Aperture Synthesis Radio Telescope Research Articles

Impact of the Correlation of Visibility Noise in Redundant Baselines on Radiometric Sensitivity of Aperture Synthesis Radio Telescope

Impact of the Correlation of Visibility Noise in Redundant Baselines on Radiometric Sensitivity of Aperture Synthesis Radio Telescope

400-to-800-MHz GaAs pHEMT-Based Wideband LNA for Radio-Astronomy Antenna-Array Feed

This letter presents the design of a two-stage 400-to-800-MHz GaAs pseudomorphic high-electron-mobility transistor wideband low-noise amplifier (LNA) to upgrade an aperture synthesis radio telescope. The first stage is based on an inductive-source-degenerated amplifier topology, uses a low voltage supply, and utilizes an intrinsic input-transistor gate–drain feedback to improve the LNA bandwidth and gain. The final stage, a common-source amplifier, provides 50- $\Omega$ match and increases the gain. Employing matching via intrinsic feedback and a low-voltage supply, the LNA exhibits gain ( $S_{21}$ ) from 41.1 to 43.3 dB, a minimum input-referred 1-dB compression point of −32.8 dBm, and a noise figure from 0.26 to 0.34 dB over the 400-to-800-MHz band. A future antenna-array feed with such LNAs is expected to exhibit 18.8–26.3 K beam-referred noise temperatures. The LNA consumes 406 mW from a 1.4-V supply.

The Pointing Self-calibration Algorithm for Aperture Synthesis Radio Telescopes

Abstract This paper is concerned with algorithms for calibration of direction-dependent effects (DDE) in aperture synthesis radio telescopes (ASRT). After correction of direction-independent effects (DIE) using self-calibration, imaging performance can be limited by the imprecise knowledge of the forward gain of the elements in the array. In general, the forward gain pattern is directionally dependent and varies with time due to a number of reasons. Some factors, such as rotation of the primary beam with Parallactic Angle for Azimuth–Elevation mount antennas are known a priori. Some, such as antenna pointing errors and structural deformation/projection effects for aperture-array elements cannot be measured a priori. Thus, in addition to algorithms to correct for DD effects known a priori, algorithms to solve for DD gains are required for high dynamic range imaging. Here, we discuss a mathematical framework for antenna-based DDE calibration algorithms and show that this framework leads to computationally efficient optimal algorithms that scale well in a parallel computing environment. As an example of an antenna-based DD calibration algorithm, we demonstrate the Pointing SelfCal (PSC) algorithm to solve for the antenna pointing errors. Our analysis show that the sensitivity of modern ASRT is sufficient to solve for antenna pointing errors and other DD effects. We also discuss the use of the PSC algorithm in real-time calibration systems and extensions for antenna Shape SelfCal algorithm for real-time tracking and corrections for pointing offsets and changes in antenna shape.

The Australian Square Kilometre Array Pathfinder: Performance of the Boolardy Engineering Test Array

AbstractWe describe the performance of the Boolardy Engineering Test Array, the prototype for the Australian Square Kilometre Array Pathfinder telescope. Boolardy Engineering Test Array is the first aperture synthesis radio telescope to use phased array feed technology, giving it the ability to electronically form up to nine dual-polarisation beams. We report the methods developed for forming and measuring the beams, and the adaptations that have been made to the traditional calibration and imaging procedures in order to allow BETA to function as a multi-beam aperture synthesis telescope. We describe the commissioning of the instrument and present details of Boolardy Engineering Test Array’s performance: sensitivity, beam characteristics, polarimetric properties, and image quality. We summarise the astronomical science that it has produced and draw lessons from operating Boolardy Engineering Test Array that will be relevant to the commissioning and operation of the final Australian Square Kilometre Array Path telescope.

Surveys of the Milky Way and Magellanic System in the λ21-cm line of atomic hydrogen

In the next three years, surveys of the Northern and Southern skies using focal plane arrays on aperture synthesis radio telescopes will lead to a breakthrough in our knowledge of the warm and cool atomic phases of the interstellar medium and their relationship with the diffuse molecular gas. The sensitivity and resolution of these surveys will give an order of magnitude or more improvement over existing interstellar medium data. The GASKAP (South) and GAMES (North) projects together constitute a complete survey of the Milky Way plane and the Magellanic Clouds and Stream in both emission and absorption in the H I 21-cm line and the OH 18-cm lines. The overall goal of this project is to understand the mechanism of galaxy evolution, through a detailed tracing of the astrophysical processes that drive the cycle of star formation in very different environments. Comparison of 21-cm emission and absorption highlights the transition from the warm, diffuse medium to cool clouds. Tracing turbulence in the Magellanic Stream shows how extra-galactic gas makes the difficult passage through the halo to replenish the disk. Finally, high resolution images of OH masers trace outflows from evolved stars that enrich the medium with heavy elements. To understand how the Milky Way was assembled and how it has evolved since, the speed and efficiency of these processes must be measured, as functions of Galactic radius and height above the plane. Observations of similar processes in the Magellanic Clouds show how differently they might have worked in conditions typical of the early universe.

Extended component in the quasar 3C 380

Results of radio interferometric observations of the quasar 3C 380 carried out on the URAN interferometers at decameter wavelengths and on the aperture synthesis radio telescope VLA at meter wavelengths are reported. The spectral index of an extended lobe about 10″ in size is considerably lower than at decimeter wavelengths. Below ∼ 100 MHz, the ratio of the emission from the compact components associated with hot spots in the radio lobe to the total flux of the source decreases due to synchrotron self-absorption at hot spots, whose flux density at 20 MHz does not exceed 65 Jy. A halo with a full width at a half-maximum of about 40″ was detected, whose angular extent considerably exceeds the total source size measured at shorter wavelengths.

LOFAR: The potential for solar and space weather studies

The Low Frequency array (LOFAR) will be a next generation digital aperture synthesis radio telescope covering the frequency range from 10 to 240 MHz. The instrument will feature full polarisation and multi-beaming capability, and is currently in its design phase. This work highlights the solar, heliospheric and space weather applications where LOFAR, with its unique and unprecedented capabilities, can provide useful information inaccessible by any other means. The relevant aspects of the LOFAR baseline design are described, and the most promising techniques of interest are enumerated. These include tracking coronal mass ejections (CMEs) out to large distances using interplanetary scintillation (IPS) methods, tomographic reconstruction of the solar wind in the inner heliosphere using IPS, direct imaging of the radio emission from CMEs and finally possible Faraday rotation studies of the magnetic field structure of the heliosphere and the CMEs. This work is a part of an effort directed towards ensuring the compatibility of LOFAR design with solar and space weather applications, in collaboration with the wider community.

New developments of MSRT’s working modes: One-dimensional radio heliograph and IPS telescope

This note introduces the newly developed working modes, i.e. one-dimensional meter-wave radio heliograph (MRH) and interplanetary scintillation (IPS) telescope, of meter-wave aperture synthesis radio telescope (MSRT) at Beijing Astronomical Observatory (BAO). The note describes briefly the scientific objectives, configurations of the hardware and software, and functions of the system. It presents the examples of observations on solar meter-wave bursts and IPS with the two new working modes. The results indicated that new modes not only can provide the information on the evolution of solar activities with space and time, but also can trace and monitor the propagation and spatial distribution of interplanetary plasma shock resulting from solar activities and the instability of the ionosphere, etc. Both modes are new facilities that could fill the gaps in scientific frontiers.

Radi bursts at 232 MHz and coronal ejection events

The aperture synthesis radio telescope of BAO has a high sensitivity, a 20ms time resolution, a 3′.8 spatial resolution, and a powerful anti-jamming capability. Using this telescope We obtained 12 large radio bursts on the Sun in 1999, of which 8 are associated with coronal mass ejection events, thus indicating the possibility of using meter wave bursts to predict CME events

Free-space optical distribution of local oscillator signals in aperture synthesis radio telescope arrays

An alternative to individually phase-stabilised links for distribution of local oscillator (LO) signals in large arrays of radio telescopes is considered. An array is illuminated from an elevated optical source, intensity modulated at the LO frequency. LO phase feedback from the centre of the illuminated zone counteracts tower and atmospheric instabilities, locking a fixed phase at zone centre, and hence a template of constant phase contours around the zone centre point. Other LO detector planes are aligned to equiphase contours, providing a phase coherent LO to all telescopes aside from differential optical path effects. A local PLL at each site reduces the remaining noise from differential slant path variations.

Adaptive Deconvolution of Astronomical Radio Images

The novel image-forming methods used in the Molonglo Observatory Synthesis Telescope (MOST) are not amenable to a direct application of the self-calibration techniques that have been so effective in improving the performance of many other aperture synthesis radio telescopes. Specifically, self-calibration of antenna-based errors and the use of phase closure conditions is not possible because vital antenna-based information is lost irretrievably as the MOST forms interferometer beams in real time. However, it is shown in this paper that suitable models of errors introduced by the atmosphere and telescope can be adaptively constructed from the properties of synthesised images. These models can then be used iteratively with standard deconvolution methods to improve significantly the fidelity of MOST images. The method would have general application to tomographic imaging systems in which non-ideal behaviour could be represented by a small number of unknown parameters.

Radio studies of giant radio galaxy 3C 236 at 326.5MHz

Radio observations at 326.5MHz of the radio galaxy 3C 236 were made using the 4 km Ooty aperture synthesis radio telescope. A radio map is presented here, together with a simple analysis and discussion, and some preliminary results.

Review of Linked Array Instruments

At cm wavelengths aperture synthesis radio-telescopes (arrays of linked antennas which synthesize an image of the sky with high angular resolution) are now becoming the dominant astronomical research tool. Major new facilities such as the VLA are in full operation, others such as the Australia Telescope are nearing completion and a number of telescopes designed to form images in real time have been converted to operate in the aperture synthesis mode (e.g. MOST, Bologna Cross). See Napier et al. (1983) for a review of modern synthesis telescopes. The high resolution, sensitivity and freedom from confusion have led the aperture synthesis telescopes into very diverse astronomical applications.

Miyun metre-wave aperture synthesis radio telescope

The Miyun metre-wave aperture synthesis radiotelescope, working at frequency of 232 MHz, consists of an E-W array of 28 elements, each of 9 m aperture. 192 baselines are effected with a full coverage of the U-V plane (Fig. 2). The longest baseline is 1,164m. This instrument is designed for source survey and detection of peculiar sources in northern declinations. A set of observations completed in 2 × 12 hours gives a thermal noise limited sensititity of 0.05 Jy and a resolution of 3'8 × 3'8 csc δ. The field of view is 8° × 8°. This should enable us to complete an overall survey of the region δ ⩾ + 30° within two years, and to carry out monitoring of selected areas. Figures 1 and 2 show the main properties and general design of the instrument and Figures 3 and 4 give some preliminary results of sky mapping.

A radio search for primordial pancakes

According to the pancake theory of formation of large-scale structure in the Universe, gaseous pancakes are expected to form in the range of redshifts 5–10. One of the ways in which they should manifest their presence is through emission in the 21-cm line by their neutral hydrogen component. An aperture synthesis radio telescope has been used at 151 MHz to map the sky north of declination 82°. Data sampling was arranged to permit a differential mapping technique to be used, which enabled the continuum emission in the field to be almost entirely cancelled, to leave a residual rms level of 5 mJy. At the 2σ level, no sources were found in a 1 per cent redshift range around z = 8.4, the nominal survey redshift. The observations are found to provide new limits on the pancake population under certain Friedmann cosmologies.

Is the Upturn in the Source Counts Caused by Primeval Radio Galaxies?

During the last decade several deep surveys have been made with various aperture synthesis radio telescopes. At milliJansky levels important contributions were made with the WSRT 3 km array and the Cambridge One Mile Telescope. At sub-milliJansky levels surveys have been done with the VLA and the WSRT down to rms noise values of ~10 μJy.

Combined MERLIN/VLA observations of the superluminal quasar 3C179

MERLIN1 (the Multi-Element Radio-Linked Interferometer Network) at Jodrell Bank and the VLA2 (the Very Large Array) in New Mexico are complementary instruments, representing the latest generation of aperture-synthesis radio telescopes; the VLA comprises 27 antennas, which, in the A-configuration, provide spacings of 0.6–36 km, whereas MERLIN is an array of six antennas with spacings of 6–134 km. Thus, MERLIN provides a factor of four greater resolution than VLA, whereas the shorter spacings of the latter provide essential information on large-scale structure. We present here the first map to be produced by combining data from MERLIN and the VLA. The new map, of the quasar 3C179 (redshift, z=0.846), is a considerable improvement on maps made from the individual arrays and reveals greater detail, particularly in the quasar's jet, which seems to be splitting or disrupting.

Rings and wiggles in Hercules A

Hercules A (3C348) is the fourth brightest extragalactic radio source in the sky at low frequencies. It is optically identified with a 18.5 mag galaxy with redshift z = 0.154 located in a faint cluster1,2. Although discovered as early as 19483, and extensively studied during the 1960s, it was not mapped with high-resolution aperture-synthesis radio telescopes because of its low declination. We report here the first results from extensive observations of Her A made with the Very Large Array (VLA)4 of the National Radio Astronomy Observatory. We find that Her A possesses an unusual jet-dominated morphology instead of the Cygnus-A-like morphology expected for such a powerful radio source. Its two jets are quite different in appearance; one appears to be a continuous twisting jet, while the other suggests repeated ejection of individual plasmons by the central object. The presence of such different morphologies in the same source raises interesting questions concerning the generation and evol ution of extended extragalactic radio sources. Variations in the central engine provide the simplest interpretation for some of the features of this object; the role of environmental effects is less clear.

The Optical and Infrared Properties of 3CR Radio Galaxies

For the last 20 years, a huge observational effort has been devoted to the identification of all 3CR radio sources lying in directions away from the Galactic plane. The project is now more-or-less complete thanks to high-resolution radio observations with aperture synthesis radio telescopes, in particular the Cambridge 5-km telescope, and the use of a CCD camera on the Palomar 5-metre telescope (see e.g. Gunn et al 1980).

Tangent‐plane aberration in aperture synthesis processing

The transform normally used for processing the measurements of a rotational aperture synthesis radiotelescope is only approximate. When other than east‐west spacings are used, it causes a reduction in the telescope's resolution. In this paper, the magnitude of the aberration is computed and shown to be significant for high resolution instruments. A new aberration‐free transform is derived.