Aperture Synthesis Telescope Research Articles

Solar image deconvolution by generative adversarial network

With aperture synthesis (AS) technique, a number of small antennas can be assembled to form a large telescope whose spatial resolution is determined by the distance of two farthest antennas instead of the diameter of a single-dish antenna. In contrast from a direct imaging system, an AS telescope captures the Fourier coefficients of a spatial object, and then implement inverse Fourier transform to reconstruct the spatial image. Due to the limited number of antennas, the Fourier coefficients are extremely sparse in practice, resulting in a very blurry image. To remove/reduce blur, “CLEAN” deconvolution has been widely used in the literature. However, it was initially designed for a point source. For an extended source, like the Sun, its efficiency is unsatisfactory. In this study, a deep neural network, referring to Generative Adversarial Network (GAN), is proposed for solar image deconvolution. The experimental results demonstrate that the proposed model is markedly better than traditional CLEAN on solar images. The main purpose of this work is visual inspection instead of quantitative scientific computation. We believe that this will also help scientists to better understand solar phenomena with high quality images.

Scalability model for the LOFAR direction independent pipeline

LOFAR is a leading aperture synthesis telescope operated in the Netherlands with stations across Europe. The LOFAR Two-meter Sky Survey (LoTSS) will produce more than 3000 14 TB data sets, mapping the entire northern sky at low frequencies. The data produced by this survey is important for understanding the formation and evolution of galaxies, supermassive black holes and other astronomical phenomena. All of the LoTSS data needs to be processed by the LOFAR Direction Independent (DI) pipeline, prefactor. Understanding the performance of this pipeline is important when trying to optimize the throughput for large projects, such as LoTSS and other deep surveys. Making a model of its completion time will enable us to predict the time taken to process large data sets, optimize our parameter choices, help schedule other LOFAR processing services, and predict processing time for future large radio telescopes. We tested the prefactor pipeline by scaling several parameters, notably number of CPUs, data size and size of calibration sky model. We present these results as a comprehensive model which will be used to predict processing time for a wide range of processing parameters. We also discover that smaller calibration models lead to significantly faster calibration times, while the calibration results do not significantly degrade in quality. Finally, we validate the model and compare predictions with production runs from the past six months, quantifying the performance penalties incurred by processing on a shared cluster. We conclude by noting the utility of the results and model for the LoTSS Survey, LOFAR as a whole and for other telescopes.

Extragalactic maser surveys

AbstractSince the IAU (maser-)Symposium 287 in Stellenbosch/South Africa (Jan. 2012), great progress has been achieved in studying extragalactic maser sources. Sensitivity has reached a level allowing for dedicated maser surveys of extragalactic objects. These included, during the last years, water vapor (H2O), methanol (CH3OH), and formaldehyde (H2CO), while surveys related to hydroxyl (OH), cyanoacetylene (HC3N) and ammonia (NH3) may soon become (again) relevant. Overall, with the upgraded Very Large Array (VLA), the Atacama Large Millimeter/submillimeter Array (ALMA), FAST (Five hundred meter Aperture Synthesis Telescope) and the low frequency arrays APERTIF (APERture Tile in Focus), ASKAP (Australian Square Kilometer Array Pathfinder) and MeerKAT (Meer Karoo Array Telescope), extragalactic maser studies are expected to flourish during the upcoming years. The following article provides a brief sketch of past achievements, ongoing projects and future perspectives.

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.

Design of Dual Circularly Polarised 2–15 GHz Feed and the Polarisation Degree Measurement for CSRH-II Antenna System

AbstractThe Chinese Spectral Radioheliograph is an aperture-synthesis telescope observing the Sun in ultra wide bandwidth on the ground. It contains two arrays Chinese Spectral Radioheliograph-I and Chinese Spectral Radioheliograph-II from 0.4 to 15 GHz. In order to obtain ultra wide-band performance, the cascaded folded dipoles are used in this feed. At the same time, in order to get circularly polarised signals coming from the Sun, a wide-band 90° hybrid is added in the output ports of the feed. This feed has characteristics of about 10 dBi gain, less than 1.5 voltage standing wave ratio. It also has characteristics with low axial ratio, fixed phase centre location, and constant beam width in operating frequencies. Through cross-correlating all combinations of two orthogonal polarisations at each antenna, the polarisation state of the waves is measured and give a differential instrumental delay between two data channels. The relationship between the own polarisation degree of the Sun and the observed polarisation degree is also measured for cross-polarised delay in observing the Sun in this paper.

High Performance Data Processing Pipeline of Chinese Solar Radio Heliograph

The Chinese Solar Radio Heliograph (CSRH) is a new generation radio heliograph would produce more than 4 terabytes data every day. As a aperture synthesis telescope, CSRH is facing the challenge of processing and storing such a vast data. Pipeline system is the key issue of data automatical processing for CSRH. In this study, to push the development of CSRH, we present a framework of high performance data processing pipeline system for saving and processing real-time observation data. The related techniques of pipeline software are presented in detail including raw data acquisition, UVFITS file, data calibration, parallel computing and data publication. The pipeline has been deployed and has played an important role for the development of CSRH.

High dynamic-range radio-interferometric images at 327 MHz

Radio astronomical imaging using aperture synthesis telescopes requires deconvolution of the point spread function as well as calibration of the instrumental characteristics (primary beam) and foreground (ionospheric/atmospheric) effects. These effects vary in time and also across the field of view, resulting in directionally-dependent (DD), time-varying gains. The primary beam will deviate from the theoretical estimate in real cases at levels that will limit the dynamic range of images if left uncorrected. Ionospheric electron density variations cause time and position variable refraction of sources. At low frequencies and sufficiently high dynamic range this will also defocus the images producing error patterns that vary with position and also with frequency due to the chromatic aberration of synthesis telescopes. Superposition of such residual sidelobes can lead to spurious spectral signals. Field-based ionospheric calibration as well as "peeling" calibration of strong sources leads to images with higher dynamic range and lower spurious signals but will be limited by sensitivity on the necessary short-time scales. The results are improved images although some artifacts remain.

A study of inductance displacement sensor on optical aperture synthesistelescope

The principle of the inductance displacement sensor on optical aperture synthesis telescope is given with a measure- ment of mutual inductance to deduce the axial distance. It illustrates an effective modelling and calculation method based on Partial Element Equivalent Circuit (PEEC). PEEC is the method to calculate mutual inductances between coils utilizing partial parameter concept. Simulation for the change of mutual inductances is done which is prepared for the next step work.

PERSONAL RECOLLECTIONS OF W.N. CHRISTIANSEN AND THE EARLY DAYS OF CHINESE RADIO ASTRONOMY

Between 1963 and 1998, Professor W.N. Christiansen visited China more than a dozen times, bringing valuable scientific information, expert guidance and all possible help to the young Chinese radio astronomy team. Here, the writer presents his memories of two typical, deeply-shared experiences, 'The Shahe Experiment' and 'The Making of the Miyun Meter Wave Aperture Synthesis Telescope', as expressions of the kind thoughts of a whole generation of Chinese researchers in astronomy.

光学综合孔径望远镜中子望远镜平行性和光程差调整的研究

光学综合孔径望远镜中子望远镜平行性和光程差调整的研究

Correcting direction-dependent gains in the deconvolution of radio interferometric images

Astronomical imaging using aperture synthesis telescopes requires deconvolution of the point spread function as well as calibration of instrumental and atmospheric effects. In general, such effects are time-variable and vary across the field of view as well, resulting in direction-dependent (DD), time-varying gains. Most existing imaging and calibration algorithms assume that the corruptions are direction independent, preventing even moderate dynamic range full-beam, full-Stokes imaging. We present a general framework for imaging algorithms which incorporate DD errors. We describe as well an iterative deconvolution algorithm that corrects known DD errors due to the antenna power patterns and pointing errors for high dynamic range full-beam polarimetric imaging. Using simulations we demonstrate that errors due to realistic primary beams as well as antenna pointing errors will limit the dynamic range of upcoming higher sensitivity instruments and that our new algorithm can be used to correct for such errors. We have applied this algorithm to VLA 1.4 GHz observations of a field that contains two ``4C'' sources and have obtained Stokes-I and -V images with systematic errors that are one order of magnitude lower than those obtained with conventional imaging tools. Our simulations show that on data with no other calibration errors, the algorithm corrects pointing errors as well as errors due to known asymmetries in the antenna pattern.

The Very Small Array

The VSA is a 14-element aperture-synthesis telescope for mapping structure in the Cosmic Microwave Background which is now being commissioned at the Teide Observatory, Tenerife. It operates at frequencies between 26 and 36 GHz, with a bandwidth of 1.5GHz. Its elements track the sky, providing fringe-rate discrimination of any instrumental effects such as antenna cross-talk or ground radiation. The instrument incorporates a single-baseline interferometer comprising two large (3.7m) dishes, which is used to provide concurrent (and same frequency) pointed flux measurements of point sources in the VSA fields, the positions of these source having been obtained previously from survey observations made with the Cambridge Ryle Telescope at 15 GHz. The VSA is now completing its commissioning programme and it will start routine observations in September 2000.

Radio Continuum Emission in Polar Ring Galaxies

We have used the Very Large Array aperture synthesis telescope to conduct a radio continuum survey of polar ring galaxies, at 20 cm and 6 cm. Forty objects were observed at 20 cm with 5'' resolution. Twenty (50%) of the program sources were detected at 20 cm, down to our 5 σ limit of 0.5 mJy beam-1. This detection rate is similar to those in surveys with comparable sensitivity for early-type galaxies without polar rings. Sixteen of the objects we detected at 20 cm were also observed at 6 cm. We show radio continuum maps for the five objects in our sample that have noticeably extended emission. Our spatial resolution was sufficient to distinguish emission originating in the host galaxy from that in the polar ring. The radio morphology of the extended sources, as well as the ratio of radio to far-infrared flux and the radio spectral indices of our detected sources, indicate that star formation, not nuclear activity, is the dominant source of the radio continuum emission in polar ring galaxies. However, the implied star formation rates are modest, and only one of our sample galaxies will consume its supply of cool gas within 500 Myr.

Computational geometry and the design of aperture-synthesis telescopes

When designing an aperture-synthesis telescope for use in astronomy it is essential to find a layout of the antennas on the site that provides good coverage of the uv-plane. Large holes in the distribution of baseline points in that plane are particularly undesirable, and so an algorithm was devised for locating all the holes in any given design and assessing whether they are unacceptably large. It finds, in each region of the uv-plane, the largest circle that does not contain any of the baseline points and then takes the properties of that empty circle, such as the radius and the position of the centre, as measures of the properties of the local hole. The algorithm is based on the Delaunay triangulation of the set of baseline points and makes use of the result that the circumscribed circle of any Delaunay triangle is always empty. The largest empty circles in the uv-plane are readily found in this way, and the algorithm selects from these circles a non-overlapping set that is a good match to the intuitive concept of the largest holes in the distribution of baseline points. Modified algorithms are also presented that rank each circle not by its absolute radius but by the radius normalized to the mean radius of the circles in the same neighbourhood of the uv-plane; these versions make proper allowance for the large-scale variation of the density of baselines caused by the taper and are therefore of greater use to the designers of a telescope. The algorithms are efficient, requiring O(NB logNB) operations where NB is the number of baselines, and may be used in several ways. One is to take the radius of the largest empty circle as a figure of demerit so that, of a set of trial configurations, the one with the smallest value may be regarded as having the best uv-plane coverage. Another is to take the baselines on the circumference of the largest circle to indicate which antennas are the most suitable for repositioning in order to improve the quality of the uv-plane coverage of a trial configuration.

The electromagnetic properties of aperture-synthesis telescopes shaped as Reuleaux triangles

The electromagnetic properties of aperture-synthesis telescopes in the form of an array of identical antennas sited on the sides of an equilateral Reuleaux triangle are studied in the limit that the number of antennas becomes arbitrarily large. The density of baselines in the uv-plane is derived exactly and is found to be highly irregular: it has seven singularities and 13 finite discontinuities, and is not even approximately circularly symmetric. The response pattern of the synthesized beam is computed numerically and is also found not to be circularly symmetric, although only to a moderate extent. The properties are compared with those of the circular ring array and found to be very similar: the main differences are that the synthesized beam of the Reuleaux triangle is 4 per cent broader than that of the circle with the same diameter whereas the average sidelobe level is 10 per cent lower. The Reuleaux triangle came to attention in a search for the shape of array that gives the most uniform coverage of baselines in the uv-plane, and a discussion is presented whether it really is optimal in this regard.

Galactic Plane Surveys at Low Frequencies

Until recently, high angular resolution and high sensitivity surveys of the radio emission from the plane of our Galaxy were available only at frequencies of several GHz, where large single dish radio telescopes provide arcminute scale angular resolution. At these frequencies thermal radiation from HII regions and diffuse ionized gas comprise a major component of the Galactic emission. Advances in wide field interferometric imaging techniques now make it possible to carry out high sensitivity surveys of the Galaxy with arcminute scale angular resolution at 1.4 GHz and below. Over the past few years initial synthesis surveys have been made. More ambitious surveys that combined sensitive continuum observations with full polarimetry and images of the 3-dimensional structure of atomic hydrogen gas at pc scales are currently underway in the northern (DRAO) and southern (ATNF) hemispheres. The interstellar medium of the Galaxy contains structure on all spatial scales, and these surveys combined data from aperture synthesis telescopes and signal dish antennas to provide full spatial frequency coverage to the resolution limit. Preliminary results reveal wide-spread features and processes in the the interstellar medium that are not readily visible by other means, including, for example, unusual atomic hydrogen structures related to the vertical transfer of matter and radiation between the disk and halo of the Galaxy, Faraday rotation structures that allow study of the magnetic field and diffuse ionized component in the plane of the Galaxy, and a cold atomic phase of the neutral medium that may provide a link between global shock phenomena in the galaxy and the formation of molecular clouds.

Interferometric Phase Correction Using 183 GHzGHz Water Vapor Monitors

The angular resolution that can be obtained by ground-based aperture synthesis telescopes at millimeter and submillimeter wavelengths is limited by phase fluctuations caused by water vapor in the Earth's atmosphere. We describe here the successful correction of such fluctuations during observations at 0.85 mm wavelength with an interferometer consisting of the James Clark Maxwell Telescope and the Caltech Submillimeter Observatory. This was achieved by using two 183 GHz heterodyne radiometers to measure the water vapor content along the line of sight of each telescope. Further development of such techniques will enable future telescopes, such as the Submillimeter Array and the Atacama Large Millimeter Array, to reach their full capability, providing a resolution of up to 001.

WSRT and VLA Observations of the 6 Centimeter and 2 Centimeter Lines of H[TINF]2[/TINF]CO in the Direction of W58C1 (ON 3) and W58C2

Absorption in the J = 211–212 transition of formaldehyde at 2 cm toward the ultracompact H II regions C1 and C2 of W58 has been observed with the Very Large Array with an angular resolution of ~02 and a velocity resolution of ~1 km s-1. The high-resolution continuum image of C1 (also known as ON) shows a partial shell that opens to the northeast. Strong H2CO absorption is observed against W58C1. The highest optical depth (τ > 2) occurs in the southwest portion of C1 near the edge of the shell, close to the continuum peak. The absorption is weaker toward the nearby, more diffuse compact H II region C2, τ ≤ 0.3. The H2CO velocity (-21.2 km s-1) toward C1 is constant and agrees with the velocity of CO emission, main-line OH masers, and the H76α recombination line, but differs from the velocity of the 1720 MHz OH maser emission (~-13 km s-1). Observations of the absorption in the J = 110–111 transition of formaldehyde at 6 cm toward W58C1 and C2 carried out earlier with the Westerbork Aperture Synthesis Telescope at lower resolution (~4'' × 7'') show comparable optical depths and velocities to those observed at 2 cm. Based on the mean optical depth profiles at 6 and 2 cm, the volume density of molecular hydrogen n(H2) and the formaldehyde column density N(H2CO) were determined. The n(H2) is ~6 × 104 cm-3 toward C1. N(H2CO) for C1 is ~8 × 1014 cm-2, while that toward C2 is ~8 × 1013 cm-2.

Cyclic variations in the angular diameter of χ Cygni

We report the direct detection of cyclic diameter variations in the Mira variable χ Cygni. Interferometric observations made between 1997 July and 1999 September, using the Cambridge Optical Aperture Synthesis Telescope (COAST) and the William Herschel Telescope (WHT), indicate periodic changes in the apparent angular diameter at a wavelength of 905 nm, with amplitude 45 per cent of the smallest value. The star appears largest at minimum light. Measurements made at a wavelength of 1.3 μm over the same period suggest much smaller size changes. This behaviour is consistent with a model in which most of the apparent diameter variation at 905 nm is caused by a large increase in the opacity of the outer atmospheric layers (which is mostly owing to titanium oxide) near minimum light, rather than by physical motions of the photosphere. The 1.3-μm waveband is relatively uncontaminated by TiO, and so much smaller size changes would be expected in this band. The latest non-linear pulsational models predict maximum physical size close to maximum light, and increases in opacity near minimum light that are too small to reproduce the diameter variation seen at 905 nm. This suggests either that the phase-dependence of the model pulsation is incorrect, or that the opacities in the models are underestimated. Future interferometric monitoring in uncontaminated near-infrared wavebands should resolve this question.

New views of Betelgeuse: multi-wavelength surface imaging and implications for models of hotspot generation

ABSTRA C T We report contemporaneous multi-wavelength interferometric imaging of the red supergiant star Betelgeuse (a Orionis), using the Cambridge Optical Aperture Synthesis Telescope (COAST) and the William Herschel Telescope (WHT), at wavelengths of 700, 905 and 1290 nm. We find a strong variation in the apparent symmetry of the stellar brightness distribution as a function of wavelength. At 700 nm the star is highly asymmetric, and can be modelled as the superposition of three bright spots on a strongly limb-darkened disc. However, at 905 nm only a single low-contrast feature is visible and at 1290 nm the star presents a featureless symmetric disc. The change in spot contrast with wavelength is consistent with a model in which the bright spots represent unobscured areas of elevated temperature, owing perhaps to convection, on a stellar disc that itself has a different appearance, i.e. geometrical extent and limb-darkening profile, at different wavelengths. The featureless centre-to-limb brightness profile seen at 1290 nm is consistent with this model and suggests that future interferometric monitoring of the star to quantify the size changes associated with radial velocity variations should be performed at similar wavelengths in the near-infrared.