44 research outputs found
The non-coplanar baselines effect in radio interferometry: The W-Projection algorithm
We consider a troublesome form of non-isoplanatism in synthesis radio
telescopes: non-coplanar baselines. We present a novel interpretation of the
non-coplanar baselines effect as being due to differential Fresnel diffraction
in the neighborhood of the array antennas.
We have developed a new algorithm to deal with this effect. Our new
algorithm, which we call "W-projection", has markedly superior performance
compared to existing algorithms. At roughly equivalent levels of accuracy,
W-projection can be up to an order of magnitude faster than the corresponding
facet-based algorithms. Furthermore, the precision of result is not tightly
coupled to computing time.
W-projection has important consequences for the design and operation of the
new generation of radio telescopes operating at centimeter and longer
wavelengths.Comment: Accepted for publication in "IEEE Journal of Selected Topics in
Signal Processing
G343.1-2.3 and PSR 1706-44
The association of G343.1-2.3 and PSR 1706-44 has been controversial from its first proposal. In this paper we present new evidence from images made by with the Australia Telescope Compact Array (ATCA), MRT and Mt. Pleasant. To cover the full extent of G343.1-2.3 with ATCA mosaicing was required, and we present the polarisation images from this experiment. The ATCA observations confirms the much larger extent of the SNR, which now encompasses the pulsar
A low frequency radio telescope at Mauritius for a Southern sky survey
A new, meter-wave radio telescope has been built in the North-East of Mauritius, an island in the Indian ocean, at a latitude of -20.14 deg. The Mauritius Radio Telescope (MRT) is a Fourier Synthesis T-shaped array, consisting of a 2048 m long East-West arm and a 880 m long South arm. In the East-West arm 1024 fixed helices are arranged in 32 groups and in the South arm 16 trolleys, with four helices on each, which move on a rail are used. A 512 channel digital complex correlation receiver is used to measure the visibility function. At least 60 days of observing are required for obtaining the visibilities up to 880 m spacing. The Fourier transform of the calibrated visibilities produces a map of the area of the sky under observation with a synthesized beam width 4'X 4.6'sec(dec+20.14) at 151.5 MHz. The primary objective of the telescope is to produce a sky survey in the declination range -70 deg to -10 deg with a point source sensitivity of about 200 mJy (3-sigma level). This will be the southern sky equivalent of the Cambridge 6C survey. In this paper we describe the telescope, discuss the array design and the calibration techniques used, and present a map made using the telescope
High Resolution, Wide Field, Narrow Band, Snapshot Imaging
We investigate the imaging performance of an interferometric array in the case of wide field, high resolution, narrow band, snapshot imaging. We find that, when uv-cell sizes are sufficiently small (ie. image sizes are sufficiently large), each instantaneous visibility record is gridded into its own uv-cell. This holds even for dense arrays, like the core of the next generation VLA. In this particular, application, Uniform weighting of the gridded visibilities approaches Natural weighting, with its often deleterious consequences on the resulting synthesized beam. For a core-dominated array, we show that the resulting image noise is highly correlated on scales comparable to the spatial frequencies of the core baselines. In general, this study accentuates the fact that, for imaging applications that require high resolution (Plains array and greater), many of the core antennas can be employed as a separate subarray for low resolution science, without sacrificing the quality of the high resolution science
On the association of G343.1-2.3 and PSR 1706-44
The association of G343.1-2.3 and PSR 1706-44 has been controversial from its
first proposal. In this paper we address the difficulties, and argue that the
association is still likely. New evidence comes from images of G343.1-2.3
obtained using the Australia Telescope Compact Array (ATCA), and the pulsar
obtained using the CHANDRA X-ray observatory. Mosaicing was required to cover
the full extent of G343.1-2.3, and we present the polarisation images from this
experiment. Also an X-ray pulsar wind nebula has been found in the archived
CHANDRA observations, with the correct morphology to support the association.
The ATCA observations confirm the much larger extent of the SNR, which now
encompasses the pulsar. The X-ray morphology points back toward the centre of
the SNR, indicating the direction of the proper motion, and that the PSR and
SNR are associated.Comment: 11 pages, 5 figures, submitted to MNRA
High Resolution, Wide Field, Narrow Band, Snapshot Imaging
We investigate the imaging performance of an interferometric array in the
case of wide field, high resolution, narrow band, snapshot imaging. We find
that, when uv-cell sizes are sufficiently small (ie. image sizes are
sufficiently large), each instantaneous visibility record is gridded into its
own uv-cell. This holds even for dense arrays, like the core of the next
generation VLA. In this particular, application, Uniform weighting of the
gridded visibilities approaches Natural weighting, with its often deleterious
consequences on the resulting synthesized beam. For a core-dominated array, we
show that the resulting image noise is highly correlated on scales comparable
to the spatial frequencies of the core baselines. In general, this study
accentuates the fact that, for imaging applications that require high
resolution (Plains array and greater), many of the core antennas can be
employed as a separate subarray for low resolution science, without sacrificing
the quality of the high resolution science.Comment: 18 pages; Next Generation VLA Memo No. 78;
https://ngvla.nrao.edu/page/memos#gen-mem
Temperature-dependent device properties of gamma-CuI and beta-Ga2O3 heterojunctions
Temperature-dependent studies of Ga2O3-based heterojunction devices are important in understanding its carrier transport mechanism, junction barrier potential, and stability at higher temperatures. In this study, we investigated the temperature-dependent device characteristics of the p-type gamma-copper iodide (gamma-CuI)/n-type beta-gallium oxide (beta-Ga2O3) heterojunctions, thereby revealing their interface properties. The fabricated gamma-CuI/beta-Ga2O3 heterojunction showed excellent diode characteristics with a high rectification ratio and low reverse saturation current at 298 K in the presence of a large barrier height (0.632 eV). The temperature-dependent device characteristics were studied in the temperature range 273-473 K to investigate the heterojunction interface. With an increase in temperature, a gradual decrease in the ideality factor and an increase in the barrier height were observed, indicating barrier inhomogeneity at the heterojunction interface. Furthermore, the current-voltage measurement showed electrical hysteresis for the reverse saturation current, although it was not observed for the forward bias current. The presence of electrical hysteresis for the reverse saturation current and of the barrier inhomogeneity in the temperature-dependent characteristics indicates the presence of some level of interface states for the gamma-CuI/beta-Ga2O3 heterojunction device. Thus, our study showed that the electrical hysteresis can be correlated with temperature-dependent electrical characteristics of the beta-Ga2O3-based heterojunction device, which signifies the presence of surface defects and interface states