Monte Carlo studies of medium-size telescope designs for the Cherenkov Telescope Array

We present studies for optimizing the next generation of ground-based imaging atmospheric Cherenkov telescopes (IACTs). Results focus on mid-sized telescopes (MSTs) for CTA, detecting very high energy gamma rays in the energy range from a few hundred GeV to a few tens of TeV. We describe a novel, flexible detector Monte Carlo package, FAST (FAst Simulation for imaging air cherenkov Telescopes), that we use to simulate different array and telescope designs. The simulation is somewhat simplified to allow for efficient exploration over a large telescope design parameter space. We investigate a wide range of telescope performance parameters including optical resolution, camera pixel size, and light collection area. In order to ensure a comparison of the arrays at their maximum sensitivity, we analyze the simulations with the most sensitive techniques used in the field, such as maximum likelihood template reconstruction and boosted decision trees for background rejection. Choosing telescope design parameters representative of the proposed Davies–Cotton (DC) and Schwarzchild–Couder (SC) MST designs, we compare the performance of the arrays by examining the gamma-ray angular resolution and differential point-source sensitivity. We further investigate the array performance under a wide range of conditions, determining the impact of the number of telescopes, telescope separation, night sky background, and geomagnetic field. We find a 30–40% improvement in the gamma-ray angular resolution at all energies when comparing arrays with an equal number of SC and DC telescopes, significantly enhancing point-source sensitivity in the MST energy range. We attribute the increase in point-source sensitivity to the improved optical point-spread function and smaller pixel size of the SC telescope design.     

A Monte Carlo template based analysis for air-Cherenkov arrays

We present a high-performance event reconstruction algorithm: an Image Pixel-wise fit for Atmospheric Cherenkov Telescopes (ImPACT). The reconstruction algorithm is based around the likelihood fitting of camera pixel amplitudes to an expected image template. A maximum likelihood fit is performed to find the best-fit shower parameters. A related reconstruction algorithm has already been shown to provide significant improvements over traditional reconstruction for both the CAT and H.E.S.S. experiments. We demonstrate a significant improvement to the template generation step of the procedure, by the use of a full Monte Carlo air shower simulation in combination with a ray-tracing optics simulation to more accurately model the expected camera images. This reconstruction step is combined with an MVA-based background rejection.Examples are shown of the performance of the ImPACT analysis on both simulated and measured (from a strong VHE source) gamma-ray data from the H.E.S.S. array, demonstrating an improvement in sensitivity of more than a factor two in observation time over traditional image moments-fitting methods, with comparable performance to previous likelihood fitting analyses. ImPACT is a particularly promising approach for future large arrays such as the Cherenkov Telescope Array (CTA) due to its improved high-energy performance and suitability for arrays of mixed telescope types.     

相控阵馈源MVDR波束合成器缓解RFI仿真

相控阵馈源(Phased array feeds, PAFs)接收机作为下一代微波接收机, 为大口径射电天文望远镜的射电干扰(Radio Frequency Interference, RFI)缓解工作带来了新的解决方法. PAFs接收机对射电望远镜焦平面的电磁波进行空域采样, 返回时域阵列信号, 使用最小方差无失真响应(Minimum Variance Distortionless Response, MVDR)波束合成器可以自适应地识别RFI的方向, 同时抑制RFI在输出信号中的功率, 从而达到提升射电望远镜灵敏度的效果. 仿真结果表明MVDR波束合成器对有源高能量的射电干扰有很强的识别能力和一定程度的缓解能力, 同时, 该波束合成器对各阵元信道中加性噪声累积引起的无源干扰有很强的抑制能力, 因此, PAFs接收机的MVDR波束合成器可以增强日益复杂电磁波环境下射电望远镜的抗干扰能力.     

Cross calibration of telescope optical throughput efficiencies using reconstructed shower energies for the Cherenkov Telescope Array

For reliable event reconstruction of Imaging Atmospheric Cherenkov Telescopes (IACTs), calibration of the optical throughput efficiency is required. Within current facilities, this is achieved through the use of ring shaped images generated by muons. Here, a complementary approach is explored, achieving cross calibration of elements of IACT arrays through pairwise comparisons between telescopes, focussing on its applicability to the upcoming Cherenkov Telescope Array (CTA). Intercalibration of telescopes of a particular type using eventwise comparisons of shower image amplitudes has previously been demonstrated to recover the relative telescope optical responses. A method utilising the reconstructed energy as an alternative to image amplitude is presented, enabling cross calibration between telescopes of varying types within an IACT array. Monte Carlo studies for two plausible CTA layouts have shown that this calibration procedure recovers the relative telescope response efficiencies at the few per cent level.     

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.     

Introducing the CTA concept

The Cherenkov Telescope Array (CTA) is a new observatory for very high-energy (VHE) gamma rays. CTA has ambitions science goals, for which it is necessary to achieve full-sky coverage, to improve the sensitivity by about an order of magnitude, to span about four decades of energy, from a few tens of GeV to above 100 TeV with enhanced angular and energy resolutions over existing VHE gamma-ray observatories. An international collaboration has formed with more than 1000 members from 27 countries in Europe, Asia, Africa and North and South America. In 2010 the CTA Consortium completed a Design Study and started a three-year Preparatory Phase which leads to production readiness of CTA in 2014. In this paper we introduce the science goals and the concept of CTA, and provide an overview of the project.     

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．     

The sensitivity of the next generation of lunar Cherenkov observations to UHE neutrinos and cosmic rays

We present simulation results for the detection of ultra-high energy (UHE) cosmic ray (CR) and neutrino interactions in the Moon by radio-telescopes. We simulate the expected radio signal at Earth from such interactions, expanding on previous work to include interactions in the sub-regolith layer for single dish and multiple telescope systems. For previous experiments at Parkes, Goldstone (GLUE), and Kalyazin we recalculate the sensitivity to an isotropic flux of UHE neutrinos. We find the published sensitivity for the GLUE experiment to be too high (too optimistic) by an order of magnitude, and consequently the GLUE limit to be too low by an order of magnitude. Our predicted sensitivity for future experiments using the Australia Telescope Compact Array (ATCA) and the Australian SKA Pathfinder (ASKAP) indicate these instruments will be able to detect the more optimistic UHE neutrino flux predictions, while the square kilometre array (SKA) will also be sensitive to all bar one prediction of a diffuse ‘cosmogenic’, or ‘GZK’, neutrino flux.Outstanding theoretical uncertainties at both high-frequency and low-frequency limits currently prevent a reliable estimate of the sensitivity of the lunar Cherenkov technique for UHE cosmic ray (CR) astronomy. Here, we place limits on the effects of large-scale surface roughness on UHE CR detection, and find that when near-surface ‘formation-zone’ effects are ignored, the proposed SKA low-frequency aperture array could detect CR events above 56 EeV at a rate between 15 and 40 times that of the current Pierre Auger Observatory. Should further work indicate that formation-zone effects have little impact on UHE CR sensitivity, observations of the Moon with the SKA would allow directional analysis of UHE cosmic rays, and investigation of correlations with putative cosmic ray source populations, to be conducted with very high statistics.     

Dark matter and fundamental physics with the Cherenkov Telescope Array

The Cherenkov Telescope Array (CTA) is a project for a next-generation observatory for very high energy (GeV–TeV) ground-based gamma-ray astronomy, currently in its design phase, and foreseen to be operative a few years from now. Several tens of telescopes of 2–3 different sizes, distributed over a large area, will allow for a sensitivity about a factor 10 better than current instruments such as H.E.S.S, MAGIC and VERITAS, an energy coverage from a few tens of GeV to several tens of TeV, and a field of view of up to 10°. In the following study, we investigate the prospects for CTA to study several science questions that can profoundly influence our current knowledge of fundamental physics. Based on conservative assumptions for the performance of the different CTA telescope configurations currently under discussion, we employ a Monte Carlo based approach to evaluate the prospects for detection and characterisation of new physics with the array.First, we discuss CTA prospects for cold dark matter searches, following different observational strategies: in dwarf satellite galaxies of the Milky Way, which are virtually void of astrophysical background and have a relatively well known dark matter density; in the region close to the Galactic Centre, where the dark matter density is expected to be large while the astrophysical background due to the Galactic Centre can be excluded; and in clusters of galaxies, where the intrinsic flux may be boosted significantly by the large number of halo substructures. The possible search for spatial signatures, facilitated by the larger field of view of CTA, is also discussed. Next we consider searches for axion-like particles which, besides being possible candidates for dark matter may also explain the unexpectedly low absorption by extragalactic background light of gamma-rays from very distant blazars. We establish the axion mass range CTA could probe through observation of long-lasting flares in distant sources. Simulated light-curves of flaring sources are also used to determine the sensitivity to violations of Lorentz invariance by detection of the possible delay between the arrival times of photons at different energies. Finally, we mention searches for other exotic physics with CTA.     

Dark matter and imaging air Cherenkov arrays

The CTA will mean a significant increase of the potential for dark matter detection, compared to present-day detectors like MAGIC, HESS and VERITAS. In particular, if – as it might be indicated from early LHC results – the dark matter sector is heavy, perhaps in the TeV mass range, imaging air Cherenkov arrays have a good opportunity to detect γ-rays from dark matter annihilation in the galactic halo, the galactic center, dwarf galaxies, or galaxy clusters. A review of the present situation is given and a few of the “miracles” that may enhance chances for detection in CTA are discussed, such as Sommerfeld enhancement and internal bremsstrahlung radiation. A few templates for dark matter are studied, and the importance of the acceptance of the detector at low energies is pointed out. Finally, the idea of a complement to CTA in the form of a high-altitude, low energy threshold dedicated dark matter array, DMA, is discussed.     

Toward high spatial resolution in solar observations

To achieve high spatial resolution capability for IR solar observations, we suggest the use of a special mask placed before the entrance aperture of large conventional optical telescopes. Such a mask would match well the resolution of a non-redundant array and would provide safe operation of a large telescope used for solar observations. The proposed solution has the additional advantage of relatively low cost because already-existing optical telescopes are used.     

Radio Astronomy Antennas by the Thousands

Large number of microwave antennas of size and surface accuracy appropriate for the Square Kilometre Array (SKA) have not been manufactured previously. To minimize total cost, the design needs to be much more carefully considered and optimized than would be affordable for a small number of antennas. The required surface area requires new methods of manufacture and production-line type assembly to be considered. A blend of past antenna construction technology, creativity, and new technology is needed to provide the best possible telescope for the proposed SKA science goals. The following key concepts will be discussed with respect to reflector antennas and many supporting photographs, figures and drawings will be included.

Surface and supporting structure – comparison of panels with a one-piece shell as produced by hydroforming.

Combined reflector and mount geometry – performance/cost materially governed by this geometry which must be optimized for SKA requirements which are significantly different from typical communications antennas

Types of fully steerable mounts – king post, turntable bearing and wheel and track

Pointing accuracy – factors effecting cost, non-repeatable and repeatable errors

Axis drive concepts – traction devices, gears, screws, etc.

Life cycle costs – maintenance and power costs must be considered

Synergistic design – all of the above factors must be considered together with the wideband feed and receiver system to optimize the whole system

     

Concept of an all-directional high-energy cosmic gamma ray telescope

Point sources in the region of hard gamma rays 30 MeV-10 GeV were identified with single young pulsars, like Crab; transient gamma ray sources and bright quasars. The main problem in this region is the small number of the photons, registrated from the source. In order to overcome this problem the new construction of a gamma telescope is proposed, which registers quanta from all directions simultaneously. In this way it is possible to increase the sensitivity by more than an order of magnitude with the same size and smaller weight, in comparison with EGRET. Analyses have shown, that such a telescope must stay without the calorimeter, what reduces its spectral resolution. The estimated number of point sources which could be discovered by such a telescope could approach one thousand. Results of Monte Carlo simulations are presented.     

射电天文中焦面阵或多波束馈源的应用

焦面阵技术或者多波束馈源系统已经日益广泛地应用于现代射电望远镜，因为它可以充分地利用同一射电望远镜反射面所能提供的信息，在观测比射电望远镜方向瓣大得多的展源时数倍乃至数十倍地提高观测的速度；当存在大气层或电离层的起伏或不均匀影响观测成像质量时，可消除这种影响，提高观测质量；利用焦面阵各单个馈源接收到的信息的互相关，则可以实时监控射电望远镜的反射面、二次反射面、指向精度，从而降低地面上大射电望远镜或空间射电望远镜的精度要求和造价。目前焦面阵已经愈来愈广泛地配置在毫米波射电望远镜和大型射电望远镜的主要波段。对此作了一个较新和全面的评述，对焦面阵应用中的限制，包括相位误差的限制和性能价格比的考虑和可能的前景作了简要的介绍。还分析了在计划中的大型主动球反射面射电望远镜（即FAST）上，配置焦面阵的相应限制、问题和难点，提出了初步的建议，并给出经中英双方讨论后初步拟定的FAST频段、波束及低噪声放大器的配置。     

Binaries with the eyes of CTA

The binary systems that have been detected in gamma rays have proven very useful to study high-energy processes, in particular particle acceleration, emission and radiation reprocessing, and the dynamics of the underlying magnetized flows. Binary systems, either detected or potential gamma-ray emitters, can be grouped in different subclasses depending on the nature of the binary components or the origin of the particle acceleration: the interaction of the winds of either a pulsar and a massive star or two massive stars; accretion onto a compact object and jet formation; and interaction of a relativistic outflow with the external medium. We evaluate the potentialities of an instrument like the Cherenkov telescope array (CTA) to study the non-thermal physics of gamma-ray binaries, which requires the observation of high-energy phenomena at different time and spatial scales. We analyze the capability of CTA, under different configurations, to probe the spectral, temporal and spatial behavior of gamma-ray binaries in the context of the known or expected physics of these sources. CTA will be able to probe with high spectral, temporal and spatial resolution the physical processes behind the gamma-ray emission in binaries, significantly increasing as well the number of known sources. This will allow the derivation of information on the particle acceleration and emission sites qualitatively better than what is currently available.     

The ARIEL Instrument Control Unit design

In this paper we present the electromagnetic modeling and beam pattern measurements of a 16-elements ultra wideband sparse random test array for the low frequency instrument of the Square Kilometer Array telescope. We discuss the importance of a small array test platform for the development of technologies and techniques towards the final telescope, highlighting the most relevant aspects of its design. We also describe the electromagnetic simulations and modeling work as well as the embedded-element and array pattern measurements using an Unmanned Aerial Vehicle system. The latter are helpful both for the validation of the models and the design as well as for the future instrumental calibration of the telescope thanks to the stable, accurate and strong radio frequency signal transmitted by the UAV. At this stage of the design, these measurements have shown a general agreement between experimental results and numerical data and have revealed the localized effect of un-calibrated cable lengths in the inner side-lobes of the array pattern.     

Scaling radio astronomy signal correlation on heterogeneous supercomputers using variousdata distribution methodologies

Next generation radio telescopes will require orders of magnitude more computing power to provide a view of the universe with greater sensitivity. In the initial stages of the signal processing flow of a radio telescope, signal correlation is one of the largest challenges in terms of handling huge data throughput and intensive computations. We implemented a GPU cluster based software correlator with various data distribution models and give a systematic comparison based on testing results obtained using the Fornax supercomputer. By analyzing the scalability and throughput of each model, optimal approaches are identified across a wide range of problem sizes, covering the scale of next generation telescopes.     

Influence of the geomagnetic field on the IACT detection technique for possible sites of CTA observatories

We investigate the influence of the geomagnetic field (GF) on the Imaging Air Cherenkov Telescope technique for two northern (Tenerife and San Pedro Martir) and three southern (Salta, Leoncito and Namibia (the H.E.S.S.-site)) site candidates for Cherenkov Telescope Array (CTA) observatories. We use the CORSIKA and sim_telarray programs for Monte Carlo simulations of gamma ray showers, hadronic background and the telescope response. We focus here on gamma ray measurements in the low energy, sub-100 GeV, range. Therefore, we only consider the performance of arrays of several large telescopes. Neglecting the GF effect, we find (in agreement with previous studies) that such arrays have lower energy thresholds, and larger collection areas below 30 GeV, when located at higher altitudes. We point out, however, that in the considered ranges of altitudes and magnetic field intensities, 1800–3600 m a.s.l. and 0–40 μT, respectively, the GF effect has a similar magnitude to this altitude effect. We provide the trigger-level performance parameters of the observatory affected by the GF effect, in particular the collection areas, detection rates and the energy thresholds for all five locations, which information may be useful in the selection of sites for CTA. We also find simple scaling of these parameters with the magnetic field strength, which can be used to assess the magnitude of the GF effect for other sites; in this work we use them to estimate the performance parameters for five sites: South Africa-Beaufort West, USA-Yavapai Ranch, Namibia-Calapanzi, Chile-La Silla and India-Hanle. We roughly investigate the impact of the geophysical conditions on gamma/hadron separation procedures involving image shape and direction cuts. We note that the change of altitude has an opposite effect at the trigger and analysis levels, i.e. gains in triggering efficiency at higher altitudes are partially balanced by losses in the separation efficiency. In turn, a stronger GF spoils both the shape and the direction discrimination of gamma rays, thus its effects at the trigger and analysis levels add up resulting in a significant reduction of the observatory performance. Overall, our results indicate that the local GF strength at a site can be equally important as its altitude for the low-energy performance of CTA.     

Chromospheric telescope modification for digital imaging

The optical and technical characteristics of the tie-in between the chromospheric AFR-2 telescope and the digital camera Sony DSC-S85 are determined by computer simulation. The optimal layout of the camera, telescope optics, and collective lens is established. The modulation transfer function is used as an optimization parameter (quality criterion for the solar disk image in H_α).