SKA Cost Model for Wide Field-of-View Options

We describe a SKA hardware component model that includes various concepts for achieving a wide field-of-view (FoV) as desired for survey speed. Costs for each of the components in the model are derived and in some cases optimum parameters are estimated. The model identifies components of major cost and allows comparison of the relative costs of the wide FoV concepts.     

Cost Effective Frequency Ranges For Multi-Beam Dishes, Cylinders, Aperture Arrays, and Hybrids

Five out of six Square Kilometre Array (SKA) science programs need extensive surveys at frequencies below 1.4 GHz and only four need high-frequency observations. The latter ones drive to expensive high surface accuracy collecting area, while the former ask for multi-beam receiver systems and extensive post correlation processing. In this paper, we analyze the system cost of a SKA when the field-of-view (Fov) is extended from 1 deg² at 1.4 GHz to 200 deg² at 0.7 GHz for three different antenna concepts. We start our analysis by discussing the fundamental limitations and cost issues of wide-band focal plane arrays (FPA) in dishes and cylinders and of wide-band receptors in aperture arrays. We will show that a hybrid SKA in three different antenna technologies will give the highest effective sensitivity for all six key science programs.     

Electronic Multi-beam Radio Astronomy Concept: Embrace a Demonstrator for the European SKA Program

ASTRON has demonstrated the capabilities of a 4 m², dense phased array antenna (Bij de Vaate et al., 2002) for radio astronomy, as part of the Thousand Element Array project (ThEA). Although it proved the principle, a definitive answer related to the viability of the dense phased array approach for the SKA could not be given, due to the limited collecting area of the array considered. A larger demonstrator has therefore been defined, known as “Electronic Multi-Beam Radio Astronomy Concept”, EMBRACE, which will have an area of 625 m², operate in the band 0.4–1.550 GHz and have at least two independent and steerable beams. With this collecting area EMBRACE can function as a radio astronomy instrument whose sensitivity is comparable to that of a 25-m diameter dish. The collecting area also represents a significant percentage area (∼10%) of an individual SKA “station.” This paper presents the plans for the realisation of the EMBRACE demonstrator.     

Active Adaptive Arrays: The ASTRON Approach to SKA

It has been argued, for a number of reasons, that the next generation radio telescope should be a multi-element interferometer with a collecting area of about 1 km². The remaining parameters of such an instrument – frequency range, angular resolution, instantaneous bandwidth, etc. – will be science driven. The requirements for propagation studies are briefly discussed, and it is pointed out how variable-source confusion may differ from the normal variety. Finally, the Dutch project to achieve a large collecting area using adaptive arrays of active antennas is described. A systematic approach has been adopted, with the construction of arrays of increasing complexity to test design features at each state. Recently, development of a low frequency array (LOFAR) has become an additional option. It would facilitate tests of some of the larger instrument's features, and provide real data on the influence of the ionosphere and interfering sources. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

Cylinder – Small Dish Hybrid For The SKA

No single technology can meet the current science specifications for the Square Kilometre Array (SKA). In this paper a hybrid option is explored that uses cylindrical reflectors to satisfy low frequency sensitivity and field-of-view requirements and small parabolic dishes to work at frequencies above 0.5 GHz.     

What Will the Next Generation Radio Telescope Detect at 1.4 GHz?

An international project is underway to design and build a radiotelescope with an effective collecting area two orders of magnitude greaterthan the largest existing instruments. One of the many scientific goals of thisinstrument will be the investigation of the extragalactic radio sourcepopulation at flux densities two to three orders of magnitude fainter thanthe limits of existing observations. We present simulations of the radiosky at 1.4 GHz down to a flux density limit of 0.1 Jy usingextrapolations of known radio luminosity functions for two differentpopulation scenarios. The resulting simulations confirm that a resolutionof 0^' _.1 is necessary to avoid formal confusion, but sourceblending may still dominate if the intrinsic size of such faint sourcesis larger than a few kiloparsecs.     

ALMA capabilities for observations of continuum emission

The Atacama Large Millimeter/submillimeter Array, ALMA, combines a large collecting area, very sensitive receivers and a location on a high dry site. ALMA’s sensitivity for continuum measurements is increased with the added feature of an 8 GHz instantaneous bandwidth. Taken together, these four factors provide unparalleled sensitivity in the millimeter/submillimeter wavelength range. With its great sensitivity and angular resolution, ALMA will transform our view of mm/sub-mm astronomy.     

The MSFC vector magnetograph

The NASA/Marshall Space Flight Center's solar vector magnetograph system is described; this sytem allows measurements of all components of the Sun's photospheric magnetic field over a 5 × 5 or 2.0 × 2.0 arc min square field-of-view with an optimum time resolution of 100 s and an optimum signal-to-noise of 1600. The basic system components are described, including the optics, detector, digital system and associated electronics. Automatic sequencing and control functions are outlined as well as manual selections of system parameters which afford unique system flexibility. Results of system calibration and performance are presented, including linearity, dynamic range, uniformity, spatial and spectral resolutions, signal-to-noise, electro-optical retardation and polarization calibration. Scientific investigations which utilize the unique characteristics of the instrument are described and typical results are presented.     

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.     

High Performance Corrugated Feed Horns for Space Applications at Millimetre Wavelengths

We report on the design fabrication and testing of a set of high performance corrugated feed horns at 30, 70, and 100 GHz, built as advanced prototypes for the Low Frequency Instrument (LFI) of the ESA Planck mission. The electromagnetic designs include linear (100 GHz) and dual shaped (30 and 70 GHz) profiles. Fabrication has been achieved by direct machining at 30 GHz and by electro-formation at higher frequencies. The measured performances on side lobe and return loss meet the stringent Planck requirements over the large (20%) instrument bandwidth. Moreover, the advantage in terms of main lobe shape and side lobe levels of the dual profiled designs has been demonstrated. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

The 40 cm Monitoring Telescope of the Universitätssternwarte Bochum

The new 40cm Bochum Monitoring Telescope (BMT) has started routine operation at the Universitätssternwarte Bochum (USB), located near Cerro Armazones in Chile. It has a 41′ × 27′ field of view (FoV) and is equipped with B and V broad band filters and three narrow band filters at 670, 680, and 690 nm. This makes the BMT ideally suited to perform photometric reverberation mapping of the Hα emission line of active galactic nuclei, where the line is redshifted into the narrow bands, and to monitor bright stars which would be saturated with large telescopes. As a complement to our Robotic Bochum Twin Telescope (RoBoTT) with 2°.7 FoV and 14 filters, the BMT is an efficient instrument to accurately study the variability of individual sources, provided that its smaller FoV covers a sufficient number of suitable comparison stars. Here we describe the telescope and its fully robotic operation, and present science verification data demonstrating the performance of the BMT. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Sky Noise Limited Snapshot Imaging in the Presence of RFI with Lofar’s Initial Test Station

The initial test station (ITS) is the first full scale prototype of a low frequency array (LOFAR) station. It operates in the 10–40 MHz range and consists of 60 sky noise limited dipoles arranged in a five-armed spiral structure offering an instantaneous synthesized aperture of almost 200 m diameter. We will present all sky snapshot images demonstrating sky-noise limited imaging capability in the presence of a strong RFI source that exceeds the all sky power by 27 dB. This result is obtained with a two stage self-calibration procedure. First, the RFI source near the horizon is used as calibrator and then subtracted, after which Cas A shows up at a level that is a factor 2000 lower and then dominates the picture with its side lobes. A second self calibration on Cas A then reveals the same extended galactic emission as found in a RFI free adjacent spectral channel. This demonstrates that a single 10 kHz channel of a 6.7 s snapshot of a single LOFAR station already provides a dynamic range of over 10⁴.     

The Australia Telescope search for cosmic microwave background anisotropy

In an attempt to detect cosmic microwave background (CMB) anisotropy on arcmin scales, we have made an 8.7-GHz image of a sky region with a resolution of 2 arcmin and high surface brightness sensitivity using the Australia Telescope Compact Array (ATCA) in an ultracompact configuration. The foreground discrete-source confusion was estimated from observations with higher resolution at the same frequency and in a scaled array at a lower frequency. Following the subtraction of the foreground confusion, the field shows no features in excess of the instrument noise. This limits the CMB anisotropy flat-band power to Q _flat < 23.6 μ K with 95 per cent confidence; the ATCA filter function (which is available at the website www.atnf.csiro.au/Research/cmbr/cmbr_atca.html) F _l in multipole l -space peaks at l _eff = 4700 and has half-maximum values at l  = 3350 and 6050.     

Pulsar sensitivity studies of the GLAST large area telescope

In this contribution we present our preliminary investigation on pulsar sensitivity of the Large Area Telescope, the main instrument aboard the GLAST mission. In particular we concentrated our attention to pulsars located at low galactic latitudes. We created a set of simulated pulsars having different fluxes in an array of galactic coordinates separated by a distance greater than the LAT Point Spread Function in order to avoid confusion between adjacent sources. Galactic gamma-ray sky background as used during the second LAT Data Challenge (DC2) is also included. We then run an automatic routine for testing periodicity for all the pulsars considering an opportune timing solution. In this way we can obtain a map of the sensitivity of the periodic searches for different fluxes and for various Galactic latitudes. Some assumptions have been made by simulating the pulsar sources, but this study is a first step toward an estimate for pulsed emission sensitivity of the GLAST LAT. The pulsed flux sensitivity profile we generate could also be input to a population synthesis code of Galactic pulsars in order to obtain more accurate predictions of the number of expected pulsar detections by GLAST. On behalf of the GLAST LAT Collaboration.     

Photo's from Proceedings of the Third MicroQuasar Workshop; Granada Worshop on Galactic Relativistic Jet Sources – Granada,Spain, 11–13 September 2000

The X-Ray Spectroscopic Explorer (XRASE) has a unique combination of features that will make it possible to address many of NASA's scientific goals. These include how galaxy clusters form, the physics and chemistry of the ISM, the heating of stellar coronae, the amount and content of intergalactic baryonic matter, the mass of black holes and the formation of disks and jets in AGN and galactic binaries. XRASE has a thin foil, multilayered telescope with a large collecting area up to 10 keV, especially in the Fe K region (1100 cm²). Its microcalorimeter array combines high energy resolution (7 eV at 6 keV) and efficiency with a field-of-view of 26 arcmin² . A deep orbit allows for long, continuous observations. Monitoring instruments in the optical (WOM-X), UV (TAUVEX) and hard X-RAY (GRAM) bands will offer exceptional opportunities to make simultaneous multi-wavelength observations.     

Clover – A B-mode polarization experiment

Clover is a new instrument being built to detect the B-mode polarization of the CMB. It consists of three telescopes operating at 97, 150, and 220 GHz and will be sited in Chile at the Llano de Chajnantor. Each telescope assembly is scaled to give a constant beam size of 8″ and feeds an array of between 320 and 512 finline-coupled TES bolometers. Here we describe the design, current status and scientific prospects of the instrument.     

Effective Sensitivity of A Non-uniform Phased Array of Short Dipoles

Short dipoles are a key element in new low frequency array antennas as proposed for LOFAR and other astronomical applications. Unfortunately standard texts on short dipole antennas are based on the effective area and do not lead to an astronomically useful sensitivity formulation in a straightforward manner. The concept of maximum effective area is applied to arrays of short dipoles and allows expressing the sensitivity as the ratio of this area over the effective sky brightness temperature as long as the output noise power is dominated by the antenna input radiation. For both quantities we only need to know the array directivity pattern that includes the mutual coupling effects when the actual loading conditions of the array elements are taken into account. Short dipole elements have a constant directivity pattern for frequencies below resonance, but they exhibit strong complex impedance variations that provide only narrow band performance when power matching is applied as required in transmit applications. However, in receive applications voltage or current sensing can be realized, for example with an active balun. Assisted by the steep increase of the sky brightness with wavelength for frequencies below 300 MHz, this can provide sky noise dominated performance over at least a three to one frequency range. Still the low frequency limit is determined by the amplifier noise contribution and the losses in the antenna and in the dielectric ground surrounding the elements. We show that for a sparse array with the elements non-uniformly distributed according to an exponential shell model, a constant sensitivity can be obtained over a frequency range of at least two octaves. In addition, such a configuration has a factor of six greater sensitivity than a rectangular array for a large part of the frequency band.