • , that is for a Southern Celestial Hemisphere radio source. Both u and v are given in wavelengths, λ.
The astronomical source is treated as a two dimensional image of intensity I(l, m) on the celestial sphere, l and m being coordinates in the plane of the celestial sphere. The projection of l and m to the plane perpendicular to the direction of the astronomical source from the Earth, as defined by the coordinate system u and v, is shown in Figure 4.
It can thus be seen that the visibility measured by an interferometer is a sample of the visibility function V (u, v), and may be expressed as the Fourier transform of the modified sky intensity I [6]:
As the visibility is sampled out to a maximum radius b max in the u, v plane, the array produces information similar to a single circular aperture of diameter D ≈ λb max . Resolution of the image will be ≈ 1/b max , but the quality of the image is determined by the u, v coverage. 3 Image recovery methods
During an observation, sources are tracked by the interferometer array and the signal at each telescope separately recorded. Data from all telescopes is then sent to a processing centre for correlation. As V is a Fourier transform of the source brightness distribution I Equation (2), the latter can be recovered by means of the inverse Fourier transformation:
As integration occurs over u and v, the more information we have about V (u, v), the more fully we can conduct numerical i
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