Space-based aperture array for ultra-long wavelength radio astronomy

The past decade has seen the advent of various radio astronomy arrays, particularly for low-frequency observations below 100 MHz. These developments have been primarily driven by interesting and fundamental scientific questions, such as studying the dark ages and epoch of re-ionization, by detecting the highly red-shifted 21 cm line emission. However, Earth-based radio astronomy observations at frequencies below 30 MHz are severely restricted due to man-made interference, ionospheric distortion and almost complete non-transparency of the ionosphere below 10 MHz. Therefore, this narrow spectral band remains possibly the last unexplored frequency range in radio astronomy. A straightforward solution to study the universe at these frequencies is to deploy a space-based antenna array far away from Earths’ ionosphere. In the past, such space-based radio astronomy studies were principally limited by technology and computing resources, however current processing and communication trends indicate otherwise. Furthermore, successful space-based missions which mapped the sky in this frequency regime, such as the lunar orbiter RAE-2, were restricted by very poor spatial resolution. Recently concluded studies, such as DARIS (Disturbuted Aperture Array for Radio Astronomy In Space) have shown the ready feasibility of a 9 satellite constellation using off the shelf components. The aim of this article is to discuss the current trends and technologies towards the feasibility of a space-based aperture array for astronomical observations in the Ultra-Long Wavelength (ULW) regime of greater than 10 m i.e., below 30 MHz. We briefly present the achievable science cases, and discuss the system design for selected scenarios such as extra-galactic surveys. An extensive discussion is presented on various sub-systems of the potential satellite array, such as radio astronomical antenna design, the on-board signal processing, communication architectures and joint space-time estimation of the satellite network. In light of a scalable array and to avert single point of failure, we propose both centralized and distributed solutions for the ULW space-based array. We highlight the benefits of various deployment locations and summarize the technological challenges for future space-based radio arrays.     

Analysis of sky contributions to system temperature for low frequency SKA aperture array geometries

The new generation of radio telescopes, such as the proposed Square Kilometer Array (SKA) and the Low-Frequency Array (LOFAR) rely heavily on the use of very large phased aperture arrays operating over wide band-widths at frequency ranges up to approximately 1.4?GHz. The SKA in particular will include aperture arrays consisting of many thousands of elements per station providing un-paralleled survey speeds. Currently two different arrays (from nominally 70?MHz to 450?MHz and from 400?MHz to 1.4?GHz) are being studied for inclusion within the overall SKA configuration. In this paper we aim to analyze the array contribution to system temperature for a number of regular and irregular planar antenna array configurations which are possible geometries for the low-frequency SKA (sparse disconnected arrays). We focus on the sub-500?MHz band where the real sky contribution to system temperature (T _sys ) is highly significant and dominants the overall system noise temperature. We compute the sky noise contribution to T _sys by simulating the far field response of a number of SKA stations and then convolve that with the sky brightness temperature distribution from the Haslam 408?MHz survey which is then scaled to observations at 100?MHz. Our analysis of array temperature is carried out by assuming observations of three cold regions above and below the Galactic plane. The results show the advantages of regular arrays when sampled at the Nyquist rate as well as their disadvantages in the form of grating lobes when under-sampled in comparison to non-regular arrays.     

一种扩大FAST视场的方法

所有的大口径射电望远镜都存在这样一个问题：在其分辨率和灵敏度提高的同时，视场变小．而且口径越大，视场越小．这成为大口径望远镜不可回避的矛盾．要解决这个矛盾，可以在望远镜的焦平面上放置Ⅳ个分立馈源．让它们同时工作，这样可以看作把视场扩大了Ⅳ倍．望远镜的工作效率提高Ⅳ倍．但是这样做的缺点是——视场不连续．且馈源数目Ⅳ受到望远镜焦比(F／D)的限制．采用致密焦面阵(dense focal plane array)就可以很好地解决这个问题．致密焦面阵的单元不是喇叭口天线，而是无方向性的Vivaldi天线(Vivaldi antenna)．要把Vivaldi阵列应用到望远镜上，需要对单个Vivaldi天线和Vivaldi阵列的电性能有清楚的认识，并能根据需要来设计照明方向图．还要知道大望远镜的焦面上电磁场的分布情况，借此判断能否应用Vivaldi阵列，以及给出Vivaldi单元的分路赋权网络．主要给出了FAST的焦面场的分布情况．并说明应用Vivaldi阵列的可能性．     

Performance of a scintillation detector array operated with LHAASO-KM2A electronics

A scintillation detector array composed of 115 detectors and covering an area of about 20000 m² was installed at the end of 2016 at the Yangbajing international cosmic ray observatory and has been taking data since then. The array is equipped with electronics from Large High Altitude Air Shower Observatory Square Kilometer Complex Array (LHAASO-KM2A) and, in turn, currently serves as the largest debugging and testing platform for the LHAASO-KM2A. Furthermore, the array was used to study the performance of a wide field-of-view air Cherenkov telescope by providing accurate information on the shower core, direction and energy, etc. This work is mainly dealing with the scintillation detector array. The experimental setup and the offline calibration are described in detail. Then, a thorough comparison between the data and Monte Carlo (MC) simulations is presented and a good agreement is obtained. With the even-odd method, the resolutions of the shower direction and core are measured. Finally, successful observations of the expected Moon’s and Sun’s shadows of cosmic rays (CRs) verify the measured angular resolution.     

Experiments on the dish verification antenna china for the SKA

The Square Kilometre Array (SKA) is expected to become the world’s most powerful radio telescope at meter and centimeter wavelength in the coming decades. The construction of SKA will be divided into two phases. The first phase (SKA1), scheduled for completion in 2023, will construct 10 % of the whole collecting area. The second phase (SKA2) will build the rest 90 % collecting area. The SKA1 consists of several types of arrays including SKA1-low and SKA1-mid. The latter is a dish array consisting of ~200 medium-size antennas. The integrated dish array in SKA2 will expand to 2500 dishes, spreading 3000 kilometers across the southern part of Africa. The demanding specifications and enormous number of the SKA dish raise challenges in the dish development such as mass production with high performance at low cost, quick installation and high reliability. Dish Verification Antenna China (DVA-C) was built as one of three initial prototypes. A novel single-piece panel reflector made of carbon fiber reinforced polymer (CFRP) was adopted. In this study, an L-band receiver is installed to make DVA-C a complete system for experiments on antenna performance test and preliminary observations. The performance of DVA-C including the system noise temperature, pointing accuracy, antenna pattern, and aperture efficiency has been tested. Preliminary observations such as pulsars and HI are then conducted, which indicates that the DVA-C can not only serve as an educational instrument and key technology test bed, but also be applied for scientific work such as pulsar timing, all-sky HI survey, multi-frequency monitoring of variable sources etc.     

光学综合孔径干涉成像的关键技术及多光束干涉仿真

闭合相位技术、U -V覆盖技术和像重构技术是光学综合孔径干涉成像的三个关键技术。文中简要介绍了光学综合孔径技术的发展历史、这三种关键技术和光学综合孔径望远镜阵的构成 ,对之进行了多光束干涉的计算机仿真和实验仿真 ,两者结果一致说明了光学综合孔径干涉成像技术中有关干涉的理论的正确性 ,指出了下一步的研究方向。     

Energy determination of gamma-ray induced air showers observed by an extensive air shower array

ASTRI SST-2M is one of the prototypes of the small size class of telescopes proposed for the Cherenkov Telescope Array. Its optical design is based on a dual-mirror Schwarzschild-Couder configuration, and the camera is composed by a matrix of monolithic multipixel silicon photomultipliers managed by ad-hoc tailored front-end electronics. This paper describes the procedures for the gain calibration on the ASTRI SST-2M. Since the SiPM gain depends on the operative voltage and the temperature, we adjust the operative voltages for all sensors to have equal gains at a reference temperature. We then correct gain variations caused by temperature changes by adjusting the operating voltage of each sensor. For that purpose the SiPM gain dependence on operating voltage and on temperature have been measured. In addition, we present the calibration procedures and the results of the experimental measurements to evaluate, for each pixel, the parameters necessary to make the trigger uniform over the whole focal plane.     

An Overview of the Veritas Prototype Telescope and Camera

The Very Energetic Radiation Imaging Telescope Array System (VERITAS) is the next-generation ground-based gamma-ray observatory that is being built in southern Arizona by a collaboration of 10 institutions in Canada, Ireland, the UK and the USA. VERITAS is designed to operate in the range from 50 GeV to 50 TeV with optimal sensitivity near 200 GeV; it will effectively overlap with the next generation of space-based gamma-ray telescopes. The first phase of VERITAS, consisting of four telescopes of 12 m aperture, will be operational by the time of the GLAST launch in 2007. Eventually, the array will be expanded to include the full array of seven telescopes on a filled hexagonal grid of side 80 m. A prototype VERITAS telescope with a reduced number of mirrors and signal channels has been built. Its design and performance is described here. The prototype is scheduled to be upgraded to a full 499 pixel camera with 350 mirrors during the autumn of 2004. The VERITAS collaboration consists of universities and institutions from Ireland, UK, USA and Canada. See for a full listing.     

Direct imaging of planetary systems with a ground-based radio telescope array

The National Radio Astronomy Observatory's proposed Millimeter Array (MMA) will bring unprecedented sensitivity, angular resolution, and image dynamic range to the millimeter wavelength region of the spectrum. An obvious question is whether such an instrument could be used to detect planets orbiting nearby stars. The techniques of aperture synthesis imaging developed for centimeter wavelength radio arrays are capable of producing images whose dynamic ranges greatly exceed the brightness ratio of a solar-type star and a Jupiter-like planet at sub-millimeter or millimeter wavelengths. The angular resolution required to separate a star and planet at a few pc distance can be obtained with baselines of several km. The greatest challenge is sensitivity. At the highest possible observing frequencies ( 300 GHz for typical high, dry sites, and 900 GHz from the Antarctic plateau), the proposed MMA will be unable to detect the thermal emission from a Jupiter-like planet a few pc away. An upgraded MMA operating near 300 GHz with twice the currently proposed number of antennas, a 20% fractional bandwidth, and improved receivers could detect Jupiter at 4 pc in a few months. Building such an array on the Antarctic plateau and operating at 900 GHz would allow Jupiter at 4 pc to be detected in approximately one day of observing time.Paper presented at the Conference onPlanetary Systems: Formation, Evolution, and Detection held 7–10 December, 1992 at CalTech, Pasadena, California, U.S.A.     

Cost-effective infrastructure in a multi-antenna telescope layout

The future Square Kilometre Array (SKA) radio telescope is an interferometer array that will use a variety of collector types, including approximately 2500 dishes distributed with separations up to a few thousand kilometres, and about 250 aperture array (AA) stations located within 200 km of the core. The data rates associated with each individual collector are vast: around 10 GBytes/s for each dish and 2 TBytes/s for an AA station. As each of these must be connected directly to a central correlator, designing a cost-effective cabling and trenching infrastructure presents a great engineering challenge. In this paper we discuss approaches to performing this optimisation. In graph theory, the concept of a minimum spanning tree (MST) is equivalent to finding the minimum total trench length joining a set of n arbitrary points in the plane. We have developed a set of algorithms which optimise the infrastructure of any given telescope layout iteratively, taking into consideration not only trenching but also cabling and jointing costs as well. Solutions for few example configurations of telescope layout are presented. We have found that these solutions depend significantly on the collectors’ output data rates. When compared to a “traditional” MST-based approach which minimises trenching costs only, our algorithms can further reduce total costs by up to 15–20%. This can influence greatly the SKA infrastructure related costs.     

Schemes for Segmenting the Main Reflector of the FAST

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Efficiency Analysis of Focal Plane Arrays in Deep Dishes

In this paper we demonstrate the practical analysis of a dense Focal Plane Array (FPA) for a deep dish radio telescope. The analytical model is used to optimize efficiency and bandwidth of deep dish system by varying design parameters such as feed size and FPA aperture field. A prototype FPA was evaluated on the Westerbork Synthesis Radio Telescope (WSRT) and the resulting efficiency is discussed.     

HARP/ACSIS: a submillimetre spectral imaging system on the James Clerk Maxwell Telescope

This paper describes a new Heterodyne Array Receiver Program (HARP) and Auto-Correlation Spectral Imaging System (ACSIS) that have recently been installed and commissioned on the James Clerk Maxwell Telescope. The 16-element focal-plane array receiver, operating in the submillimetre from 325 to 375 GHz, offers high (three-dimensional) mapping speeds, along with significant improvements over single-detector counterparts in calibration and image quality. Receiver temperatures are ∼120 K across the whole band, and system temperatures of ∼300 K are reached routinely under good weather conditions. The system includes a single-sideband (SSB) filter so these are SSB values. Used in conjunction with ACSIS, the system can produce large-scale maps rapidly, in one or more frequency settings, at high spatial and spectral resolution. Fully sampled maps of     size can be observed in under 1 h.
The scientific need for array receivers arises from the requirement for programmes to study samples of objects of statistically significant size, in large-scale unbiased surveys of galactic and extra-galactic regions. Along with morphological information, the new spectral imaging system can be used to study the physical and chemical properties of regions of interest. Its three-dimensional imaging capabilities are critical for research into turbulence and dynamics. In addition, HARP/ACSIS will provide highly complementary science programmes to wide-field continuum studies and produce the essential preparatory work for submillimetre interferometers such as the Submillimeter Array (SMA) and Atacama Large Millimeter/Submillimeter Array (ALMA).     

The Atacama Large Millimeter/Submillimeter Array: overview & status

The Atacama Large Millimeter/Submillimeter Array (ALMA) is an international millimeter-wavelength radio telescope under construction in the Atacama Desert of northern Chile. ALMA will be situated on a high-altitude site at 5000 m elevation which provides excellent atmospheric transmission over the instrument wavelength range of 0.3 to 3 mm. ALMA will be comprised of two key observing components—a main array of up to sixty-four 12-m diameter antennas arranged in a multiple configurations ranging in size from 0.15 to ∼18 km, and a set of four 12-m and twelve 7-m antennas operating in a compact array ∼50 m in diameter (known as the Atacama Compact Array, or ACA), providing both interferometric and total-power astronomical information. High-sensitivity dual-polarization 8 GHz-bandwidth spectral-line and continuum measurements between all antennas will be available from two flexible digital correlators. At the shortest planned wavelength and largest configuration, the angular resolution of ALMA will be 0.005″. The instrument will use superconducting (SIS) mixers to provide the lowest possible receiver noise contribution, and special-purpose water vapor radiometers to assist in calibration of atmospheric phase distortions. A complex optical fiber network will transmit the digitized astronomical signals from the antennas to the correlators in the Array Operations Site Technical Building, and post-correlation to the lower-altitude Operations Support Facility where the array will be controlled, and initial construction and maintenance of the instrument will occur. ALMA Regional Centers in the US, Europe, Japan and Chile will provide the scientific portals for the use of ALMA; early science observations are expected in 2010, with full operations in 2012.     

Digital array scanned interferometers for astronomy

We report investigations of digital array scanned interferometers (DASI) with silicon CCD array detectors to define the operational capabilities of a mapping (polarimetric) spectrometer for astronomical applications based on these instruments. For spectral mapping, we demonstrate spatially resolved spectra using a cylindrical lens to image in the interferometer's redundant coordinate. We demonstrate enhanced spectral resolution with a band limiting filter and standard aliasing techniques. The signal-to-noise characteristics of the Fourier transformed data are demonstrated with regard to the effects of a rectangular sampling function, spectral multiplexing and the pixel-to-pixel variation of the CCD array.These data indicate that DASIs can offer simple, versatile (polarization) mapping spectrometers suitable for spectral mapping observations from the ultraviolet to the infrared of extended sources at variable spatial resolution, particularly where long term stable operation is essential, as for spaceraft instruments.This research was supported by NASA under grants NAGW-122 and NAGW-1801.     

Five-hundred-meter Aperture Spherical Telescope project

A Five hundred meter Aperture Spherical Telescope (FAST) is proposed to be built in the unique karst area of southwest China, and will act, in a sense, as a prototype for the Square Kilometer Array (SKA). It will be over twice as large as the Arecibo telescope coupled with much wider sky coverage. Some results from site surveys for such a SKA concept are briefly reported. Technically, FAST is not simply a copy of the existing Arecibo telescope but has rather a number of innovations. Firstly, the proposed main spherical reflector, by conforming to a paraboloid of revolution in real time through actuated active control, enables the realization of both wide bandwidth and full polarization capability while using standard feed design. Secondly, a feed support system which integrates optical, mechanical and electronic technologies will effectively reduce the cost of the support structure and control system. Pre-research on FAST has become a key project in the CAS. This revised version was published online in July 2006 with corrections to the Cover Date.     

SCUBA-2 CCD-STYLE IMAGING FOR THE JCMT

SCUBA-2 will replace SCUBA (Submillimetre Common User Bolometer Array) on the James Clerk Maxwell Telescope in 2006 and will be the first CCD-style camera for submillimetre astronomy. The instrument will simultaneously image at 850 and 450 microns using two focal plane arrays of 5120 pixels each. SCUBA-2 will map the submillimetre sky 1000 times faster than SCUBA to the same signal-to-noise ratio. This paper introduces the detector technology and the challenges faced in reading out a detector array cooled to ～120 mK.     

Phased array observations with infield phasing

We present results from pulsar observations using the Giant Metrewave Radio Telescope (GMRT) as a phased array with infield phasing. The antennas were kept in phase throughout the observation by applying antenna based phase corrections derived from visibilities that were obtained in parallel with the phased array beam data, and which were flagged and calibrated in real time using a model for the continuum emission in the target field. We find that, as expected, the signal to noise ratio (SNR) does not degrade with time. In contrast observations in which the phasing is done only at the start of the observation show a clear degradation of the SNR with time. We find that this degradation is well fit by a function of the form SNR\((\tau ) = \alpha + \beta e^{-(\tau /\tau _{0})^{5/3}}\), which corresponds to the case where the phase drifts are caused by Kolmogorov type turbulence in the ionosphere. We also present general formulae (i.e. including the effects of correlated sky noise, imperfect phasing and self noise) for the SNR and synthesized beam size for phased arrays (as well as corresponding formulae for incoherent arrays). These would be useful in planning observations with large array telescopes.     

Monte Carlo design studies for the Cherenkov Telescope Array

The Cherenkov Telescopes Array (CTA) is planned as the future instrument for very-high-energy (VHE) gamma-ray astronomy with a wide energy range of four orders of magnitude and an improvement in sensitivity compared to current instruments of about an order of magnitude. Monte Carlo simulations are a crucial tool in the design of CTA. The ultimate goal of these simulations is to find the most cost-effective solution for given physics goals and thus sensitivity goals or to find, for a given cost, the solution best suited for different types of targets with CTA. Apart from uncertain component cost estimates, the main problem in this procedure is the dependence on a huge number of configuration parameters, both in specifications of individual telescope types and in the array layout. This is addressed by simulation of a huge array intended as a superset of many different realistic array layouts, and also by simulation of array subsets for different telescope parameters. Different analysis methods – in use with current installations and extended (or developed specifically) for CTA – are applied to the simulated data sets for deriving the expected sensitivity of CTA. In this paper we describe the current status of this iterative approach to optimize the CTA design and layout.