CTA: Cherenkov Telescope Array

Manel Martínez

The Cherenkov Telescope Array (CTA) is the next generation ground-based observatory for gamma-ray astronomy at very-high energies. With more than 100 telescopes located in the northern (ORM Observatory, La Palma) and southern (ESO Paranal Observatory, Chile) hemispheres, CTA will be the world’s largest and most sensitive high-energy gamma-ray observatory. IFAE participates in CTA with high impact and visibly, keeping important responsibilities also at the management level.

Introduction

CTA will probe energies inaccessible to man-made accelerators like the Large Hadron Collider

The Cherenkov Telescope Array will look at the sky in higher energy photons than ever measured before. In fact, the cosmic particle accelerators CTA will probe can reach energies inaccessible to man-made accelerators like the Large Hadron Collider.

CTA’s unique capabilities will help us to address some of the most perplexing questions in astrophysics. CTA will seek to understand the impact of high-energy particles in the evolution of cosmic systems and to gain insight into the most extreme and unusual phenomena in the Universe. CTA will search for annihilating dark matter particles and deviations from Einstein’s theory of relativity and even conduct a census of particle acceleration in the Universe.

Figure 1: Artistic rendering of CTA’s northern hemisphere site located on the Observatorio del Roque de los Muchachos on the island of La Palma.

Progress of the CTA project in 2017

During 2017, besides key political decisions such as the choice of an ERIC (European Research Infrastructure Consortium) as the legal entity form for the CTA Observatory, and key scientific outcomes as the “Science with CTA” book currently in press, the main progress in the CTA project has been the completion and testing of the prototypes for the different telescope and camera types. Among them it was specially impressive the fast progress of the construction of the first LST prototype at La Palma, de-facto the first CTA telescope in a CTA observatory site. During 2017 the gamma ray group at IFAE was composed by about 15 physicists with similar proportion of senior scientists, postdocs and PhD students and shared its time between the CTA project and the MAGIC Telescopes. In addition about 10 engineers and technicians (software, mechanics and electronics) expended a good fraction of their time for the CTA project at IFAE.

During 2017 IFAE has had two representatives at the highest management level of the LST Collaboration

In 2017 IFAE has continued participating with high impact and visibly in CTA, keeping important responsibilities also at the management level: during 2017 IFAE has had two representatives at the highest management level of the LST Collaboration: J. Cortina has been the Co-PI of the project and M.Martinez has served as the Chair of its Steering Committee. Additionally O.Ballester has been the LST Systems Engineer, O.Blanch the Camera Coordinator, A.Moralejo the Software Coordinator and J.Cortina the Infrastructure Coordinator. All them have been members of the Executive Board of the LST project.

In addition in 2017 A. Moralejo was appointed CTA’s Deputy Coordinator of the Analysis and Simulations working group. M.Martinez continued as well as the leader of the 12 Spanish groups that presently constitute the CTA-Spain consortium.

####CTA activities at IFAE during 2017

During the last few years, IFAE has played an important role in the CTA Analysis and Simulations working group, with the development of an independent analysis chain within the framework of the MAGIC reconstruction software (MARS). The results from the simulations have been instrumental in the evaluation of the proposed locations for CTA north and south, and in the definition of the so-called “baseline designs” for the two arrays. In 2017 the IFAE group led the publication “Monte Carlo Performance Studies for the Site Selection of the Cherenkov Telescope Array” (Hassan et al, Astropart. Phys. 93), in which the main results used in the site selection process are presented.
In what concerns Data, continuing its participation in the European Commission H2020 project ASTERICS, IFAE has actively contributed to the development of the first open, high-level data format for Atmospheric Cherenkov telescopes data, allowing for the first time joint analysis between different IACT experiments.
As for Science, IFAE members have led the production of the CTA internal document “Conventions for CTA dark matter searches”, that will serve as the basis for dark matter prospect studies and data analyses.
IFAE is responsible for the integration of the first LST camera, which started during 2017. Several sub-systems were installed in the camera and validated. In particular the full power system, which has been developed at IFAE, is already operational inside the camera. In parallel, IFAE has produced the trigger decision system, which will be integrated in the camera during 2018, and developed the camera control software.
For the IFAE/UAB Raman LIDAR project, during the year 2017 the polychromator construction was completed and characterized in the lab with respect to its capability to separate the different wavelengths (355nm, 387nm, 532nm, 607nm). It was found to operate according to specifications, particularly light leakage from the elastic channels (532nm and 355nm) into the much dimmer Raman channels (387 nm and 607nm) could be excluded to at least less than one per mile.

The LST bogies

IFAE was responsible of designing and building the undercarriage of the Large Size Telescope.

During 2017 IFAE has been coordinating all infrastructure deployment at the ORM. We also produced a preliminary design for the foundations of the next three LSTs at ORM, with an eye especially to their location at the observatory.

IFAE was responsible to design and build the undercarriage of the LST. The undercarriage allows the telescope to rotate in azimuth. It consists of a circular rail and six wheel bogies running on top of it. Similar to the foundation, the undercarriage is also designed to tolerate strong vertical forces both in the downward and upward directions while keeping the needed precision for an accurate pointing. The design finished in 2015 and was partly validated using a test setup at IFAE in 2016.

The construction of the different mechanical parts for the six bogies took place between 2016 and 2017 at different institutes belonging to the LST collaboration (IFAE, CIEMAT, INFN Padova, MPI Munich and University of Hamburg did produce different pieces for the bogies) under IFAE’s supervision. The parts were later transported to IFAE, where they were assembled and integrated at the mechanical workshop until beginning of 2017.

Figure 2: Two of the 6 bogies for LST1 designed and assembled at IFAE and ready for shipment to ORM

Figure 4: The undercarriage structure of LST1 being erected on top of the 6 bogies by the time the first dynamic tests took place (October 2017)

The bogies were installed in the LST1 rail at ORM by the IFAE team in August 2017 after a whole week of very intense work in which some unexpected tolerance issues had to be solved on the fly. A first commissioning in dynamic conditions was done a few weeks later and confirmed that the bogies were working well.

In parallel in 2017 IFAE completed the design of the Azimuth Locking System, a complex mechanical system able to lock automatically the LST in the parking position and hold it in the worst meteorological condition expected. It will be installed before the end of 2018.

Figure 3: August 2017: the 6 bogies already installed in the LST1 rail, and the teams from IFAE and MPI in front.