192 research outputs found
Aperture Array Configurations for SKA1 Core
This memo considers some aspects of the configuration of the SKA1 Low
Frequency Aperture Array, both at the element and station level. At the element
level I propose a possible scenario for forming station beams where elements
are shared between stations and apodisation is implemented, with the aim of
improving filling factor, overall sensitivity and sidelobe performance; the
disadvantages of such a scheme with regards to beam former requirements and
shortest available baseline are also discussed. At the station level, a
randomised configuration within a filled central region together with spiral
arms is explored
Investigating the source of Planck-detected AME: high resolution observations at 15 GHz
The Planck 28.5 GHz maps were searched for potential Anomalous Microwave
Emission (AME) regions on the scale of or smaller, and several
new regions of interest were selected. Ancillary data at both lower and higher
frequencies were used to construct spectral energy distributions (SEDs), which
seem to confirm an excess consistent with spinning dust models. Here we present
higher resolution observations of two of these new regions with the Arcminute
Microkelvin Imager Small Array (AMI SA) between 14 and 18 GHz to test for the
presence of a compact (10 arcmin or smaller) component. For
AME-G107.1+5.2, dominated by the {\sc Hii} region S140, we find evidence for
the characteristic rising spectrum associated with the either the spinning dust
mechanism for AME or an ultra/hyper-compact \textsc{Hii} region across the AMI
frequency band, however for AME-G173.6+2.8 we find no evidence for AME on
scales of arcmin.Comment: 13 pages, 8 figures, 4 tables. Submitted to Advances in Astronomy AME
Special Issu
Characterization of Optical Frequency Transfer Over 154 km of Aerial Fiber
We present measurements of the frequency transfer stability and analysis of
the noise characteristics of an optical signal propagating over aerial
suspended fiber links up to 153.6 km in length. The measured frequency transfer
stability over these links is on the order of 10^-11 at an integration time of
one second dropping to 10^-12 for integration times longer than 100 s. We show
that wind-loading of the cable spans is the dominant source of short-timescale
noise on the fiber links. We also report an attempt to stabilize the optical
frequency transfer over these aerial links.Comment: 4 pages, submitted to Optics Letter
Frequency Reference Stability and Coherence Loss in Radio Astronomy Interferometers Application to the SKA
The requirements on the stability of the frequency reference in the Square
Kilometre Array (SKA), as a radio astronomy interferometer, are given in terms
of maximum accepted degree of coherence loss caused by the instability of the
frequency reference. In this paper we analyse the relationship between the
characterisation of the instability of the frequency reference in the radio
astronomy array and the coherence loss. The calculation of the coherence loss
from the instability characterisation given by the Allan deviation is reviewed.
Some practical aspects and limitations are analysed.Comment: 14 page
Effect of gain and phase errors on SKA1-low imaging quality from 50-600 MHz
Simulations of SKA1-low were performed to estimate the noise level in images
produced by the telescope over a frequency range 50-600 MHz, which extends the
50-350 MHz range of the current baseline design. The root-mean-square (RMS)
deviation between images produced by an ideal, error-free SKA1-low and those
produced by SKA1-low with varying levels of uncorrelated gain and phase errors
was simulated. The residual in-field and sidelobe noise levels were assessed.
It was found that the RMS deviations decreased as the frequency increased. The
residual sidelobe noise decreased by a factor of ~5 from 50 to 100 MHz, and
continued to decrease at higher frequencies, attributable to wider strong
sidelobes and brighter sources at lower frequencies. The thermal noise limit is
found to range between ~10 - 0.3 Jy and is reached after ~100-100 000 hrs
integration, depending on observation frequency, with the shortest integration
time required at ~100 MHz.Comment: 23 pages, 11 figures Typo correcte
Cosmology from Cluster SZ and Weak Lensing Data
Weak gravitational lensing and the Sunyaev-Zel'dovich effect provide
complementary information on the composition of clusters of galaxies.
Preliminary results from cluster SZ observations with the Very Small Array are
presented. A Bayesian approach to combining this data with wide field lensing
data is then outlined; this allows the relative probabilities of cluster models
of varying complexity to be computed. A simple simulation is used to
demonstrate the importance of cluster model selection in cosmological parameter
determination.Comment: 4 pages, 4 figures, to appear in proceedings of XXXVIIth Rencontres
de Moriond, "The Cosmological Model"; h-depebndence corrected, typos fixe
Astronomical verification of a stabilized frequency reference transfer system for the Square Kilometre Array
In order to meet its cutting-edge scientific objectives, the Square Kilometre
Array (SKA) telescope requires high-precision frequency references to be
distributed to each of its antennas. The frequency references are distributed
via fiber-optic links and must be actively stabilized to compensate for
phase-noise imposed on the signals by environmental perturbations on the links.
SKA engineering requirements demand that any proposed frequency reference
distribution system be proved in "astronomical verification" tests. We present
results of the astronomical verification of a stabilized frequency reference
transfer system proposed for SKA-mid. The dual-receiver architecture of the
Australia Telescope Compact Array was exploited to subtract the phase-noise of
the sky signal from the data, allowing the phase-noise of observations
performed using a standard frequency reference, as well as the stabilized
frequency reference transfer system transmitting over 77 km of fiber-optic
cable, to be directly compared. Results are presented for the fractional
frequency stability and phase-drift of the stabilized frequency reference
transfer system for celestial calibrator observations at 5 GHz and 25 GHz.
These observations plus additional laboratory results for the transferred
signal stability over a 166 km metropolitan fiber-optic link are used to show
that the stabilized transfer system under test exceeds all SKA phase-stability
requirements under a broad range of observing conditions. Furthermore, we have
shown that alternative reference dissemination systems that use multiple
synthesizers to supply reference signals to sub-sections of an array may limit
the imaging capability of the telescope.Comment: 12 pages, accepted to The Astronomical Journa
Detection of a CMB decrement towards a cluster of mJy radiosources
We present the results of radio, optical and near-infrared observations of
the field of TOC J0233.3+3021, a cluster of milliJansky radiosources from the
TexOx Cluster survey. In an observation of this field with the Ryle Telescope
(RT) at 15 GHz, we measure a decrement in the cosmic microwave background (CMB)
of Jy on the RT's 0.65 k baseline. Using
optical and infrared imaging with the McDonald 2.7-m Smith Reflector, Calar
Alto 3.5-m telescope and UKIRT, we identify the host galaxies of five of the
radiosources and measure magnitudes of , , .
The CMB decrement is consistent with the Sunyaev-Zel'dovich (SZ) effect of a
massive cluster of galaxies, which if modelled as a spherical King profile of
core radius has a central temperature decrement
of K. The magnitudes and colours of the galaxies are consistent with
those of old ellipticals at . We therefore conclude that TOC
J0233.3+3021 is a massive, high redshift cluster. These observations add to the
growing evidence for a significant population of massive clusters at high
redshift, and demonstrate the effectiveness of combining searches for AGN
`signposts' to clusters with the redshift-independence of the SZ effect.Comment: Six pages; accepted for publication in MNRAS. Version with
full-resolution UV plot available from
http://www.mrao.cam.ac.uk/~garret/MB185.p
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