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The prototype 4-meter telescope developed by the ASTRI/CTA project. It is located at the observing station operated by INAF Catania Astrophysical Observatory, at Serra La Nave, Mount Etna, where it was inaugurated in September 2014
The ASTRI project, led by the Italian National Institute of Astrophysics (INAF), produced the first prototype of a Small Size Telescope (SST) in 2014, now undergoing testing at the Serra La Nave observing station on Mount Etna in Sicily.
On 16th October 2016, the ASTRI telescope prototype, a proposed Small-Size Telescope for the Cherenkov Telescope Array, passed its severest test: demonstrating a constant angular resolution of a few arcmin (as needed for astronomy with Cherenkov telescopes) over a field of view of ten degrees.
Cherenkov Telescope Array (CTA, for short) is the next-generation ground-based gamma-ray observatory. CTA will have, among its other objectives, the detection of the very highest energy radiation known in the Universe, namely, gamma rays with energies as high as 100 Tera Electron Volts (TeV). To successfully observe these rather elusive cosmic messengers, many relatively Small-Size Telescopes (SSTs), like ASTRI prototype, with a diameter of 4 m, each with good resolution across a wide field of view, will be required.
In the picture the three classes of telescopes to be used in the CTA project can be seen. An array based on a large number of telescopes of different sizes is needed in order to cover the entire energy range of gamma-rays (20 GeV to 300 TeV) that produce Cherenkov light when they strike the Earth’s atmosphere. As many as 8 Large-Size telescopes (LSTs) of 23 m diameter, 40 Medium-Size Telescopes (MSTs) of 12 m diameter, and 70 Small-Size Telescopes (SSTs) of 4 m diameter are planned to be installed at the two CTA sites. The ASTRI prototype represents a solution for the SSTs to be installed at the southern site of CTA. Credit: G. Pérez, IAC, SMM
The technical idea is not new. In 1905, German astrophysicist Karl Schwarzschild proposed a design for a two-mirror telescope intended to eliminate much of the optical aberration across the field of view. This idea, enhanced in 1926 by André Couder, lay dormant for almost a century, consigned to suspended animation in specialized journals and texts on astrophysical optics, and considered too difficult and expensive to build.
Much has changed in a hundred years, however: progress in mirror-making technology has made Schwarzschild-Couder telescopes practical and Italian scientists and industry, along with their international collaborators in Brazil, South Africa and Germany, have made them real.
Two weeks ago, the INAF astronomers involved in the ASTRI project presented their results at the CTA Consortium General Meeting at the site of the future headquarters of CTA in Bologna. They hope that ASTRI’s demonstrated success will lead to a sub-array of tens of dual-mirror telescopes of SST class at the southern observing site of CTA in Chile, with a precursor mini-array of about a dozen ASTRI clones installed there by 2018.
The picture shows the result obtained with the ASTRI telescope prototype. Polaris, the North Star, has been observed with different offsets from the optical axis of the telescope. For a Cherenkov telescope it is not necessary to have very fine imaging capability, in contrast to conventional optical telescopes. It should instead be able to see "reasonably well" across a wide field of view (much wider than that needed for optical astronomy), in order to determine the morphology of the showers due to the cosmic rays responsible for the Cherenkov light emission. With this information, one can correctly reconstruct the direction of gamma-ray photons emitted from celestial sources. The recorded images have the same angular size, each one from a different observational direction in the field of view (from 0 to 4.5 degrees from each side with respect to the central optical axis). Credits: Enrico Giro, Rodolfo Canestrari, Salvo Scuderi and Giorgia Sironi, INAF Padova, Brera and Catania
The ASTRI project, funded by the Italian Ministry of University and Research (MIUR) and by further government funding allocated under the Finance Act of 2015, is led by INAF with the collaboration of a number of Italian Universities, of the Universidade de São Paulo (USP, Brazil) and the North-West University (South Africa), the Italian National Institute of Nuclear Physics (INFN) and several industrial partners, in particular EIE, Galbiati Group, Media Lario, Zaot (Italy), Tomelleri srl, BCV Progetti and Flabeg FE GMBH (Germany). INFN, the Italian National Institute for Nuclear Physics (http://home.infn.it/en/), is studying the triggering scheme of the array for the ASTRI telescopes but is mainly involved in developing innovative light sensors for CTA’s Large- and Medium-Size Telescope projects .
The CTA project, an international collaboration among scientific institutes from 32 countries, is building an observatory of more than a hundred telescopes to be installed at two observing sites, one in the northern, and one in the southern hemisphere. CTA will be able to study the sky in the high-energy gamma-ray range with a sensitivity approximately ten times greater than that of existing Cherenkov experiments. The southern site of the observatory will be located in the Atacama Desert of Chile, near ESO’s world-class, state-of-the-art optical observatories, the VLT, and the E-ELT, while the northern site will be located in La Palma, Canary Islands.
We asked Giovanni Pareschi, Astronomer at the INAF- Brera Astronomical Observatory and Principal Investigator of the ASTRI project, what are the most innovative aspects of the telescope prototype:
"The telescope was built and completed for the first time ever adopting the Schwarzschild-Couder configuration. This kind of telescope has never been realized until now, mainly due to technological difficulties. However, recent advances in technology (in particular for the realization of the primary and secondary mirrors) have made the implementation of this design practicable (thanks also to the study by Prof. Vladimir Vassiliev at UCLA) for the observation of Cherenkov light emitted by the atmospheric showers generated by cosmic gamma rays. It is also the first time that a Cherenkov telescope with two focusing mirrors has been completely characterized from the opto-mechanical point of view. It is an important result because it allows us to move immediately to the next step: to mount a Cherenkov camera by December 2016 aiming to observe the first gamma-ray light with the ASTRI dual-mirror telescope. “
This press release was issued by the Italian National Institute for Astrophysics (INAF) jointly with CTA Observatory gGmbH (CTAO gGmbH).
The Italian version is available at: http://www.media.inaf.it/2016/11/11/telescopio-cta-ci-vede-benissimo/
Gamma ray astronomy with ground-based Cherenkov Telescopes
The Earth's atmosphere protects us from harmful cosmic radiation but, at the same time, keeps us from directly observing it. On the other hand, it has been understood that the atmosphere can be used as a detector for high-energy gamma rays. For photons that are even ten trillion times more energetic than those of visible light, the reduced size of the detectors mounted on satellites, together with an important reduction of the signal intensity as the photon energy increases, makes it very difficult to measure gamma-rays directly. However, it becomes possible to detect gamma rays using ground-based telescopes through the effects of their interaction with the atmosphere. This interaction produces a cascade of secondary charged particles, mainly electrons and positrons, which propagate at a speed higher than that of light in the atmosphere. It should be noted that this does not violate at all the special theory of relativity. In fact, in a medium such as the air, the light propagates more slowly than in the vacuum. However in a medium like the atmosphere (or water) the speed of a particle with a mass, while it is not able reach the speed of light in vacuum, will instead get close to it indefinitely with increasing energy and, in certain conditions, is larger the speed of the same photons. When this phenomenon occurs (similar, in some aspects, to the supersonic bang) a flash of bluish light is emitted as a secondary effect. This emission is called Cherenkov light, in honor of the Russian physicist who first observed and correctly interpreted this phenomenon, receiving the Nobel Prize in 1958 for this discovery.
INAF involvement in CTA
Rendering of the mini-array in the Atacama Desert based on nine replicas of the ASTRI telescope prototype. Credits: Antonio Stamerra
INAF, the National Institute of Astrophysics, leads the Italian participation in the realization of CTA with a contribution exceeded only by Germany. The INAF institutions directly involved in the project are the Brera Observatory, Catania, Padova, Capodimonte (Naples), Rome and Turin, the IASF institutions in Bologna, Milan, Palermo and Rome IAPS. Also several Italian universities are involved, including a major contribution from the University of Perugia in collaboration with the Universities of Genova, Padova and the Politecnico University of Milan. Even the National Institute of Nuclear Physics INFN is collaborating in this project.
INAF and its Italian partners will not be alone in this challenge but are working in synergy with the CTA Observatory and CTA Consortium and with the support of the University of São Paulo and FAPESP (Brazil), with the North-West University (South Africa).
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In October 2016, the ASTRI telescope prototype (pictured below), a novel dual-mirror Schwarzschild-Couder telescope design proposed for the Cherenkov Telescope Array (CTA), passed its biggest test yet by demonstrating a constant point-spread function of a few arc minutes over a large field of view of 10 degrees.
Three classes of telescope types are required to cover the full CTA very-high energy range (20 GeV to 300 TeV): Medium-Size Telescopes will cover CTA’s core energy range (100 GeV to 10 TeV) while the Large-Size Telescopes and Small-Size Telescopes (SSTs) are planned to extend the energy range below 100 GeV and above a few TeV, respectively.
The ASTRI telescope is one of three proposed SST designs being prototyped and tested for CTA’s southern hemisphere array. The ASTRI telescope uses an innovative dual-mirror Schwarzschild-Couder configuration with a 4.3 m diameter primary mirror and a 1.8 m monolithic secondary mirror. In 1905, the German physicist and astronomer Karl Schwarzschild proposed a design for a two-mirror telescope intended to eliminate much of the optical aberration across the field of view. This idea, enhanced in 1926 by André Couder, lay dormant for almost a century because it was considered too difficult and expensive to build. It was in 2007 that a study by Vladimir Vassiliev and colleagues at the University of California Los Angeles (UCLA) demonstrated the design’s usefulness for atmospheric Cherenkov telescopes.
The ASTRI prototype, the first Schwarzschild-Couder telescope to be built and tested, was inaugurated in September 2014 and has been undergoing testing at the Serra La Nave observing station on Mount Etna in Sicily ever since. The technical challenges of the design were overcome by recent advances, particularly in dual-mirror technology, making it a feasible implementation for the observation of Cherenkov light.
Pictured below, Polaris, the North Star, as observed by ASTRI with different offsets from the optical axis of the telescope. The recorded images have approximately the same angular size, each one from a different observational direction in the field of view (from 0 to 4.5 degrees from each side with respect to the central optical axis). These images show that the optical point-spread function of the telescope is approximately constant across the full field of view. This information will allow scientists to reconstruct the direction of gamma-ray photons emitted from celestial sources.
Image credits: Enrico Giro, Rodolfo Canestrari, Salvo Scuderi and Giorgia Sironi, INAF Padova, Brera and Catania.
“This is also the first time that a Cherenkov telescope with two focusing mirrors has been completely characterized from the opto-mechanical point of view,” said Giovanni Pareschi, astronomer at the INAF-Brera Astronomical Observatory and principal investigator of the ASTRI project. “This is an important result because it allows us to move immediately to the next step: to mount a Cherenkov camera by December 2016 with the aim to observe the first gamma-ray light with ASTRI.”
The ASTRI project is led by the Italian National Institute of Astrophysics (INAF) with the collaboration of a number of Italian universities, the Italian National Institute of Nuclear Physics (INFN), Universidade de São Paulo in Brazil and North-West University in South Africa.
The SSTs will outnumber all the other telescopes with 70 planned to be spread out over several square kilometres in the southern hemisphere array. Since very high-energy gamma-ray showers (between a few TeV and 300 TeV) produce a large amount of Cherenkov light, it is sufficient to build telescopes with small mirrors to catch that light. The SSTs’ wide coverage and large number, spread over a large area, will improve CTA’s chances of detecting the highest energy gamma rays. The Schwarzschild-Couder design is being used in two additional CTA prototypes (the SST-2M GCT and the SCT), but the ASTRI is the first to conclusively demonstrate the viability of the system.
For more information, including Italian language content, go to: http://www.inaf.it/en/inaf-news/astri-telescope-2020-vision.