The Cherenkov Telescope Array (CTA) is the major ground-based gamma-ray observatory under construction. The CTA South observatory is foreseen to consist of Large-, Medium-, and Small-sized imaging atmospheric Cherenkov telescopes (IACTs). The innovative Schwarzschild-Couder Telescope (SCT) is a candidate IACT and a proposed major U.S. contribution for the Medium-sized, 10m aperture telescopes for CTA. The SCT is designed to simultaneously achieve 8 degrees field of view and high imaging resolution with unprecedented 11,328 pixels camera by implementing novel, aplanatic, segmented dual-mirror optics and compact silicon photomultiplier detectors. This presentation will provide an overview of the SCT program in the U.S. including the construction of a full-scale prototype instrument by an international consortium of scientists with the focus on the alignment of the segmented primary and secondary mirrors and the ongoing upgrade of the camera to full scale.
The VERITAS Imaging Air Cherenkov Telescope array (IACT) was augmented in 2019 with high-speed focal plane electronics to create a new Stellar Intensity Interferometry (SII) observational capability (VERITAS-SII, or VSII). VSII operates during bright moon periods, providing high angular resolution observations ( < 1 mas) in the B photometric band using idle telescope time. VSII has already demonstrated the ability to measure the diameters of two B stars at 416 nm (Bet CMa and Eps Ori) with < 5% accuracy using relatively short (5 hours) exposures.1 The VSII instrumentation was recently improved to increase instrumental sensitivity and observational efficiency. This paper describes the upgraded VSII instrumentation and documents the ongoing improvements in VSII sensitivity. The report describes VSII’s progress in extending SII measurements to dimmer magnitude stars and improving the VSII angular diameter measurement resolution to better than 1%.
The prototype Schwarzschild-Couder Telescope (pSCT) is a candidate for a medium-sized telescope in the Cherenkov Telescope Array. The pSCT is based on a dual-mirror optics design that reduces the plate scale and allows for the use of silicon photomultipliers as photodetectors. The prototype pSCT camera currently has only the central sector instrumented with 25 camera modules (1600 pixels), providing a 2.68-deg field of view (FoV). The camera electronics are based on custom TARGET (TeV array readout with GSa/s sampling and event trigger) application-specific integrated circuits. Field programmable gate arrays sample incoming signals at a gigasample per second. A single backplane provides camera-wide triggers. An upgrade of the pSCT camera that will fully populate the focal plane is in progress. This will increase the number of pixels to 11,328, the number of backplanes to 9, and the FoV to 8.04 deg. Here, we give a detailed description of the pSCT camera, including the basic concept, mechanical design, detectors, electronics, current status, and first light.
The Cherenkov Telescope Array (CTA) is the next-generation ground-based observatory for very-high-energy gamma rays. One candidate design for CTA's medium-sized telescopes consists of the Schwarzschild-Couder Telescope (SCT), featuring innovative dual-mirror optics. The SCT project has built and is currently operating a 9.7-m prototype SCT (pSCT) at the Fred Lawrence Whipple Observatory (FLWO); such optical design enables the use of a compact camera with state-of-the art silicon photomultiplier detectors. A partially-equipped camera has recently successfully detected the Crab Nebula with a statistical significance of 8.6 standard deviations. A funded upgrade of the pSCT focal plane sensors and electronics is currently ongoing, which will bring the total number of channels from 1600 to 11328 and the telescope field of view from about 2.7° to 8° . In this work, we will describe the technical and scientific performance of the pSCT.
The novel 9.7m Schwarzschild-Couder Telescope (SCT), utilizing aspheric dual-mirror optical system, has been constructed as a prototype medium size x-ray telescope for the Cherenkov Telescope Array (CTA) observatory. The prototype SCT (pSCT) is designed to achieve simultaneously the wide (≥ 8°) field of view and the superior imaging resolution (0.067 per pixel) to significantly improve scientific capabilities of the observatory in conducting the sky surveys, the follow-up observations of multi-messenger transients with poorly known initial localization and the morphology studies of x-ray sources with angular extent. In this submission, we describe the hardware and software implementations of the telescope optical system as well as the methods specifically developed to align its complex optical system, in which both primary and secondary mirrors are segmented. The pSCT has detected Crab Nebula in June 2020 during ongoing commissioning, which was delayed due to worldwide pandemic and is not yet completed. Verification of pSCT performance is continuing and further improvement of optical alignment is anticipated.
Over recent years, several independent groups have pursued the realization of a modern stellar intensity interferometry (SII) system to perform high angular resolution observations at optical wavelengths. Here, we present a general purpose SII observation planner (ASIIP) that can be used to aid in SII observational efforts. ASIIP can be used to coordinate and prioritize SII observations based on observational and instrumental parameters. ASIIP constructs a master catalog by gathering information from several stellar catalogs, and targets within the master catalog are ranked based on the ability to make stellar diameter estimates using a Monte Carlo analysis. The Monte Carlo analysis takes into account the estimated angular diameter, apparent brightness, a target’s uv-plane baseline coverage for a given observation, and instrumental sensitivity.
For the first time in the history of ground-based y-ray astronomy, the on-axis performance of the dual mirror, aspheric, aplanatic Schwarzschild-Couder optical system has been demonstrated in a 9:7-m aperture imaging atmospheric Cherenkov telescope. The novel design of the prototype Schwarzschild-Couder Telescope (pSCT) is motivated by the need of the next-generation Cherenkov Telescope Array (CTA) observatory to have the ability to perform wide (≥8°) field-of-view observations simultaneously with superior imaging of atmospheric cascades (resolution of 0:067 per pixel or better). The pSCT design, if implemented in the CTA installation, has the potential to improve significantly both the x-ray angular resolution and the off-axis sensitivity of the observatory, reaching nearly the theoretical limit of the technique and thereby making a major impact on the CTA observatory sky survey programs, follow-up observations of multi-messenger transients with poorly known initial localization, as well as on the spatially resolved spectroscopic studies of extended x-ray sources. This contribution reports on the initial alignment procedures and point-spread-function results for the challenging segmented aspheric primary and secondary mirrors of the pSCT.
The first prototype of the Schwarzschild Couder Medium Size Telescope (pSCT) proposed for the CTA observatory has been installed in 2018 at the Fred Lawrence Whipple Observatory. The pSCT camera is composed of 25 modules with 64 channels each, covering only a small portion of the full focal plane of the telescope. The Italian Institute of Nuclear Physics (INFN) has developed and characterized in collaboration with Fondazione Bruno Kessler (FBK) a new generation of Silicon Photomultipliers (SiPMs) sensitive to the Near Ultraviolet wavelengths, based on the High Density technology (NUV-HD devices). The latest generation of 6×6 mm2 SiPMs (called NUV-HD3) have been used to equip a subsection of 9 out of 25 modules of the pSCT camera. An upgrade of this camera is foreseen between 2019 and 2020 using the same sensors, aiming to equip the full focal plane with 177 modules, for a total of more than 11000 pixels. We will present a full characterization of the performance of these devices, highlighting why they are suitable for Cherenkov light detection. An overview on the overall behavior of the installed sensors will be also given, providing information on the uniformity of the sensors and of the performance of the camera.
A modern implementation of a stellar intensity interferometry (SII) system on an array of large optical telescopes would be a highly valuable complement to the current generation of optical amplitude interferometers. The SII technique allows for observations at short optical wavelengths (U/B/V bands) with potentially dense (u,v) plane coverage. We describe a complete SII system that is used to measure the spatial coherence of a laboratory source which exhibits signal to noise ratios comparable to actual stellar sources. A novel analysis method, based on the correlation measurements between orthogonal polarization states, was developed to remove unwanted effects of spurious correlations. Our system is currently being tested in night sky observations at the StarBase Observatory (Grantsville, Utah) and will soon be ported to the VERITAS (Amado, AZ) telescopes. The system can readily be integrated with current optical telescopes at minimal cost. The work here serves as a technological pathfinder for implementing SII on the future Cherenkov Telescope Array.
The construction of a prototype Schwarzschild-Couder telescope (pSCT) started in early June 2015 at the Fred Lawrence Whipple Observatory in Southern Arizona, as a candidate medium-sized telescope for the Cherenkov Telescope Array (CTA). Compared to current Davies-Cotton telescopes, this novel instrument with an aplanatic two-mirror optical system will offer a wider field-of-view and improved angular resolution. In addition, the reduced plate scale of the camera allows the use of highly-integrated photon detectors such as silicon photo multipliers. As part of CTA, this design has the potential to greatly improve the performance of the next generation ground-based observatory for very high-energy (E>60 GeV) gamma-ray astronomy. In this contribution we present the design and performance of both optical and alignment systems of the pSCT.
Stephan LeBohec, Ben Adams, Isobel Bond, Stella Bradbury, Dainis Dravins, Hannes Jensen, David Kieda, Derrick Kress, Edward Munford, Paul Nuñez, Ryan Price, Erez Ribak, Joachim Rose, Harold Simpson, Jeremy Smith
Experiments are in progress to prepare for intensity interferometry with arrays of air Cherenkov
telescopes. At the Bonneville Seabase site, near Salt Lake City, a testbed observatory has been set
up with two 3-m air Cherenkov telescopes on a 23-m baseline. Cameras are being constructed, with
control electronics for either off- or online analysis of the data. At the Lund Observatory (Sweden),
in Technion (Israel) and at the University of Utah (USA), laboratory intensity interferometers simulating stellar observations have been set up and experiments are in progress, using various analog and digital correlators, reaching 1.4 ns time resolution, to analyze signals from pairs of laboratory telescopes.
Sub milli-arcsecond imaging in the visible band will provide a new perspective in stellar astrophysics. Even
though stellar intensity interferometry was abandoned more than 40 years ago, it is capable of imaging and
thus accomplishing more than the measurement of stellar diameters as was previously thought. Various phase
retrieval techniques can be used to reconstruct actual images provided a sufficient coverage of the interferometric
plane is available. Planned large arrays of Air Cherenkov telescopes will provide thousands of simultaneously
available baselines ranging from a few tens of meters to over a kilometer, thus making imaging possible with
unprecedented angular resolution. Here we investigate the imaging capabilities of arrays such as CTA or AGIS
used as Stellar Intensity Interferometry receivers. The study makes use of simulated data as could realistically
be obtained from these arrays. A Cauchy-Riemann based phase recovery allows the reconstruction of images
which can be compared to the pristine image for which the data were simulated. This is first done for uniform
disk stars with different radii and corresponding to various exposure times, and we find that the uncertainty
in reconstructing radii is a few percent after a few hours of exposure time. Finally, more complex images are
considered, showing that imaging at the sub-milli-arc-second scale is possible.
Building on technological developments over the last 35 years, intensity interferometry now appears a feasible option by which to achieve diffraction-limited imaging over a square-kilometer synthetic aperture. Upcoming Atmospheric Cherenkov Telescope projects will consist of up to 100 telescopes, each with ~100m2 of light gathering area, and distributed over ~1km2. These large facilities will offer thousands of baselines from 50m to more than 1km and an unprecedented (u,v) plane coverage. The revival of interest in Intensity Interferometry has recently led to the formation of a IAU working group. Here we report on various ongoing efforts towards implementing modern Stellar Intensity Interferometry.
The accurate determination of the elemental composition of cosmic rays at high energies is expected to provide crucial clues on the origin of these particles. Here we discuss a technique that has become possible through the use of modern ground-based Cherenkov imaging detectors. We combine a measurement of the Cherenkov light produced by the incoming cosmic-ray nucleus in the upper atmosphere with an estimate of the total nucleus energy produced by the extensive air shower initiated when the particle interacts deeper in the atmosphere. The emission regions prior to and after the first nuclear interaction can be separated by an imaging Cherenkov system with sufficient angular and temporal resolution. Monte Carlo simulations indicate a widely space array of 10m diameter imaging Cherenkov detectors should have charge resolution of ΔZ/Z <5% for incident iron nuclei in the region of the "knee" of the cosmic-ray energy spectrum. This technique also has the intriguing possibility to unambiguously discover nuclei heavier than iron at energies above 1014 eV. We describe a strawman detector design for a future observatory dedicated to high resolution cosmic ray measurements. This observatory can also serve as a wide field of view TeV gamma-ray survey instrument.
S. Barwick, James Beatty, David Besson, John Clem, Stephane Coutu, Michael DuVernois, Paul Evenson, Peter Gorham, Francis Halzen, Abram Jacobson, David Kieda, John Learned, Kurt Liewer, Stephen Lowe, Charles Naudet, Allen Odian, David Saltzberg, David Seckel
The ANITA project is designed to investigate ultra-high energy (>1017 eV) cosmic ray interactions throughout the universe by detecting the neutrinos created in those interactions. These high energy neutrinos are detectable through their interactions within the Antarctic ice sheet, which ANITA will use as a detector target that effectively converts the neutrino interactions to radio pulses. This paper will give an overview of the project including scientific objectives, detection description and mission design.
We will report on the details of the ANITA instrument. This instrument is fundamentally a broadband antenna, which is arrayed and constructed in such a way as to be optimized for the detection and characterization of high-energy neutrino cascades. The requirement to maximize the detector view of the Antarctic ice fields implies low gain antennas yet the need for maximum sensitivity dictates using the highest gain possible. Since the Cherenkov signal increases quadratically at higher frequencies suggesting that the optimal selection is an antenna with constant gain as a function of frequency. The baseline design will be a linearly polarized log-periodic zigzag (LPZZ) antenna.
David Kieda, S. Biller, Patrick Boyle, I. Bond, S. Bradbury, James Buckley, D. Carter-Lewis, Stephen Criswell, W. Cui, P. Dowknott, C. Duke, A. Falcone, D. Fegan, S. Fegan, John Finley, L. Fortson, J. Gaidos, S. Gammell, K. Gibbs, A. Hillas, J. Holder, D. Horan, M. Kertzman, J. Knapp, F. Krennrich, S. LeBohec, J. Lloyd-Evans, P. Moriarity, D. Moeller, P. Ogden, Rene Ong, D. Petry, J. Quinn, P. Reynolds, H. Rose, M. Schroedter, J. Smith, G. Sembrowski, Simon Swordy, V. Vassiliev, Scott Wakely, G. Walker, Trevor Weekes, J. Hall
The Very Energetic Radiation Imaging Telescope Array System (VERITAS) is an array of seven 10m aperture telescopes used for gamma-ray astronomy in the 50 GeV to 50 TeV (1 TeV= 1012 electron Volt) energy range. The gamma rays are detected by measuring the optical Cherenkov light emitted by the cascade of electromagnetic particles that is generated by interactions of the high energy gamma-ray with the Earth's Atmosphere. This paper describes the science goals of the VERITAS array, a description of the array, and expected performance of the instrument.
Lawrence Wiencke, Richard Gray, J. Reid Mumford, T. Abu-Zayyad, Y. Au, G. Archbold, K. Belov, J. Belz, D. Bergman, J. Boyer, Z. Cao, G. Chen, R. Clay, B. Dawson, B. Dieterle, B. Jones, C.C. Jui, D. Kieda, K. Kiyong, B. Knapp, W. Lee, Eugene Loh, E. Mannel, G. Martin, John Matthews, Joseph Meyer, K. Reil, J. Smith, Pierre Sokolsky, C. Song, W. Springer, Shanshan Taylor, Stan Thomas, T. Vanderveen, X. Zhang
Monitoring the aerosol component of the lowest 10 km of the atmosphere at UV wavelengths (300 - 400 nm) is an important part of the High Resolution Fly's Eye astrophysics experiment. Our method of atmospheric monitoring uses a frequency tripled YAG laser and a steering system that can point the beam anywhere in the sky. The same detector that measures scintillation light from high energy cosmic rays also measures light scattered from this laser system over a range of laser energies, geometries, and polarizations. This paper describes the technique, the laser system, and some recent measurements.
Lawrence Wiencke, D. Bird, G. Chen, R. Clay, H. Dai, B. Dawson, M. Huang, C.C. Jui, M. Kidd, D. Kieda, S. Ko, C. Larsen, Eugene Loh, J. Smith, Pierre Sokolsky, Paul Sommers, J. Tang, Stan Thomas, C. Wilkinson, S. Yoshida
The High Resolution Fly's Eye (HiRes) cosmic ray detector at Dugway Utah, measures UV scintillation light from extensive air showers. The detection technique is calorimetric in that the amount of light produced is proportional to the energy of the primary particle. Primary particle energies range from 1017 to more than 1020 electron volts. The detector can measure air showers more than 30 km away, a distance of several atmospheric extinction lengths. Variations in the atmosphere can cause significant variations in the amount of light reaching the detector. Atmospheric monitoring is extremely important. This paper discusses a method, under development, that uses the HiRes detector to measure light scattered from pulsed collimated xenon flashbulb sources (Flashers). Discussion includes a description of the light sources and a preliminary data analysis to extract a measurement of atmospheric extinction length and scale height.
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