The Cherenkov Telescope Array Observatory (CTAO) will include telescopes of three different sizes, the smallest of which are the Small-Sized Telescopes (SSTs). In particular, the SSTs will be installed at the southern site of CTAO, on the Chilean Andes, and will cover the highest energy range of CTAO (up to ~300 TeV). The SSTs are developed by an international consortium of institutes that will provide them as an in-kind contribution to CTAO. The optical design of the SSTs is based on a Schwarzschild-Couder-like dual-mirror polynomial configuration, with a primary aperture of 4.3m diameter. They are equipped with a focal plane camera based on SiPM detectors covering a field of view of ~9°. The preliminary design of the SST telescopes was evaluated and approved during the Product Review (PR) organised with CTAO in February 2023. The SST project is now going through a consolidation phase leading to the finalisation and submission of the final design to the Critical Design Review (CDR), expected to occur late 2024, after which the production and construction of the telescopes will begin leading to a delivery of the telescopes to CTAO southern site starting at the end of 2025-early 2026 onward. In this contribution we will present the progress of the SST programme, including the results of the PDR, the consolidation phase of the project and the plan up to the on-site integration of the telescopes.
The prototype of Gamma-ray Cherenkov Telescope (pGCT) is an Imaging Atmospheric Cherenkov Telescope (IACT) developed to detect Very High Energy (VHE) from various cosmic sources emitting gamma rays. It is based on a Schwarzschild-Couder (SC) dual-mirror configuration resulting in a compact telescope and an optimised PSF over a wide field of view. Moreover, some innovative features were implemented with the aim to ease and to fasten the assembly, integration, test (AIT) and maintenance activities. Both mirrors, the 4-meter tessellated primary mirror and the secondary mirror, are aspherical lightweight mirrors made by a subtractive manufacturing method. This process was improved several times leading to enhanced optical performance of the telescope in terms of Point Spread Function and improved signal to noise ratio. This paper deals with the optomechanical design of the pGCT and its latest measured performance. After a brief overview of the mechanical design of the telescope, the updated manufacturing process of the new generation mirrors and the optical performance of pGCT are given. Finally, recent operational performance of the telescope is given.
In 2015, the pGCT, a telescope prototype planned for the Cherenkov Telescope Array (CTA) gets its first Cherenkov light in the Meudon site of the Observatoire de Paris. As a part of the small-sized telescopes of the CTA, this telescope was designed to detect showers of secondary particles produced when very high energy gamma rays and cosmic rays enter in the upper atmosphere and interact with the atmospheric gas. It is now dedicated to a test bench for Cherenkov observation and to educational purposes. Within this last framework and in order to propose to the general public an easy way to observe high energy particles, we started in 2021 the development of a Langsdorf cloud chamber based on a previous model using Peltier cells and developed for students in Tours. Cloud chambers provide a convenient way to observe signatures of charged particles related to cosmic rays since they allow a direct detection also during daytime and are easier to use for the visits by the general public to concretely illustrate the existence of these high energy particles. This paper describes mechanical and electronic designs of this cloud chamber. Some results for educational purposes are also given.
The Small Sized Telescope (SST) is one of the three types of telescopes that will constitute the Cherenkov Telescope Array (CTA). For the CTA Southern site, 37 SST will be realized for the first alpha CTA configuration. Their design is based on the ASTRI-Horn dual-mirror telescope. Some modifications of the design are currently under study searching for possible improvements of the behaviour of the telescope. Amongst them, there are studies on the primary mirror dish (M1 Dish) led by a team of the Observatoire de Paris. The main purpose of these studies is to optimize the mass stiffness ratio of this structure. It means reducing its total mass while keeping its performance, mainly its stiffness, and taking into account existing constraints related to dependent fixed subsystems (counterweight, secondary mirror...) or environment (gravity, wind). This problem can be described as a classical optimization problem in the way it aims at finding an optimal mass distribution by minimizing the compliance with a constraint of mass reduction and under given boundary conditions. This methodology, previously used by the Observatoire de Paris for the design of lightweight mirrors and of components of another Cherenkov telescope, is applied to propose an alternative option to the ASTRI-Horn baseline design of the M1 Dish. Its lays on the use of structural optimization tools, which can help to get more quickly an accurate mass distribution and to improve the design process by reducing the number of iterations between phases of design definition under computer-aided design (CAD) and phases of design validation under finite-element analysis. This methodology and the corresponding results are presented in this paper.
The Cherenkov Telescope Array Observatory (CTAO) consists of three types of telescopes: large-sized (LST), mediumsized (MST), and small-sized (SST), distributed in two observing sites (North and South). For the CTA South “Alpha Configuration” the construction and installation of 37 (+5) SST telescopes (a number that could increase up to 70 in future upgrades) are planned. The SSTs are developed by an international consortium of institutes that will provide them as an in-kind contribution to CTAO. The SSTs rely on a Schwarzschild-Couder-like dual-mirror polynomial optical design, with a primary mirror of 4 m diameter, and are equipped with a focal plane camera based on SiPM detectors covering a field of view of ~9°. The current SST concept was validated by developing the prototype dual-mirror ASTRI-Horn Cherenkov telescope and the CHEC-S SiPM focal plane camera. In this contribution, we will present an overview of the SST key technologies, the current status of the SST project, and the planned schedule.
Optimization techniques are powerful tools for producing lightweight structures with the maximum structural stiffness. They allow an optimized design to be produced directly for a given structure and, in this way, save considerable time in the design phase of a structure by avoiding multiple iterations between the definition of the design under computer-aided design (CAD) and the verification of the performance under finite-element (FE) analysis. There are three classes of optimization: size optimization, shape optimization and topology optimization. The topology optimization technique aims to find an optimal distribution of material given boundary conditions, i.e. the fixing points and the external loads. It starts from an initial volume representing a blank of the structure and removes the most unused material to meet the objective of mass reduction. In optomechanical engineering, this technique is met in the design of lightweight mirrors and especially in the design of their core-cell shapes. To provide reliable and useable results, this technique requires a fine and regular mesh of the mirror as well as a postprocessing of the results by the mechanical engineers. These constraints, combined with the necessity of using 3-D models, contribute an increase in the computation time and complicate the meshing. We propose here an innovative approach to this design problem by using topography optimization instead of topology optimization. Topography optimization, also named bead optimization, is a branch of the shape optimization and consists in introducing beads to a surface in order to increase its structural stiffness. The main advantage of this technique is that shell models can be used instead of solid models, easing the meshing operation and decreasing the number of degrees of freedom in the FE model, and thereby reducing computation cost. This paper presents an example of the application of this technique to the design of the primary mirror panel of the GCT (Gamma-ray Cherenkov Telescope), a dual-mirror 4-meter telescope proposed for the future Cherenkov Telescope Array. FE models and optimizations are made with MD.Patran and MD.Nastran respectively.
The Gamma-ray Cherenkov Telescope (GCT) is one of the telescopes proposed for the Small Sized Telescope (SST) section of CTA. Based on a dual-mirror Schwarzschild-Couder design, which allows for more compact telescopes and cameras than the usual single-mirror designs, it will be equipped with a Compact High-Energy Camera (CHEC) based on silicon photomultipliers (SiPM). In 2015, the GCT prototype was the first dual-mirror telescope constructed in the prospect of CTA to record Cherenkov light on the night sky. Further tests and observations have been performed since then. This report describes the current status of the GCT, the results of tests performed to demonstrate its compliance with CTA requirements, and the optimisation of the design for mass production. The GCT collaboration, including teams from Australia, France, Germany, Japan, the Netherlands and the United Kingdom, plans to install the first telescopes on site in Chile for 2019-2020 as part of the CTA pre-production phase.
Lasers with sub-hertz line-width and fractional frequency instability around 1×10-15 for 0.1 s to 10 s averaging time are currently realized by locking onto an ultra-stable Fabry-Perot cavity using the Pound-Drever-Hall method. This powerful method requires tight alignment of free space optical components, precise polarization adjustment and spatial mode matching. To circumvent these issues, we use an all-fiber Michelson interferometer with a long fiber spool as a frequency reference and a heterodyne detection technique with a fibered acousto optical modulator (AOM)1. At low Fourier frequencies, the frequency noise of our system is mainly limited by mechanical vibrations, an issue that has already been explored in the field of optoelectronic oscillators.2,3,4
J. L. Dournaux, A. Abchiche, D. Allan, J. P. Amans, T. P. Armstrong, A. Balzer, D. Berge, C. Boisson, J.-J. Bousquet, A. Brown, M. Bryan, G. Buchholtz, P. Chadwick, H. Costantini, G. Cotter, L. Dangeon, M. Daniel, A. De Franco, F. De Frondat, D. Dumas, J. P. Ernenwein, G. Fasola, S. Funk, J. Gironnet, J. Graham, T. Greenshaw, B. Hameau, O. Hervet, N. Hidaka, J.A. Hinton, J.M. Huet, I. Jégouzo, T. Jogler, T. Kawashima, M. Kraush, J. Lapington, P. Laporte, J. Lefaucheur, S. Markoff, T. Melse, L. Mohrmann, P. Molyneux, S. Nolan, A. Okumura, J. Osborne, R. Parsons, S. Rosen, D. Ross, G. Rowell, C. Rulten, Y. Sato, F. Sayède, J. Schmoll, H. Schoorlemmer, M. Servillat, H. Sol, V. Stamatescu, M. Stephan, R. Stuik, J. Sykes, H. Tajima, J. Thornhill, L. Tibaldo, C. Trichard, J. Vink, J. Watson, R. White, N. Yamane, A. Zech, A. Zink
The GCT (Gamma-ray Cherenkov Telescope) is a dual-mirror prototype of Small-Sized-Telescopes proposed for the Cherenkov Telescope Array (CTA) and made by an Australian-Dutch-French-German-Indian-Japanese-UK-US consortium. The integration of this end-to-end telescope was achieved in 2015. On-site tests and measurements of the first Cherenkov images on the night sky began on November 2015. This contribution describes the telescope and plans for the pre-production and a large scale production within CTA.
A. Brown, A. Abchiche, D. Allan, J.-P. Amans, T. Armstrong, A. Balzer, D. Berge, C. Boisson, J.-J. Bousquet, M. Bryan, G. Buchholtz, P. Chadwick, H. Costantini, G. Cotter, M. Daniel, A. De Franco, F. de Frondat, J.-L. Dournaux, D. Dumas, G Fasola, S. Funk, J. Gironnet, J. Graham, T. Greenshaw, O. Hervet, N. Hidaka, J. Hinton, J.-M. Huet, I. Jégouzo, T. Jogler, M. Kraus, J. Lapington, P. Laporte, J. Lefaucheur, S. Markoff, T. Melse, L. Mohrmann, P. Molyneux, S. Nolan, A. Okumura, J. Osborne, R. Parsons, S. Rosen, D. Ross, G. Rowell, Y. Sato, F. Sayede, J. Schmoll, H. Schoorlemmer, M. Servillat, H. Sol, V. Stamatescu, M. Stephan, R. Stuik, J. Sykes, H. Tajima, J. Thornhill, L. Tibaldo, C. Trichard, J. Vink, J. Watson, R. White, N. Yamane, A. Zech, A. Zink, J. Zorn
The Gamma-ray Cherenkov Telescope (GCT) is proposed for the Small-Sized Telescope component of the Cherenkov Telescope Array (CTA). GCT's dual-mirror Schwarzschild-Couder (SC) optical system allows the use of a compact camera with small form-factor photosensors. The GCT camera is ~ 0:4 m in diameter and has 2048 pixels; each pixel has a ~ 0:2° angular size, resulting in a wide field-of-view. The design of the GCT camera is high performance at low cost, with the camera housing 32 front-end electronics modules providing full waveform information for all of the camera's 2048 pixels. The first GCT camera prototype, CHEC-M, was commissioned during 2015, culminating in the first Cherenkov images recorded by a SC telescope and the first light of a CTA prototype. In this contribution we give a detailed description of the GCT camera and present preliminary results from CHEC-M's commissioning.
The Cherenkov Telescope Array (CTA) project, led by an international collaboration of institutes, aims to create the world's largest next generation Very High-Energy (VHE) gamma-ray telescope array, devoted to observations in a wide band of energy, from a few tens of GeV to more than 100 TeV. The Small-Sized Telescopes (SSTs) are dedicated to the highest energy range. Seventy SSTs are planned in the baseline array design with a required lifetime of about 30 years.
The GCT (Gamma-ray Cherenkov Telescope) is one of the prototypes proposed for CTA's SST sub-array. It is based on a Schwarzschild-Couder dual-mirror optical design. This configuration has the benefit of increasing the field-of-view and decreasing the masses of the telescope and of the camera. But, in spite of these many advantages, it was never implemented before in ground-based Cherenkov astronomy because of the aspherical and highly curved shape required for the mirrors.
The optical design of the GCT consists of a primary 4 meter diameter mirror, segmented in six aspherical petals, a secondary monolithic 2-meter mirror and a light camera. The reduced number of segments simplifies the alignment of the telescope but complicates the shape of the petals. This, combined with the strong curvature of the secondary mirror, strongly constrains the manufacturing process. The Observatoire de Paris implemented metallic lightweight mirrors for the primary and the secondary mirrors of GCT. This choice was made possible because of the relaxed requirements of optical Cherenkov telescopes compared to optical ones. Measurements on produced mirrors show that these ones can fulfill requirements in shape, PSF and reflectivity, with a clear competition between manufacturing cost and final performance.
This paper describes the design of these mirrors in the context of their characteristics and how design optimization was used to produce a lightweight design. The manufacturing process used for the prototype and planned for the large scale production is presented as well as the performance, in terms of geometric and optical properties, of the produced mirrors. The alignment procedure of the mirrors is also detailed. This technique is finally compared to other manufacturing techniques based on composite glass mirrors within the framework of GCT mirrors specificities.
J. L. Dournaux, J. P. Amans, L. Dangeon, G. Fasola, J. Gironnet, J. M. Huet, P. Laporte, A. Abchiche, S. Barkaoui, J. J. Bousquet, G. Buchholtz, D. Dumas, J. Gaudemard, I. Jégouzo, P. Poinsignon, L. Vergne, H. Sol
The Cherenkov Telescope Array (CTA) project aims to create the next generation Very High-Energy (VHE) gamma-ray telescope array. It will be devoted to the observation of gamma rays from 20 GeV to above 100 TeV. Because of this wide energy band, three classes of telescopes, associated with different energy ranges and different mirror sizes, are defined. The Small Size Telescopes (SSTs) are associated with the highest energy range. Seventy of these telescopes are foreseen on the Southern site of the CTA. The large number of telescopes constrains their mechanical structure because easy maintenance and reduced cost per telescope are needed. Moreover, of course, the design shall fulfill the required performance and lifetime in the environment conditions of the site.
The Observatoire de Paris started design studies in 2011 of the mechanical structure of the GCT (Gamma-ray Cherenkov Telescope), a four-meter prototype telescope for the SSTs of CTA, from optical and preliminary mechanical designs made by the University of Durham. At the end of 2014 these studies finally resulted in a lightweight (~8 tons) and stiff design. This structure was based on the dual-mirror Schwarzschild-Couder (SC) optical design, which is an interesting and innovative alternative to the one-mirror Davies-Cotton design commonly used in ground-based Cherenkov astronomy. The benefits of such a design are many since it enables a compact structure, lightweight camera and a good angular resolution across the entire field-of-view. The mechanical structure was assembled on the Meudon site of the Observatoire de Paris in spring 2015. The secondary mirror, panels of the primary mirror and the Telescope Control System were successfully implemented afterwards leading now to a fully operational telescope.
This paper focuses on the mechanics of the telescope prototype. It describes the mechanical structure and presents its performance identified from computations or direct measurements. Upgrades of the design in the context of the preproduction and the large scale CTA production are also discussed.
The Cherenkov Telescope Array (CTA) project aims to create the next generation Very High Energy (VHE) gamma-ray
telescope array. It will be devoted to the observation of gamma rays over a wide band of energy, from a few tens of GeV
to more than 100 TeV. Two sites are foreseen to view the whole sky where about 100 telescopes, composed of three
different classes, related to the specific energy region to be investigated, will be installed. Among these, the Small Size
class of Telescopes, SSTs, are devoted to the highest energy region, to beyond 100 TeV. Due to the large number of
SSTs, their unit cost is an important parameter.
At the Observatoire de Paris, we have designed a prototype of a Small Size Telescope named SST-GATE, based on the
dual-mirror Schwarzschild-Couder optical formula, which has never before been implemented in the design of a
telescope. Over the last two years, we developed a mechanical design for SST-GATE from the optical and preliminary
mechanical designs made by the University of Durham. The integration of this telescope is currently in progress.
Since the early stages of mechanical design of SST-GATE, finite element method has been used employing shape and
topology optimization techniques to help design several elements of the telescope. This allowed optimization of the
mechanical stiffness/mass ratio, leading to a lightweight and less expensive mechanical structure. These techniques and
the resulting mechanical design are detailed in this paper. We will also describe the finite element analyses carried out to
calculate the mechanical deformations and the stresses in the structure under observing and survival conditions.
The Observatoire de Paris is involved in the Cherenkov Telescope Array (CTA) project by designing and constructing on
the site of Meudon a Small Size Telescope prototype, named SST-GATE, in collaboration with the CHEC team
(Compact High Energy Camera) which is providing the camera. The telescope structure is based on the Schwarzschild-
Couder optical design which has never been adopted before in the design of a ground-based telescope. This concept
allows a larger field of view and cheaper and smaller telescope and camera design with improved performance compared
to the Davies-Cotton design traditionally used in very high energy gamma-ray telescopes.
The SST-GATE telescope has been designed with the prime objectives of being light, versatile and simple to assemble
with a minimal maintenance cost. This papers aims at reviewing the SST-GATE telescope structure from mechanics to
optics along with the control command architecture; several innovative developments implemented within the design are
discussed. Updates of the project status and perspectives are made.
The Cherenkov Telescope Array (CTA) project aims to create a next generation Very High Energy (VHE)γ-ray
telescope array, devoted to the observation in a wide band of energy, from a few tens of GeV to more than 100 TeV.
Two sites are foreseen to view the whole sky, with the main one in the Southern Hemisphere where about 100 telescopes
of three different classes, related to the specific energy region to be investigated, will be installed. Among these, the
Small Size class of Telescopes, SSTs, are 4-meter telescopes and are devoted to the highest energy region, from 1 TeV to
beyond 100 TeV.
Some of these sites considered for CTA exhibit strong seismic constraints. At the Observatoire de Paris, we have
designed a prototype of a Small Size Telescope named SST-GATE, based on the dual-mirror Schwarzschild-Couder
optical formula, which was never before implemented in the design of a Cherenkov telescope. The integration of this
telescope on the site of the Observatoire de Paris is currently in progress.
Technical solutions exist in the literature to protect structures from dynamic loads caused by earthquakes without
increasing the mass and cost of the structure. This paper presents a state of the art of these techniques by keeping in mind
that the operational performance of the telescope should not be compromised. The preliminary seismic analysis of SSTGATE
performed by the finite element method is described before.
Jean-Laurent Dournaux, Christophe Berthod, David Horville, Jean-Michel Huet, Philippe Laporte, Martina Wiedner, Alexia Romanow, Jean-Michel Krieg, Laurent Pagani, Jean Evrard, Albert Gomes, Martine Jouret
Astronomers require more and more precise instruments for their observations. Here we describe the challenges encountered in the optical and mechanical designs of the CIDRE (Campagne d’Identification du Deutérium par Réception hEtérodyne) project, which was to be flown on a high altitude balloon at 40 km. The project aimed to measure the transitions of the HD molecule at 2.675 THz band and some other THz lines in our galaxy. The astronomers asked to fly the biggest possible telescope in a standard balloon gondola, and required high pointing accuracy (7 arcsec). In January 2014, the technical project, including the optical and mechanical designs, was evaluated to be of excellent standard, but, for all that, the project was cancelled because of financial constraints. Nevertheless the phase A study allowed us to identify the optical and mechanical challenges of balloon projects and we were able to come up with a simple design, that fulfilled all the requirements. The 900 mm primary mirror and the rest of the optics were designed to be supported by a sandwich-panel composite structure with carbon epoxy skins and aluminum honeycomb core to improve the mechanical stiffness and the thermal behavior of the instrument without increasing its mass or its complexity.
In this paper, we describe the optical design and the mechanical structure of the instrument. Finite element analysis is carried out to estimate the gravitational flexure and the thermal deformations, which can both harm the pointing accuracy and the performances of the instrument. These simulations show that the proposed design would fulfill the different requirements (pointing accuracy, landing survival as well as the dynamic behavior).
The Observatoire de Paris is constructing a prototype Small-Sized Telescope (SST) for the Cherenkov Telescope Array
(CTA), named SST-GATE, based on the dual-mirror Schwarzschild-Couder optical design. Considering the mirrors size
and its specific curvature and the optical requirements for the Cherenkov imaging telescope, a non-conventional process
has been used for designing and manufacturing the mirrors of the SST-GATE prototype. Based on machining, polishing
and coating of aluminium bulk samples, this process has been validated by simulation and tests that will be detailed in
this paper after a discussion on the Schwarzschild-Couder optical design which so far has never been used to design
ground based telescopes.
Even if the SST-GATE is a prototype for small size telescopes of the CTA array, the primary mirror of the telescope is 4
meters diameter, and it has to be segmented. Due to the dual-mirror configuration, the alignment is a complex task that
needs a well defined and precise process that will be discussed in this paper.
The Cherenkov Telescope Array (CTA) is an international collaboration that aims to create the world's
largest (ever) Very High Energy gamma-ray telescope array, consisting of more than 100 telescopes
covering an area of several square kilometers to observe the electromagnetic showers generated by
incoming cosmic gamma-rays with very high energies (from a few tens of GeV up to over 100 TeV).
Observing such sources requires - amongst many other things - a large FoV (Field of View). In the
framework of CTA, SST-GATE (Small Size Telescope - GAmma-ray Telescope Elements) aims to
investigate and to build one of the two first CTA prototypes based on the Schwarzschild-Couder (SC)
optical design that delivers a FoV close to 10 degrees in diameter. To achieve the required
performance per unit cost, many improvements in mirror manufacturing and in other technologies are
required. We present in this paper the current status of our project. After a brief introduction of the
very high energy context, we present the opto-mechanical design, discuss the technological tradeoffs
and explain the electronics philosophy that will ensure the telescopes cost is minimised without
limiting its capabilities. We then describe the software nedeed to operate the telescope and conclude
by presenting the expected telescope performance and some management considerations.
In order to mitigate the risks of development of the M4 adaptive mirror for the E-ELT, CILAS has proposed to build a
demonstration prototype and breadboards dedicated to this project. The objectives of the demonstration prototype
concern the manufacturing issues such as mass assembly, integration, control and polishing but also the check the global
dynamical and thermal behaviour of the mirror. The local behaviour of the mirror (polishing quality, influence function,
print through...) is studied through a breadboard that can be considered as a piece of the final mirror. We propose in this
paper to present our breadboard strategy, to define and present our mock-up and to comment the main results and lessons
learned.
CILAS proposes a M4 adaptive mirror (M4AM) that corrects the atmospheric turbulence at high frequencies and residual
tip-tilt and defocus due to telescope vibrations by using piezostack actuators. The design presents a matrix of 7217
actuators (triangular geometry, spacing equal to 29 mm) leading to a fitting error reaching the goal. The mirror is held by
a positioning system which ensures all movements of the mirror at low frequency and selects the focus (Nasmyth A or B)
using a hexapod concept. This subsystem is fixed rigidly to the mounting system and permits mirror displacements. The
M4 control system (M4CS) ensures the connection between the telescope control/monitoring system and the M4 unit - positioning system (M4PS) and piezostack actuators of the M4AM in particular. This subsystem is composed of
electronic boards, mechanical support fixed to the mounting structure and the thermal hardware. With piezostack
actuators, most of the thermal load is minimized and dissipated in the electronic boards and not in the adaptive mirror.
The mounting structure (M4MS) is the mechanical interface with the telescope (and the ARU in particular) and ensures
the integrity and stability of M4 unit subsystems. M4 positioning system and mounting structure are subcontracted to
AMOS company.
Increasing dimensions of ground based telescopes and adaptive optics needs for these instruments require wide
deformable mirrors with a high number of actuators to compensate the effects of the atmospheric turbulence on the wave
fronts. The new dimensions and characteristics of these deformable mirrors lead to the apparition of structural vibrations,
which may reduce the rejection band width of the adaptive optics control loop.
The aim of this paper is the study of the dynamic behavior of a
1-meter prototype of E-ELT's deformable mirror in order
to identify its eigenmodes and to propose some ways to control its vibrations. We first present the first eigenmodes of the
structure determined by both finite element analysis and experimental modal analysis. Then we present the frequency
response of the prototype to a tilt excitation to estimate the effects of its vibrations on the adaptive optics loop. Finally
we suggest a method to control the dynamics of the deformable mirror.
ESA's cornerstone mission Gaia will construct a billion-star catalogue down to magnitude 20 but will only provide
detailed chemical information for the brighter stars and will be lacking radial velocity at the faint end due to
insufficient Signal-to-Noise Ratios (SNR). This calls for the deployment of a ground spectrograph under time
scales coherent with those of Gaia for a complementary survey.
The GYES instrument is a high resolution (~ 20,000) spectrometer proposed for installation on the Canada-
France-Hawaii Telescope (CFHT) to perform this survey in the northern hemisphere. It exploits the large Field
of View (FoV) available at the prime focus together with a high multiplex (~ 500 fibres) to achieve a SNR of 30
in two hours at magnitude 16 and render the survey possible on the order of 300 nights. The on-going feasibility
study aims at jointly optimising all components of the system: the field corrector, the positioner, the fibres
and the spectrograph. The key challenges consist in accommodating the components in the highly constrained
environment of the primary focus, as well as in achieving maximum efficiency thanks to high transmission
and minimum reconfiguration delays. Meanwhile, for GYES to have its first light at the time of Gaia's initial
data release (2014-2015), it is mandatory to keep its complexity down by designing a predominantly passive
instrument.
Increasing dimensions of ground based telescopes while implementing Adaptive Optics systems to cancel both structural
deformations and atmospheric effects require very large diameters deformable mirrors (DM) and a high number of
actuators with large strokes. This has led for the future E-ELT to a 2.5 m diameter DM getting about 8000 actuators.
This paper presents a local and a global model of the DM in order to both study its influence function and its dynamical
behavior. In the first part, influence function of the mirror is calculated. Results obtained by an analytical way are
compared to those obtained numerically. In the second part, modal analysis of the mirror is presented. Results are limited
to the first modes. Modal analysis is also only made for the base plate to derive the specific influence of DM's
components on the global dynamic behavior. In the last part, optimization methods are used to help designing a 1 m
prototype of the DM.
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