The metal quadrupod responsible for supporting and positioning the 2-metric-ton camera of the mid-sized telescopes in the Cherenkov Telescope Array (CTA), an international initiative to build the largest gamma-ray observatory by 2020, will be manufactured in accordance with the specifications of a prototype designed in the state of São Paulo. Developed during the last four years by Luiz Vitor de Souza Filho’s team at the Physics Institute of the University of São Paulo, São Carlos (IFSC-USP), together with the Federal University of the ABC (UFABC) and a company in São José dos Campos, Orbital Engenharia, the mechanical support made in Brazil nudged out a French rival that had developed a similar structure. The team’s prototype was chosen to equip 40 telescopes that will be assembled and operated by the observatory. “Our prototype met all the requirements,” states Souza Filho, who received two grants from FAPESP for the design and construction of the metal quadrupod.
Comprising 84 distinct parts, the tubular structure is made of carbon steel, weighs 4.5 metric tons and is 16 meters (m) high. In November 2014, it was sent to Germany, unassembled, to be installed together with the other parts of the prototype 12-m-diameter telescope and underwent tests. The support proved to be extremely stable, as required of a component whose main function is to connect the mirror’s base to the telescope’s camera and maintain the correct alignment of these two parts. “It works well,” says particle physicist Stefan Schlenstedt, of Deutsches Elektronen-Synchrotron (Desy), a research center near Berlin responsible for developing CTA’s mid-sized telescopes. “We hope that Brazil can provide at least 25 of these structures.” The others, even if built outside Brazil or with funding from other CTA partners, should use the same design and have the same quality as the prototype made in São Paulo state, according Schlenstedt. The manufacturing cost of the mechanical quadrupod developed by IFSC and Orbital, a company dedicated primarily to building solar panels for satellites, is about R$500,000 each. The project to develop the prototype for the telescope camera support structure was the subject of a patent application by its developers.
With a total budget estimated at €270 million, CTA is an international consortium made up of 1,200 scientists and engineers from 170 research institutions and 31 countries. Its purpose is to install about 120 Cherenkov-type telescopes, ideal for gamma-ray observations of the most energetic phenomena in the universe, such as the collision of dark matter particles, the nature of events capable of accelerating cosmic rays (such as collisions between stars and the action of supermassive black holes) and occurrences that violate the constancy of the speed of light. There will be three telescope sizes: the largest will measure 24 m, the mid-sized ones will measure 12 m and the smallest will measure 4 m. Approximately 70 telescopes will be of the smallest size and 40 will be mid-sized. There will probably not be more than a dozen of the largest.
Each type of telescope observes events in different energy ranges, from a few dozen gigaelectron-volts (GeV) to 100 teraelectron-volts (TeV), and has a different field of view. “The higher energies are captured by the small telescopes and the lower energies by the big telescopes,” explains Souza Filho. When they are 100% operational, CTA’s observation instruments should increase the capacity of physicists and astrophysicists to record high-energy events in gamma-ray frequencies tenfold, argue the consortium’s supporters.
Sites in Atacama and La Palma
The CTA telescopes will be located at two astronomical sites, one in the Southern hemisphere and the other in the Northern. In July 2015, the international consortium decided on potential sites. A set or array of 100 telescopes (including 25 mid-sized ones) will be installed in Paranal, in the Atacama Desert in Chile, where one of the European Southern Observatory (ESO) units is located. A second, smaller set with 20 telescopes (15 of them mid-sized) will be erected in the Roque de los Muchachos Observatory in La Palma, a popular site for astronomical observations in the Canary Islands, Spain, located 2,400 meters above sea level.
The large tubular structure is not impressive, due to its simple appearance. A layman would say that it appears to be made of two giant tripods with some extra features. The lower part has six legs—four thick and two thin—that fit into the panel where the telescope mirror is located. The upper part has 10 supports—four main ones and six auxiliary ones—that attach to the camera or to the camera support. “It is not a conventional structure, easy to design and manufacture,” explains mechanical engineer Celio Costa Vaz, president of Orbital, who directly participated in developing the prototype. “It had to satisfy a series of specific prerequisites.”
The most draconian of the requirements was with respect to the stability and precision of the mechanical quadrupod after being installed on the telescope. The structure had to support a camera weighing 2 metric tons without deforming more than 19 millimeters (mm), be resistant to snow and withstand winds of up to 200 kilometers per hour. When the telescope is in operation, it cannot vibrate more than 3mm. “The telescope rotates to any position within 50 seconds and the camera support structure must remain stable,” comments Ronald Cintra Shellard, a researcher in high-energy experimental physics at the Brazilian Center for Physics Research (CBPF) in Rio de Janeiro, which represents Brazil on the CTA finance board.
A solid, closed structure would be easier to design, but it would limit the telescope mirror’s field of view. This led the USP researchers and Orbital engineers to decide to develop a hollow, tubular mechanical quadrupod. “This way, the shadow generated by the structure affects less than 10% of the mirror’s area,” says physicist Marcelo Leigui, of UFABC, another researcher involved in the project.
Plug and play
Other smaller requirements also guided the design and manufacturing work for the mechanical quadrupod. The CTA telescopes will be installed at two sites that are relatively difficult to reach. In this context, the camera support structure needed to be easy to transport and assemble, not need constant maintenance, and have a service life of at least two decades. “This is why we created a plug-and-play type of structure,” says Vaz, from Orbital, jokingly. The tubular support parts are assembled using 584 screws. The carbon steel parts are hot-dip galvanized, a technique that insures protection even on the internal surfaces of the tubes. If the initial schedule is maintained, the mechanical quadrupod prototype for 12-m telescopes developed in São Carlos and São José dos Campos will probably be sent to the southern CTA site in the Atacama Desert in the second half of 2016.
According to physicist Souza Filho, of IFSC, it was not easy to find a private partner in Brazil willing to participate in the mechanical quadrupod development process. “Companies other than Orbital have the technical capacity to develop a design with these characteristics,” affirms the researcher from USP São Carlos. “But most of them cannot adapt their production lines, dedicated to the manufacture of commercial products, to invest in developing a structure for a very specific purpose.” In his contacts with engineering firms, Souza Filho recounts that he met with a company that, like Orbital, was capable of designing and building the mechanical quadrupod for the CTA telescope. There was only one problem: its production line was almost completely dedicated to manufacturing parabolic antennas, a market with strong demand in Brazil.
In addition to having designed and manufactured the prototype for the mechanical quadrupod to be used with the 12-m CTA telescopes, Brazil is also working on projects for the small and large telescopes for the future gamma-ray observatory. Researchers from CBPF and the Federal University of Rio de Janeiro (UFRJ) are working on developing and building interface boards for the mechanical devices that align the 24-m telescope mirrors. Astrophysicist Elisabete de Gouveia Dal Pino, of the USP Institute of Astronomy, Geophysics and Atmospheric Sciences (IAG), coordinates the Brazilian contribution to the Astri Mini-Array, an initiative that provides for the installation of nine 4-m telescopes at the CTA site in Paranal, Chile, in 2016–2017. At a cost of about €3 million, FAPESP is paying for the manufacture of three small telescopes in Italy for this first CTA array, based on a prototype developed by the Italian National Institute of Astrophysics with contributions from Brazilian engineers.
1. Design and construction of a prototype for the Cherenkov Telescope Array Observatory (nº 2010/19514-6); Grant Mechanism Regular Research Grant; Principal Investigator Luiz Vitor de Souza Filho (IFSC-USP); Investment R$ 128,568.00.
2. Design and construction of prototype for a CTA telescope structure (nº 2012/22540-4); Grant Mechanism Regular Research Grant; Principal Investigator Luiz Vitor de Souza Filho (IFSC-USP); Investment R$ 406,449.00.