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The Preparatory Phase for the Cherenkov Telescope Array
(CTA-PP)

Final Report Summary - CTA-PP (The Preparatory Phase for the Cherenkov Telescope Array<br/>(CTA-PP))

Executive Summary:
The Cherenkov Telescope Array (CTA) is an observatory for very-high-energy gamma-ray astronomy, which will provide observers with data on astrophysical objects over a very wide range of energies, between 20 GeV and 300 TeV. The CTA Observatory – operating arrays of Cherenkov telescopes on two sites, one in each hemisphere – is intended to provide a service to a wide scientific community, beyond those institutes currently involved in the design of the instrument and in the preparation of its construction.
CTA will be a targeted instrument following a standard model for observatory operations. Observation proposals from scientists will be reviewed and selected based on scientific merit, and CTA observation time may be allocated, observations performed, and data reduced and delivered to the proposer, together with appropriate analysis tools.

Supported by the EC Preparatory Phase grant (CTA-PP), the design and construction of the CTA Observatory is prepared by the CTA Consortium, currently encompassing 28 countries, over 170 institutes and over 1000 members. CTA-PP support allowed establishment of the CTA Project Office covering project management and systems engineering, and supported planning of governance, legal aspects, finance, procurement, outreach as well as of industrial development of key components.
The goals and design of the CTA Observatory are described in the documents (i) Design Concepts for the Cherenkov Telescope Array (2010), (ii) Seeing the High-Energy Universe with the Cherenkov Telescope Array - The Science Explored with the CTA, Astroparticle Physics special volume, 43 (2013), (iii) CTA Preliminary Technical Design Report (Oct. 2013) and (iv) CTA Technical Design Report (TDR, Reviewer version July 2014). The multi-volume TDR also summarizes plans for governance, procurement etc. as worked out in the context of CTA-PP.

The basic concept of CTA has been assessed and recommended by the ESFRI Roadmap and the European road maps of Astroparticle Physics (by ASPERA) and of Astronomy (by ASTRONET), as well as by numerous national roadmaps. On basis of an evaluation by an ESFRI expert group, CTA was found eligible for support under H2020 – INFRADEV 2015-3.

Representatives of ministries and funding agencies of a number of countries involved in CTA formed the CTA Resource Board. In July 2012, 13 countries - accounting for 84% of the members of the CTA Consortium – signed an Declaration of Intent expressing their common interest in participating in the construction and operation of CTA.
Three major reviews were agreed towards approval of CTA construction: (i) Science Performance and Preliminary Requirement Review (SPPRR), (ii) Preliminary Design Review (PDR), (iii) Critical Design Review (CDR). The SPPRR was successfully concluded in February 2013, the PDR in November 2013. The CDR is planned for early 2015, on the basis of the TDR.

To operate the project office after the end of CTA-PP, to provide a legal framework for site negotiations and agreements regarding in-kind contributions to CTA construction, and to prepare the founding agreement for CTA construction and operation, the CTA Observatory GmbH – a non-profit limited liability organization – was founded in July 2014.
Project Context and Objectives:
Context
Very high-energy (VHE) electromagnetic radiation reaches Earth from a large part of the Cosmos, carrying crucial and unique information about the most energetic phenomena in the Universe. Yet, only for the last 25 years have we had instruments that can “see” this radiation. Over that time, imaging air Cherenkov telescopes (IACTs) and air shower detectors have developed very rapidly, and opened up a new window for the exploration of the Universe. Current IACT instruments, such as the H.E.S.S. MAGIC and VERITAS telescope systems, together with the very successful Fermi and AGILE satellites and the gamma-ray detecting air shower experiments Milagro, Tibet AS-gamma and ARGO-YBJ , have produced a wealth of exciting results and demonstrated that VHE phenomena are ubiquitous throughout the Universe.
However, many of the results from these instruments have raised new questions, which require more and better data to reach a deeper understanding of the underlying phenomena.

CTA hence aims to advance the state of the art in VHE gamma ray astronomy in a number of decisive ways, all of which are unprecedented in this field:

- Worldwide integration: CTA will for the first time bring together the experience of virtually all groups worldwide working with IACTs, with an interest in astrophysics at the highest energies.

- Performance of the instrument: CTA aims at providing full-sky coverage, via a southern and northern site, with unprecedented sensitivity, spectral coverage, angular, energy and timing resolution. The large number of telescopes will also allow for independent operation of a large number of subarrays, each with the sensitivity of current instruments or better, which are either pointed at individual sources or are staggered to cover a very large area of the sky. These observing modes provide a high degree of flexibility of operation.

- Operation as an open observatory: CTA will, for the first time in this field, be operated as a true observatory, open to a wide scientific community, and providing support for easy access and analysis of data. Data will be made publicly available and will be accessible through Virtual Observatory tools. Services to professional astronomers will be supplemented by outreach activities and interfaces to the data which are suitable for laypersons.

- Technical implementation, operation, and data access: While based on existing and proven techniques, the goals of CTA imply significant advances in terms of efficiency of construction and installation, in terms of the reliability of the telescopes, and in terms of data preparation and dissemination. Therefore, the CTA observatory is qualitatively different from all earlier VHE instruments and its increase in capability goes well beyond anything that could be achieved through an expansion or upgrade of the existing instruments.

CTA builds on the proven technique of detecting gamma-ray induced particle cascades in the atmosphere through their Cherenkov radiation, simultaneously imaging each cascade stereoscopically with multiple telescopes, and reconstructing the properties of the primary gamma ray from those images. Through deployment of a large number of Cherenkov telescopes per site at two sites in the southern and the northern hemispheres CTA will achieve full-sky coverage.

Key science issues to be addressed using CTA can be grouped in the main areas:

Theme 1: Cosmic Particle Acceleration, Propagation and Impact
- What are the sites of particle acceleration in our own galaxy, filling the Milky Way with particles of up to PeV energies?
- Where exactly are the sites of particles acceleration in the jets and lobes of active galaxies?
- What is the mechanism, or mechanisms, for cosmic particle acceleration up to very high energies in many astrophysical systems? And of particle transport?
- What role do accelerated particles play in feedback on star formation and galaxy evolution?

Theme 2: Probing Extreme Environments
- What physical processes are at work close to neutron stars and black holes?
- What happens in the relativistic jets, winds and explosions?
- What are the contents - radiation fields, magnetic fields, particles - of cosmic voids, and how did these evolve over 
the history of the Universe?
- What role do relativistic particles play in the energetics of starburst regions, normal galaxies, active galaxy jets and lobes, and in clusters of galaxies?

Theme 3: Physics Frontiers
- What is the nature of Dark Matter? How is it distributed?
- Is the speed of light a constant for high-energy photons?
- Do axion-like particles exist?

The CTA project was initiated and is currently being developed by a group of institutes known as the CTA Consortium. The CTA Consortium formed in 2008 and is organised by a Memorandum of Understanding (MoU). The CTA Consortium comprises over 170 institutes in 28 countries, with over 1000 members. In 2010, the EU funded CTA Preparatory Phase (CTA-PP) started. The ministries and funding agencies of a number of countries involved in CTA have signed a Declaration of Intent (DoI) to express their common interest in participating in the construction and operation of CTA. With the signature of the DoI, CTA entered the Pre-Construction Phase (CTA-PCP), which overlaps until summer 2014 with the CTA-PP. Once a funding and construction agreement is reached, the CTA Construction Phase (CTA-CP) will be entered.

Objectives

The principal objective of the CTA Preparatory Phase (CTA-PP) was to elaborate a detailed plan regarding the implementation of CTA. Specific objectives of CTA-PP included:

- To explore possible legal frameworks and related governance schemes that provide the means for agencies to jointly establish, construct and operate CTA as a new science infrastructure, and to propose an implementation;

- To explore funding schemes and funding sources for CTA; this includes proposal of financial models that allow the combination of direct financial as well as in-kind contributions towards the construction and operation of CTA, and of schemes to administer shared expenses and a common fund;

- To identify, qualify and rank possible sites for CTA, based on their optical quality, environmental characteristics, logistics and availability, including administrative, political, financial and environmental considerations;

- To prepare site agreements and to plan site development and the implementation of the infrastructure required to operate the CTA observatory;
- To define schemes to approach procurement of components in the many participating countries and to efficiently involve industry in the optimization of design and the preparation of the production of CTA components;

- To enhance and intensify outreach activities and strategic links with the user communities of CTA and with related facilities in the relevant fields; to continuously refine and prepare the science goals and science optimization of the instrument as well as the user interfaces and user services;

- To prepare detailed technical designs and cost estimates of the different telescopes employed in CTA, with prototypes or component demonstrators where appropriate, and to provide a final optimization of the array layout giving the detailed characteristics and costs of the telescopes;

- To install project management and systems engineering to address issues such as high-level coordination and interface management, definition of requirements and specifications, system verification and validation, system integration, quality assurance and risk management, and documentation, and to implement a coherent approach to costing.

Driven by the goal of providing high-quality observatory services to external users, and also by the large number of telescopes to be deployed, CTA project management, systems engineering and quality assurance had to take a major step forward compared to current instruments, operated as ‘experiments’ and serving an expert user circle. This was to be achieved by using CTA-PP support for creating a central CTA Project Office (PO) providing project management and system engineering capacity, with the specific tasks to

- coordinate and verify the overall system design and system engineering to ensure the project satisfies user objectives and requirements;

- coordinate work towards an optimized and fully costed technical design of the telescopes and required infrastructure;

- coordinate and support the development of technical implementation plans for the observatory and its operation;

- define and maintain the relevant requirements definitions, specifications, work breakdown structures (WBS), interface definitions and develop a coherent management strategy;

- Implement the CTA-PP organizational structure, allowing for evolution towards future phases of the project;

- Implement complete and transparent internal communication and documentation for the project;

- Interfacing with ministries and agencies regarding future commitments concerning the construction and operation of CTA;

- Instigation and coordination of reviews;

- Support the steering of the CTA Resource Board and its subcommittees (AFAC: Administrative and Funding Advisory Committee, STAC: Scientific and Technical Advisory Committee, and SSC: Site Selection Committee);

The PO also served to coordinate and support of the development and implementation of legal frameworks, governance and funding schemes.
Project Results:
The objectives of CTA-PP were mainly of managerial, strategic, legal, financial and governmental nature concerning the implementation of the CTA Observatory. Thus the project itself did not focus on scientific or technical development which is mainly supported by national funding and is reported elsewhere.
The primary scientific and technical output of CTA-PP is the Technical Design Report (TDR), work towards which was managed under CTA-PP. However, with help of the funding for subcontracts within CTA-PP, certain critical S&T results were generated in cooperation with industry are reported later in this section.

Science Definition

The science goals and science requirements for CTA have been defined on the basis of input from within the CTA Consortium, and using input from the wider community, gathered at the LINK workshops supported by CTA-PP. Three LINK workshops served to address different communities:

- Dark matter / particle physicists and CTA (Oxford, UK, November 2010: http://indico.cern.ch/conferenceDisplay.py?confId=105442). This workshop took stock of the physics potential of the forthcoming Cherenkov Telescope Array regarding the detection of dark matter annihilation/decay signals, tests of Lorentz invariance violation and other new phenomena beyond the Standard Model.

- GeV, cosmic-ray, and neutrino observatories and observers: acceleration of particles from a MW perspective (Buenos Aires, Argentina, November 2012: http://www.ctalinks.iteda.cnea.gov.ar/). The workshop centred on exploring the relationship of CTA with neutrino and cosmic-ray experiments.

- X-ray and radio observatories and observers, common problems, future perspectives (Hakone, Japan, November 2013: http://www-conf.kek.jp/past/link13/program.html). With a special focus on the relationship of next-generation observatories of the near future such as the Cherenkov Telescope Array and ASTRO-H, this workshop aimed to bring together experts in X-ray and gamma-ray astrophysics, in order to overview the latest observational and theoretical developments and intensify the inherently synergistic interplay between these fields.

In addition, a joint discussion “The Highest-Energy Gamma-ray Universe” as a Session of the International Astronomical Union General Assembly in 2012 has been organized.
The purpose of this Joint Discussion (JD) session was to discuss the highest-energy gamma-ray universe as observed with Cherenkov telescope arrays with particular focus on the fruitful scientific interactions of such arrays and their capabilities with the broader astronomical community. This JD allowed, on one hand, non-gamma-ray astronomers to have a direct, compact, and precise view of the forthcoming possibilities with the Cerenkov Telescope Array (CTA) and facilities alike; and on the other, gamma-ray astronomers to benefit from the fresh views on possible joint ventures coming from the larger astronomical community. More details and slides of all talks can be found here: http://www.lsw.uni-heidelberg.de/cta/iau2012.php?lang=en.

Activities of science Task (a) and Task (b) were coordinated under CTA-PP:

- Task (a): provide a summary of detailed physics goals to be investigated with CTA;
- Task (b): quantitatively use the possible CTA's configuration response(s) to answer how well CTA will be able to tackle these science issues being quantitative in assertions reached.

These results were used as a part of the decision making process for which kind of array / sub-array to build.

Results regarding science goals and science requirements are condensed in the document “Science Requirements for CTA”, which provides both an introduction to the science of CTA, its main expectations and unique reach, and defines the minimum system parameters that would allow such science to be carried out.

The results of the work of the LINK, PHYS (Physics) and MC (Monte-Carlo) work packages as described above (Task (a) and (b)) are summarised a series of articles published in a special volume of the journal “Astroparticle Physics” that was devoted to CTA, “Seeing the High-Energy Universe with the Cherenkov Telescope Array , J. Hinton, S. Sarkar, D. F. Torres, & J. Knapp (Editors), 2013, ISSN: 0927-6505, Astroparticle Physics Special Issue, Volume 43, 356 pages. Part A of the special issue is devoted to invited reviews from leading experts on the key science topics, while Part B presents the detailed studies performed by various CTA Working Groups using Monte Carlo simulations of possible array configurations.

CTA Requirements

As a prerequisite for the final design, CTA requirements were established on the basis of science goals and technical feasibility. Requirements are defined and documented at several levels (see table below): the top level requirements, level A and level B requirements were established at a global level. Level A requirements concern the CTA Observatory as a whole. The Level B requirements define i) key telescope parameters ii) requirements on the optical and mechanical systems, iii) camera requirements and iv) requirements regarding synchronization, trigger, and data readout as well as Observatory operation and Observatory infrastructure. To the extent possible, requirements were defined in an implementation-independent fashion. Requirements at lower levels flow down from higher-level requirements, and are managed via a database. Level C lists product specifications defined by the product teams on the basis of the requirements.

Top level CTA requirements:
- The CTA Observatory
- CTA Basic Definitions
- Science Requirements

Level A CTA requirements:
- Performance Requirements
- Reliability, Availability, Mantainability and Safety (RAMS) Requirements
- Environmental Requirements
- End-User Requirements

Level B CTA requirements:
- Observatory Operations Requirements
- Infrastructure Requirements
- Data Management Requirements
- Array Control Requirements
- LST Requirements
- MST Requirements
- SCT Requirements
- SST Requirements

Level C Specifications:
- Observatory Operations Specifications
- Infrastructure Specifications
- Data Management Specifications
- Array Control Specifications
- LST Specifications
- MST Specifications
- SCT Specifications
- SST Specifications

Together with the requirements and specifications, procedures for change control and for validation and verification were established.

Site Characterisation and Site Selection

The Site Evaluation Summary (SES) report summarizes extensive work on site characterisation carried out with support of CTA-PP, to assist in the decision where to locate the CTA arrays.

The criteria considered in the SES for the site ranking are science performance, which depends on the average annual observation time (AAOT) available and on the performance of the array per unit time at a given site, as well as costs and risks. Factors which may depend on negotiations with the hosts, such as contributions to construction and operation, levies, taxes and fees, are not considered in the SES.

To evaluate the AAOT on each site, many different sources of data are used. Instruments have been placed on the sites to record in-situ weather data with data covering a period of typically one year. Other data from various satellites and weather simulations were are covering longer periods up to 10 years.

The main factor influencing the AAOT is the fraction of cloud free night hours, but losses due to environmental conditions are also significant in some cases. The cloud free fraction is derived from many sources, which together provide a coherent picture for each of the sites, with values ranging from 71% to 89%. Due to bad weather (primarily high winds and high relative humidity) and also very high levels of aerosols, losses vary from 1% to 9% and 0% to 7%, respectively. The available dark time before losses also depends on the latitude of the site, an effect of up to 6%.

For a fixed array configuration, the performance per unit observation time depends on site altitude, geomagnetic field, characteristic aerosol optical depth and night sky background level. Simulations identified altitude as the primary driver of science performance, with superior overall performance at lower altitude, but with some gain in energy threshold for higher altitude sites. The level of anthropogenic night sky background varies significantly between sites but has modest impact on performance for observations except at large zenith angles.

Using the AAOT and the performance per unit time, a Figure of Merit (FoM) is defined. Figure of merit values for the northern sites are consistent within uncertainties. For the southern sites, figures of merit vary significantly: from 1.6 for the best sites to 1.2 for the worst; a site just meeting science requirements would have a figure of merit of 1.0.

Regarding site-dependent hazards and risks, independent and objective assessments have been made wherever possible. Economic and socio-political risks vary considerably between sites.

According to a detailed and comprehensive study by external consultants, the infrastructure cost (including landscaping, roads, buildings, telescope foundations and appropriate earthquake mitigation measures for these elements) is independent of site within the uncertainty of 30%. The cost estimate for Infrastructure implementation were found to be of similar level in all places thanks to very limited civil engineering difficulties.

Operations costs differ somewhat reflecting local salary levels and power costs. Several scenarios were studied to handle the peak power, including green solutions.

The SES served as input for the site ranking by the CTA Consortium and by the external Site Selection Committee, on which basis the CTA agency board selected (for the south) resp. will select (for the north) host countries with which negotiations are carried out.

CTA Technical Design Report

As key deliverables of CTA-PP, the Observatory Technical Design Report, the Infrastructure Technical Design Report and the Technical Design Reports relating to the Large-Size, Medium-Size and Small-Size CTA telescopes were produced.
The Observatory Technical Design Report describes the plans for operation of the Observatory and contains

- Scientific Justification and Performance 
including a discussion of the Key Science Projects;
- Observatory Governance, Legal Structure, Policies 
addressing Observatory governance, organization, legal implementation, contributions and resulting rights and duties of parties, sharing of operating costs, data policy and data rights, access policy and publication policy;

- Planning and cost estimates for Central Activities such as management and organisation, facilities and support services, science management, and technical operations;

- Planning and cost estimates for Site Operations with local management and organization, on-site support services, instrument operations, and instrument maintenance;

- Plans relating to procurement, logistics, installation, commissioning, science verification and decommissioning;

- A discussion of Risks of various types,

- A discussion of External Engagement, including economic impact, industry and innovation interaction, increase of skills base, knowledge generation and outreach and education.

The Infrastructure Technical Design Report covers

- Site Infrastructure General Characteristics;

- Infrastructure for a Generic Site, including civil construction, power distribution, data networks, safety and security infrastructure etc.;

- Cost Estimates for site infrastructure;

- A discussion of Assessment Parameters for all Sites;

- For all site candidates a summary of standards and regulations, availability of services up to the fence, logistics constraints, environmental or cultural heritage protection, physical constraints, layout adaptation to sites and civil works.

The LST-, MST-, SST Technical Design Reports describe design and prototyping of the different telescope types which are used in CTA arrays, the plans for their construction, and cost estimates. These Technical Design Reports contain sections on

- Design and Prototyping, describing the design, interfaces, and prototypes and tests;

- Design Validation and Product Acceptance regarding safety, performance and reliability;

- Plans for Construction including procurement, logistics, integration and testing;

- Description of Management Structures including PBS, WBS, organization chart, scheduling;

- Milestones which allow to gauge progress;

- Construction Costs broken down along the WBS;

- Maintenance and Operations plans including maintenance needs and schedules and life-time costs;

- Assumptions, Dependencies and Caveats;

- Risks and their mitigation;

- Lessons Incorporated from previous projects and as a result of prototyping;

- The Full Product Breakdown Structure;

- The Full Work Breakdown Structure.

The MST Technical Design Report includes two versions of focal plane instrumentation, the NectarCam camera with readout of photomultiplier signals through a signal-sampling analogue ASIC, and the FlashCam camera with readout of signals with flash-ADC following by digital signal processing and trigger generation.

The SST Technical Design Report includes dual-mirror SST versions which provide a very compact telescope and a very compact camera, and a more conventional single-mirror version, which is less demanding regarding mirror manufacturing. All SSTs use silicon light sensors.

The additional Array Control Technical Design Report and the Data Management Technical Design Report describe the control and steering of the CTA arrays and the management, processing and dissemination of CTA data; however, these areas were not or only marginally supported under CTA-PP.

Technology results

Telescope Mirrors: Large-scale low-cost production of telescope mirror facets conforming to CTA specifications represents a significant challenge. Relevant R&D in cooperation the SMEs received significant attention under CTA-PP, with emphasis on techniques for reproducing mirrors using molds rather than individual grinding.

A high performing mould for the manufacturing of the mirror prototypes for the MST telescope was designed, developed and manufactured. The mirrors were manufactured by means of the technology “cold glass slumping” developed by INAF in the recent past. Here, a thin glass sheet is bent by vacuum suction and is made to adhere to a mould having a highly precise shape. The complete panel is then assembled by gluing a reinforcing core structure and a second thin glass sheet onto the bent glass sheet. Aluminium honeycombs are typically used for the core structure. After the glue is polymerized, the vacuum suction can be released and the substrate properly coated. The coating typically consists of two layers deposited by plasma vapour deposition - one of evaporated aluminium and another of thin quartz, as protection against reflectivity losses and scratches. The mirror is then finished by sealing its edges to prevent damage from water infiltration. The material selected for the mould is a particular type of aluminium that shows a considerable reduction of the internal stress.

This helps in maintaining the shape for long periods of time after the machining. Moreover, the design has been optimized in order to reduce the mass of the mould while keeping the requested stiffness against gravity deformations. The mass reduction achieved is the 45%. This achievement helps both in simplifying the handling of the mould and, even more important, by strongly reducing the production cycle of the mirrors because of a lower thermal inertia. The design has been validated through the implementation of a Finite Element Model and related analysis. The mould has been finished by means of a high precision diamond milling process. Its final surface accuracy is better than 50 μm P-V over the entire area.

For the MST, 20 mirrors with aluminium coating and triple layer protection were tested on their resistance to stripping, stability of the radius of curvature and water tightness. The results are very promising. A long-term study is underway.

For the dual-mirror SST with its aspherical, more strongly curved mirrors a detailed Finite Element Model that reproduces the mirror manufacturing process by cold slumping has been implemented. By means of parametric Finite Element Analyses, the behaviour of the mechanical parameters of the materials involved in the process has been investigated. Particular care has been devoted to the stress state developed into the glass by the bending process with respect to shape, thickness and dimension of the glass under test, including investigations of the mechanical parameters of the materials involved as well as the structural behaviour under different loading conditions of gravity, wind and temperature. With these models also the mechanical structure of the entire mirror panel was described: the honeycomb core with two glass skins, the bonding glue and the interfaces to the telescope. These models are used to investigate the structural behaviour of the mirror as a function of the core thickness and interfaces position. Different loading conditions have been applied: gravity, winds and temperatures.

The designs have been optimized by means of Finite Element Analyses. The results showed that the very short radii of curvature needed for the primary and secondary mirrors of the SST dual-mirror telescope cannot be produced with the baseline technology initially envisaged. A thermal pre-shaping of the glass has to be implemented before the standard “cold glass slumping” process is applied.

Together with an industrial partner, CTA has furthermore performed studies to enhance both the reflectance and the long-term durability of mirror surfaces using more sophisticated coatings.

Investigated were purely dielectric coatings without any metallic layer, circumventing the rather low adhesion of aluminum on glass. These show a reflectance greater than 95% in the wavelength region of interest and very low reflectance of only a few percent elsewhere. With this the night-sky background above 550 nm can be significantly suppressed, which is of special interest if silicon detectors rather than the standard photomultipliers are used, since they have a higher sensitivity at these wavelengths. Extensive temperature and humidity cycling as well as abrasion tests indicate a more stable long-term behaviour of these purely dielectric coatings. The disadvantage is that mirrors with these coatings are more likely to form condensation. Modifications to improve this are currently under investigation.

In parallel, different dielectric coating layouts were simulated and small coating samples were realized in a suitably adapted coating chamber at one of the CTA institutes. These samples were then evaluated against expected reflectance and durability performance. A preferred coating layout was identified, which performs according to CTA requirements, specifically not relying on substrate heating which would be prohibitive for many CTA mirror structure candidates. Commercial application of the result for CTA mirror production may be possible.

To qualify mirror technologies for use in CTA, a suite of mirror testing procedures has been defined and suitable test facilities have been prepared. Tests have well-defined ‘passing’ criteria and include

- Tests of optical performance, including local reflectance, level of scattered light, directed reflectance into the focal spot, measurement of focal length and point spread function (PSF), and measurement of temperature dependence of surface shape and PSF;

- Tests of mechanical stability, under mechanical impact, water immersion, and wind load;

- Tests of long-term durability, considering coating adhesion, temperature and humidity cycling, solar irradiation, salt fog tests, abrasion tests, bird faeces tests, and on-site outdoor testing.

Several cycles of mirror prototypes have been evaluated during CTA-PP, with significant improvements in mirror performance due to the feedback from the tests.

Mirror Actuators: CTA telescopes rely on actuators to align mirror facets; requirements for actuators – which need to be produced in very large volume – include low cost and long-term reliability. AAT / Tübingen has developed actuators for the adjustment of mirror segments at CTA single mirror telescopes, focusing its efforts on the CTA medium sized telescope (MST). The actuator development profits from experience gained from the actuators for the 28 m telescope of H.E.S.S. phase II. To adapt that actuator system to CTA requirements, the electronics concept was completely redesigned. The goal of the project was to also redesign the actuator mechanics to adapt it to CTA specifications, and towards industrial production with the goal of lowering production costs. The work was performed in collaboration with a SME, co-financed through CTA-PP funding. 35 pre-series actuators were delivered under supervision of the SME.

Photo Sensors: The CTA telescopes empliy very large numbers of photo-sensor pixels to image the gamma-ray induced particle cascades. Optimisation of these sensors and of their mass production was a key theme of CTA-PP technology developments.

Currently the standard light sensors for imaging atmospheric Cherenkov telescopes are the classical photo multiplier tubes that are using bialkali photo cathodes. The peak Quantum Efficiency (QE) is at the level of 25-27%. New PMTs appeared with a peak QE of 35%; these have received the name “super-bialkali”. Due to the CTA-PP-supported collaborative developments with Hamamatsu and Electron Tubes resulting in improvements of manufacturing techniques and of quality control, PMTs with average peak QE of approximately 40% are now available.
The QE curves of these PMTs match well with the simulated Cherenkov light spectrum. Also, the photo electron collection efficiency of the previous generation PMTs of 80- 90% has been enhanced towards 95-98% for the new ones. The after-pulsing of the novel PMTs has been reduced towards the level of 0.02% for the set threshold of 4 photo electrons; low after-pulsing levels are critical for optimal performance of the telescopes. This has been achieved by shielding the dynodes since light emission between dynodes was identified as the cause for after-pulses.

To avoid gaps between photo-sensor pixels and to reduce impact of stray light, light concentrators in front of the pixels are used. Two different options of light concentrators (LC) for the MST cameras were developed: hollow Winston cone and lenses. A first generation of prototypes (20 lenses and 20 cones) were manufactured, partly using CTA-PP support. This first generation of prototypes allowed validating performance and design. A batch of second-generation cones with improved design and coating performances was produced and tested.

Telescope Structures: Specific technical R&D aspects of CTA telescope structures have been addressed under CTA-PP, in cooperation with SMEs. One example is the rather unconventional camera support structure (CSS) for the CTA Large Size Telescopes (LSTs).

The design derived for the CSS for the LST is based on an elliptical arch geometry reinforced along its orthogonal projection by two symmetric sets of stabilizing ropes. The main requirements in terms of minimal camera displacement, minimal weight, minimal shadowing on the telescope mirror, maximal strength of the structures and fast dynamical stabilization have led to the application of Carbon Fibre Plastic Reinforced (CFPR) technology. In order to eliminate of thermal expansion difference issues, all junction elements between CSS components are made of CFPR. Stainless steel parts are used only in the arch/dish interfaces for higher linking efficiency and production process purposes.

The manufacturing process proposed to use for the arch components and the camera frame is the pre-preg technology, used for state-of-the-art structures such as racing boats and cars, new generation aircraft etc. The main advantages of this technique for the LST purposes are:

- the possibility of achieving any fibre orientation;
- the possibility of reinforcing the structures on well localised critical parts;
- a lower resin-to-fibre ratio;
- a better compaction of the layers enabled by the curing process (autoclave);
- a higher glass transition temperature guaranteeing stable performance at high environmental temperatures over a long time period.

The CCS mechanical behaviour was studied under different static, dynamic, and thermal expansion load cases. Furthermore, critical situations affecting the data-taking phases of the telescopes were also simulated, namely the telescope behaviour under the action of turbulent wind, after fast repositioning, e.g. following-up a received GRB alert, and fast emergency braking.
Potential Impact:
The main product of the CTA Preparatory Phase is the proposal to construct and operate the Cherenkov Telescope Array as a new facility for high energy astrophysics; the proposal will be substantiated by a multi-volume Technical Design Report regarding facility construction, and proposals regarding the organizational form, governance and funding of the facility and its operation. The preparation of these accompanying documents was conducted throughout the Preparatory Phase. It is assumed that on the basis of this material, agencies will approve CTA construction during 2015. An interim legal entity was founded to overcome the legal and financial gap between the end of the Preparatory Phase and the constitution of a final legal entity.

Technical developments in the context of CTA, such as the development of improved photomultipliers or high-volume production techniques for mirror facets are starting to find applications in other fields.

During the reporting period, the CTA Consortium kept growing, demonstrating the large and growing interest in the community.
Like the CTA consortium, the community of users interested in CTA – while already large and spanning the fields of particle physics, astroparticle physics, astronomy and astrophysics as well as cosmology – is steadily increasing. Through the development of new simply and flexible data formats and of analysis algorithms, CTA also influences currently operating instruments and contributes to their steady improvement.

CTA has had an increasing impact on the scientific community at large during the Preparatory Phase, via CTA science workshops, conferences and scientific publications.

A joint discussion meeting organised by the CTA Consortium was held at the International Astronomical Union (IAU) General Assembly in Beijing in August 2012, and several open meetings have been held targeting scientists working in other disciplines and covering topics such as physics beyond the Standard Model, the physics of active galactic nuclei, links with cosmic ray physics and X-ray astronomy missions, gamma-ray bursts etc. It is estimated that over 1000 non-CTA scientists have been reached using this method.

CTA scientists have disseminated the work undertaken during the Preparatory Phase at 126 national and international conferences on 5 continents, from Hawaii to Japan and from Lapland to Zululand. An impression of the increase in activity and interest may be gauged by the number of papers presented at conferences by CTA members as registered with CTA’s Speakers and Publications Office. This is shown in Figure 1 below; at the time of writing (November 2014) there were already 124 CTA contributions to conferences in 2014 registered. Many of the conference contributions have been invited talks, a good indication of the wider impact of CTA.

Another dissemination route to the scientific community is of course via journal publications. A highlight has been the publication of the special edition of Astroparticle Physics in March 2013, containing 346 pages of papers about CTA and its scientific impact written by members of the CTA Consortium and experts from outside the Consortium. Despite being published for only just over a year, these papers have received 180 citations. To date, according to the NASA Astrophysics Data System there have been over 300 publications which explicitly mention CTA and its scientific properties; Figure 2 shows how the number of these papers has increased during the Preparatory Phase. Many of these papers are written by scientists who are not members of the CTA Consortium, but who have been attracted by the scientific possibilities offered by CTA.

Figure 1: Number of conference contributions from CTA Consortium members by year (attachment)


Figure 2: Number of published papers mentioning CTA by year. Data from the NASA Astrophysics Data System (attachment)

Finally, CTA has received coverage in the broader scientific press, including Nature, Science and CERN Courier, and the CTA management is regularly contacted by science journalists regarding new about CTA, e.g. concerning site selection.

A further area for impact, dissemination and exploitation involves industry. A website introducing CTA to industry was produced as one of the first actions during the Preparatory Phase; it can be found here: http://www.cta-industries.org/. Industrial research and development workshops have been organised by CTA, one in Madrid in 201, one in Rome in 2012, and a joint work shop with SKA was conducted in Bonn in April 2014. These were attended by over 40 different industries from around Europe, covering areas from mechanical construction to high-speed electronics. In addition, many CTA workshops have had industrial attendees, including the recent Silicon Photomultiplier Workshop held in Geneva. Individual CTA member states have also held local industrial workshops, such as the Industrial Co-operation Meeting in Warsaw in April 2014, and CTA has presented at wider industrial engagement events, such as Spacetech (UK).

CTA groups are working with a wide range in industries, primarily in Europe, on prototyping elements for CTA. They are too numerous to provide a complete list here, but include: BTE Bedampfungstechnik GmbH (Germany), Electron Tubes (UK), Hamamatsu (Japan), Kerdry (France), Media Lario (Italy), NTE (Spain) and Philips (Netherlands) . The impact on these industries is derived not solely from orders placed by CTA but also from working closely with CTA scientists to improve techniques and technologies. For example, the requirement for mirrors that will survive the considerable temperature variation expected at the CTA sites has resulted in Kerdry creating several different mirror designs and improving their manufacturing techniques. This experience will likely have a positive impact on other projects. Some industries, such as Airworks (Italy) are using their CTA work to advertise their skills - http://www.air-works.eu/portfolio/cta-lst/

Science Education

CTA represents a tremendous opportunity for educating young people (and indeed the wider community) about science. Here, we identify two distinct areas: higher education and schools. The general public are considered in the next section.

One of the most important outputs of a large, international project such as CTA is human; people who have high-level skills in science, mathematics and computing which can be applied outside the confines of the CTA project and across national boundaries. During the Preparatory Phase, CTA has been instrumental in the education of both undergraduate and postgraduate students and a large number of final year undergraduate, diploma, masters and doctoral theses have been produced, with subjects ranging from ‘The Misting of Cherenkov Telescope Mirrors’ to ‘Is the Speed of Light Really Constant?’. Many of these students and early-stage researchers have remained within CTA, but others have gone on to teach in schools or to work in industries such as Airbus Defence and Space or IBM.

In the modern age, one of the major means of education is via websites. The CTA website www.cta-observatory.org contains some suitable educational material, but for schools and the general public information is usually required that is appropriate to the local curriculum and is in the native language of the country concerned. To that end, a number of native-language CTA websites have been set up by CTA groups, some national, some based around institutions:

CTA Argentina: http://astrum.frm.utn.edu.ar/CTA-Argentina/
CTA Paris (France): http://cta.obspm.fr/
CTA Zeuthen (Germany): http://astro.desy.de/index_ger.html
CTA Milan (Italy): http://www.iasf-milano.inaf.it/Divulgazione/divulgazione.php?pg=gamma_strum_ceren&mn=gamma&lin=gamma_strum_ceren
CTA Japan: http://www.cta-observatory.jp/
CTA Poland: http://www.obserwatorium-cta.pl/
CTA Spain: http://www.observatorio-cta.es/
CTA Zurich (Switzerland): http://cta.physik.uzh.ch/
CTA UK: https://www.cta-observatory.ac.uk/

CTA scientists have also created material which is being used in schools. One example is the lesson plan and material about choosing the location for telescopes. This is highly relevant to CTA’s Preparatory Phase and also happens to be part of an option in the UK’s GCSE Physics syllabus. An example slide from the material is shown in Figure 3.

Figure 3: Sample slide from the teaching material supporting UK GCSE Physics (attachment)

Although the web is a very powerful educational tool, it is still the case that school pupils greatly appreciate meeting scientists and hearing about what they do. Many CTA scientists have visited schools, or had schools visit their laboratories for open days. The Max-Planck-Institut für Physik in Munich, for example, held an open day which was attended by 1200 people. Among other displays they used a ‘sandbox’ model of CTA, showing how Cherenkov radiation is incident on the array, as shown in Figure 4.


Figure 4: A ‘sandbox’ model of CTA used at the open day of the Max-Planck-Institut für Physik in Munich. The coloured dots indicate CTA telescopes, and the light shows how the Cherenkov radiation from a proton-induced shower would impact on CTA (attachment)

Media and Communication to the General Public

Media and communication to the general public covers several different areas: CTA-produced print media, exhibitions and other events, press coverage and the use of ‘new’ media.

Right from the beginning of the Preparatory Phase, it was felt important to have a physical leaflet about CTA that could be handed to interested people at various events with a CTA presence. The leaflet was produced in English initially and then in French, German, Spanish, Italian, Portuguese and Polish. Figure 5 shows the French version of the leaflet. Over 35,000 leaflets have been produced and distributed in Europe, Asia and the USA.


Figure 5: Front and rear pages of the French version of the CTA leaflet (attachment)

In addition to this, a longer brochure was to explain CTA in more detail to interested parties.

CTA has had a presence at many exhibitions and other events attended by members of the public during the Preparatory Phase. Of particular note are the European Weeks of Astronomy and Space Science in Finland and in Switzerland, the Royal Society Summer Science Exhibition in London, BBC Stargazing Live, the American Astronomical Society meeting in California and the IAU General Assembly in Beijing. Posters and exhibits suitable for a general audience have been created for such events. Overall, CTA has reached some 20,000 members of the public at these and similar events.


Figure 6: CTA stand at the European Week of Astronomy and Space Science in Finland (attachment)

Another method by which CTA communicates with the wider public is via articles in the press, some of which are written by CTA members, some not. CTA has received press coverage over a wide range of journals and newspapers including Il Sole 24Ore and Le Stelle (Italy), Business Day (South Africa), El Pais (Spain), The Arizona Sun (USA) and even The Hindu (India). There has also been coverage in online journals, such as International Science Grid This Week and Softpedia.

Finally, CTA is exploiting ‘new media’ to communicate with the public. CTA scientists participate in science blogs, such as http://desayunoconfotones.org/2014/04/25/los-fotones-mas-energeticos-que-conocemos/ . CTA has a Facebook presence, https://www.facebook.com/ctaobservatory?fref=ts and Wikipedia entries in English, French and Spanish. A CTA YouTube channel has been started, https://www.youtube.com/channel/UC0IHTTfgiiyCFLp-SH8egMw , which contains several podcasts from the UK and a movie of a computer simulation of CTA produced by the Instituto de Astrofisica de Canarias.

List of Websites:
Website address: www.cta-observatory.org
Contact: Werner Hofmann, , +49 6221 516 330