65 research outputs found
Understanding synthesis imaging dynamic range
We develop a general framework for quantifying the many different
contributions to the noise budget of an image made with an array of dishes or
aperture array stations. Each noise contribution to the visibility data is
associated with a relevant correlation timescale and frequency bandwidth so
that the net impact on a complete observation can be assessed. All quantities
are parameterised as function of observing frequency and the visibility
baseline length. We apply the resulting noise budget analysis to a wide range
of existing and planned telescope systems that will operate between about 100
MHz and 5 GHz to ascertain the magnitude of the calibration challenges that
they must overcome to achieve thermal noise limited performance. We conclude
that calibration challenges are increased in several respects by small
dimensions of the dishes or aperture array stations. It will be more
challenging to achieve thermal noise limited performance using 15 m class
dishes rather than the 25 m dishes of current arrays. Some of the performance
risks are mitigated by the deployment of phased array feeds and more with the
choice of an (alt,az,pol) mount, although a larger dish diameter offers the
best prospects for risk mitigation. Many improvements to imaging performance
can be anticipated at the expense of greater complexity in calibration
algorithms. However, a fundamental limitation is ultimately imposed by an
insufficient number of data constraints relative to calibration variables. The
upcoming aperture array systems will be operating in a regime that has never
previously been addressed, where a wide range of effects are expected to exceed
the thermal noise by two to three orders of magnitude. Achieving routine
thermal noise limited imaging performance with these systems presents an
extreme challenge. The magnitude of that challenge is inversely related to the
aperture array station diameter.Comment: 27 pages, 24 figures, accepted in A&A, final versio
SKA antenna systems; outlook for non-astronomy applications
The globally endorsed Square Kilometre Array project primarily aims to advance high sensitivity radio astronomy using a distributed collection of radio telescope stations spiraling outward from the core along three to five arms out to 3000km. This planned highly sensitive instrument covering a frequency range from 70MHz up to 10GHz will be used as wideband, high resolution, wide observing field interferometer of which the first phase will be realized this decade. With these SKA telescope capabilities and with the underlying technologies, there are many space related applications outside the immediate radio astronomy domain especially in the ground segment area. Examples are tracking space debris, precision orbit determination, simultaneous deep space tracking of multiple spacecrafts, GNSS and for other ground segment applications such as search and rescue tracking. After a brief introduction to the SKA, this paper will explore these potential application areas using the SKA based on its underlying approaches in the antenna and receiving subsystems
Extragalactic Radio Continuum Surveys and the Transformation of Radio Astronomy
Next-generation radio surveys are about to transform radio astronomy by
discovering and studying tens of millions of previously unknown radio sources.
These surveys will provide new insights to understand the evolution of
galaxies, measuring the evolution of the cosmic star formation rate, and
rivalling traditional techniques in the measurement of fundamental cosmological
parameters. By observing a new volume of observational parameter space, they
are also likely to discover unexpected new phenomena. This review traces the
evolution of extragalactic radio continuum surveys from the earliest days of
radio astronomy to the present, and identifies the challenges that must be
overcome to achieve this transformational change.Comment: To be published in Nature Astronomy 18 Sept 201
SKA antenna systems; Outlook for non-astronomy applications
The globally endorsed Square Kilometre Array project primarily aims to advance high sensitivity radio astronomy using a distributed collection of radio telescope stations spiraling outward from the core along three to five arms out to 3000km. This planned highly sensitive instrument covering a frequency range from 70MHz up to 10GHz will be used as wideband, high resolution, wide observing field interferometer of which the first phase will be realized this decade. With the SKA telescope capabilities and with the underlying technologies, there are many space related applications outside the immediate radio astronomy domain. Examples are tracking space debris, precision orbit determination, simultaneous deep space tracking of multiple spacecrafts, GNSS and other ground segment applications, such as search and rescue tracking. After a brief introduction to the SKA, this paper will explore these potential application areas using the SKA based on its underlying approaches in the antenna and receiving subsystems
HI observations of the nearest starburst galaxy NGC 253 with the SKA precursor KAT-7
We present HI observations of the Sculptor Group starburst spiral galaxy NGC
253, obtained with the Karoo Array Telescope (KAT-7). KAT-7 is a pathfinder for
the SKA precursor MeerKAT, under construction. The short baselines and low
system temperature of the telescope make it very sensitive to large scale, low
surface brightness emission. The KAT-7 observations detected 33% more flux than
previous VLA observations, mainly in the outer parts and in the halo for a
total HI mass of M. HI can be found at
large distances perpendicular to the plane out to projected distances of ~9-10
kpc away from the nucleus and ~13-14 kpc at the edge of the disk. A novel
technique, based on interactive profile fitting, was used to separate the main
disk gas from the anomalous (halo) gas. The rotation curve (RC) derived for the
HI disk confirms that it is declining in the outer parts, as seen in previous
optical Fabry-Perot measurements. As for the anomalous component, its RC has a
very shallow gradient in the inner parts and turns over at the same radius as
the disk, kinematically lagging by ~100 km/sec. The kinematics of the observed
extra planar gas is compatible with an outflow due to the central starburst and
galactic fountains in the outer parts. However, the gas kinematics shows no
evidence for inflow. Analysis of the near-IR WISE data, shows clearly that the
star formation rate (SFR) is compatible with the starburst nature of NGC 253.Comment: 18 pages, 20 figures, 8 Tables. Accepted for publication to MNRA
WALLABY – an SKA Pathfinder H i survey
The Widefield ASKAP L-band Legacy All-sky Blind surveY (wallaby) is a next-generation survey of neutral hydrogen (H i) in the Local Universe. It uses the widefield, high-resolution capability of the Australian Square Kilometer Array Pathfinder (ASKAP), a radio interferometer consisting of 36 × 12 -m dishes equipped with Phased-Array Feeds (PAFs), located in an extremely radio-quiet zone in Western Australia. wallaby aims to survey three-quarters of the sky (− 90 ∘\u3c δ\u3c + 30 ∘) to a redshift of z≲ 0.26 , and generate spectral line image cubes at ∼30 arcsec resolution and ∼1.6 mJy beam−1 per 4 km s−1 channel sensitivity. ASKAP’s instantaneous field of view at 1.4 GHz, delivered by the PAF’s 36 beams, is about 30 sq deg. At an integrated signal-to-noise ratio of five, wallaby is expected to detect around half a million galaxies with a mean redshift of z∼ 0.05 (∼200 Mpc). The scientific goals of wallaby include: (a) a census of gas-rich galaxies in the vicinity of the Local Group; (b) a study of the H i properties of galaxies, groups and clusters, in particular the influence of the environment on galaxy evolution; and (c) the refinement of cosmological parameters using the spatial and redshift distribution of low-bias gas-rich galaxies. For context we provide an overview of recent and planned large-scale H i surveys. Combined with existing and new multi-wavelength sky surveys, wallaby will enable an exciting new generation of panchromatic studies of the Local Universe. — First results from the wallaby pilot survey are revealed, with initial data products publicly available in the CSIRO ASKAP Science Data Archive (CASDA)
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