The Dome of the Extremely Large Telescope (ELT) is under construction at Cerro Armazones, in the Chilean Andes. It is constituted by a concrete pier, with an 86 m diameter concrete wall, and a rotating enclosure on it; the maximum height is about 80 m. The Dome will protect the 40 m class optical telescope, inside it, and must withstand wind speeds of over 40 m/s, as well as strong earthquakes. The whole structure is seismically isolated at the base, for an overall seismic mass of about 35000 t. The rotating enclosure main elements are a truss steel structure, having a base ring and a series of arch girders. It has a hemispherical shape to enhance the aerodynamic behavior and it weighs close to 6500 t. Two slit doors allow the telescope observation, guaranteeing a 42 m wide and 64 m long opening. The enclosure’s Azimuth Rotation Mechanism is constituted by 36 trolleys, installed on the top beam of the concrete pier, on a diameter of 86 m. Cladding covers the Dome structure and it is designed in order to provide proper thermal insulation and to withstand the harsh environmental site conditions. A windscreen, made of four permeable panels, having 42 m span and 10 m height each, protects the Telescope during observation and controls the airflow around it, together with a series of 89 louvers, placed both on the rotating and the fixed part of the Dome. In the Auxiliary Building, which is a ring surrounding the pier, technical rooms to operate and maintain the telescope are hosted. A custom HVAC system controls the temperature with a ±2 °C precision inside the Telescope chamber having about 300000 m3 volume.
The telescope Main Structure and the Dome of the ELT are being procured by ESO as an integrated system together with the auxiliary building and the service plants, and are supplied by the ACe Consortium. From the start of the contract, signed in May 2016, major progress has been achieved. During the design phase various challenging technical issues had to be solved, not least given the size of the ELT and the environmental conditions at Cerro Armazones in Chile. As per today the system has been developed, detail designed, reviewed, and is completing the Final Design Review process. Procurement of long lead or schedule critical items was assured by using specific Critical Design Reviews. Manufacturing in Europe and construction in Chile have started and are proceeding, although delays were encountered due to various issues including the Covid-19 pandemic. In this paper we will describe the advancement reached by the project and discuss some technical aspects associated to the design. The status of the design, of the manufacturing and of the construction on site will be described.
The Dome and the telescope Main Structure of the ELT are being procured by ESO as an integrated unit, which includes also the technical buildings needed to host the various system plants and the primary mirror maintenance facilities. At conclusion of a Call for Tender, ESO has signed in May 2016 a contract with the Italian Consortium ACe for the design, manufacture, transport and construction at Cerro Armazones in Chile of the Dome and Telescope Structure (DMS) of the ELT. The Consortium is constituted by Astaldi S.p.A. and Cimolai S.p.A. The first step of the design phase, namely the Preliminary Design Review (PDR) of both Dome and Main Structure has been completed, and rapid progress is being achieved toward the completion of the detail design. For some long lead items, the procurement via Critical Design Reviews (CDRs) is taking place in parallel to the detail design phase. The Contractor was given access to the Armazones site, including the levelled ELT platform in June 2017, and he has prepared at the base of the Armazones peak a large construction camp with dormitories and construction facilities. At the top of the mountain the excavation for the ELT foundations are completed, and foundation construction is starting. The overall status of the procurement of the DMS and some key aspects of the design will be described herein.
The 25 European antennas of ALMA were delivered by ESO to the ALMA project in Chile between April 2011 and September 2013. Their combined time of operation is already significant and allows us to draw conclusions regarding their ability to fulfil the original specification, in terms of both scientific performance and operational availability. In this paper, we will summarize the experience gained during the past five years of operation. We will characterize the performance of the antennas in routine operation and compare with the data obtained during acceptance testing. We will also describe the few technical issues experienced while operating at 5000m and the way in which these were treated during these first years of operation. We will evaluate the effective reliability obtained in service based on field data and draw some conclusions as to the way in which reliability and maintainability aspects were covered during the process which led to the final design of the antenna. We will discuss the smart use of software to handle redundancy in a flexible way and to exclude failed components without affecting overall antenna operability. The use of low-level diagnostics enabled by remote access allows us to shorten the trouble-shooting cycle and to optimise physical interventions on the antennas. Finally, the paper will cover Antenna maintenance manuals edited using an industrial interactive standard. It will be explained why this advanced and innovative concept has not achieved the success that was expected, and why the traditional form is preferred at the ALMA Observatory.
The Atacama Large Millimeter Array (ALMA) consists of a large number of 12 m diameter antennas that will operate up
to 950GHz. The mechanical performances in terms of surface accuracy, pointing stability and residual delay are very
tight. The antennas must work at full performances in free air during night and day with also the request to observe the
sun. The mechanical performances are affected by all the not repeatable error sources and in particular by the
temperature variations and wind component blowing from different directions. The design of the antenna has been done
in order to have a very light and stiff structure, in particular all the elevation structure is in carbon fibre with also a very
low thermal expansion coefficient, but to achieve the ALMA specification, two different systems able to predict the
above error sources have been implemented in the control of the antenna.
The first system is composed by a determined number of thermal sensors distributed in the alidade of the antenna (is the
only part in steel ) and compensates the elevation axis deformation due to the temperature variation by means of a
deformation matrix.
The second system is based on two high accuracy inclinometers with a very short recovery time opportunely placed on
the antenna and correct the wind induced errors.
These innovative systems and instruments have been design and tested in the prototype antenna to the production phase.
In the ALMA project, in order to achieve the specification, very important is the use of a metrology instrument able to
correct the non repeatable errors. Errors induced by wind or temperature variations inside the structure that are non
systematic have to be predicted and corrected by an active system. During design phase, CFD and thermal analyses have
been performed in order to identify the whole behaviour of the structure and the relative corrections formulas. Innovative
instruments and layout have been studied from the prototype to the production phase.
The first European ALMA production antenna is in the final stage of manufacturing and pre-assembly. Precommissioning
of parts of this antenna will start soon. The series production is partly ongoing or in the final stage of
preparation. Design features of the European ALMA antennas w.r.t. to manufacturing, assembly and integration in
Europe and on site, the status of the manufacturing works and
pre-commissioning are presented. An outlook to the next
steps of the realization of the antennas is given.
Systems Engineering has been used throughout the development of the Visible and Infrared Survey Telescope for Astronomy (VISTA). VISTA was originally conceived as being a classic 4m telescope with wide-field imaging capability. The UK Astronomy Technology Centre (UK ATC) radically changed this thinking by treating the whole design as one system, integrating the camera optics into the telescope design.
To maximise the performance, an f/1 primary mirror was adopted resulting in a very compact telescope and enclosure. Amongst other benefits, this reduced the overall mass of the telescope from 250 to 90 tonnes. During this optimisation process, the concept of a direct imaging K-short camera was developed. This development, in conjunction with an increase in IR field of view, produced a system with uniform image quality and throughput across a 350 mm diameter focal plane, 1.65 degree field.
While this has presented some major engineering challenges, the approach has produced a system which is both scientifically rewarding and achievable. The optimisation, design trade-offs and Technical Specification developed in the conceptual design phase were achieved through a systems analysis approach.
This paper describes some of the key systems engineering decisions and the tools employed to achieve them. Current systems engineering activities are described and future plans outlined.
The design of VISTA (Visible and Infrared Survey Telescope for Astronomy) requires close interaction between the science requirements, the optical and active mechanical design of the telescope and its instrumentation with the wavefront sensing. The optical design is based on an integrated approach of the telescope with tow separate cameras, one working in the IR waveband and the other working in the Visible waveband. The large field of view (2 degrees in the visible and 1.65 degrees in the IR), the seeing-limited resolution required (FWHM of 0.4 arcsec for the visible and 0.5 arcsec for the IR), the technological advance in active telescopes and large IR arrays and the f/1 quasi Ritchey-Chretien telescope design, makes this telescope a very powerful tool in performing high resolution and large astronomical surveys. A system analysis, modeling the various sources of errors such as optical aberrations, surface errors, control errors, environmental effects and detector effects is presented in this paper.
This paper explains the overall mechanical functions of the Very Large Telescope Secondary Mirror Unit (VLT-M2 Unit), and the method of their verification by tests inside two different test adapters. The cinematic mechanisms of the M2 Unit are the Focusing Stage, Centering Stage and the Chopping Assembly which enables to adjust the position of the secondary mirror along 5 degrees of freedom. All these mechanisms are characterized by very high stiffness and accuracy, long lifetime and high reliability. The presentation of the finally achieved performance test results of the M2 Unit, realized by their high precision mechanisms, are the major points of this presentation.
KEYWORDS: Mirrors, Telescopes, Digital signal processing, Control systems, Beryllium, Computer programming, Manufacturing, Signal processing, Kinematics, Mechanics
The mechanics of the secondary unit are located behind the secondary mirror as seen from the telescope focus. The secondary is undersized and defines the pupil of the telescope. There is a focusing and centering drive for slow adjustments of the secondary mirror. In addition, the secondary mirror has a fast beam steering mechanism for chopping and rapid guiding to remove atmospheric wavefront tilt during observations. The specified square wave chopping frequency is 5 Hz with a duty cycle larger than 80%. To achieve a high bandwidth, the secondary mirror is manufactured of light-weighted beryllium coated with nickel. The beam steering mechanism has a counter-vibrating mass to compensate for dynamic forces and moments. The chopping mechanism has been successfully tested. The code of the digital control used during the tests was generated using Matlab real time toolbox. The servos were implemented on a digital signal processor card equipped with a TMS 320C40. To compensate for resonances inside the bandwidth of the servos, a special filter is applied in the velocity loop. The design of the secondary unit is now completed and fabrication and assembly have begun.
The primary mirror cell of the very large telescope supports the primary mirror, the tertiary tower and mirror, and the Cassegrain instrumentation. Stringent requirements have been set to achieve the desired image quality, flexibility of use, and the necessary mirror safety. This paper describes the most important requirements set on the system and some of the design solutions which were chosen.
REOSC has been selected for the design, manufacturing and integration of the four ESO very large telescope (VLT) secondary mirrors. The VLT secondary mirrors are 1.12 m lightweight convex hyperbolic mirrors made of beryllium. Despite the VLT active optics correction capabilities, the use of a metal for the mirror structure implies specific manufacturing processes and associated design rules in order to ensure its dimensional stability during the telescope required life time. This paper describes how the fabrication process of the VLT secondary mirror has been optimized in order to maximize the dimensional stability of its structure. The beryllium properties are analyzed in parallel with the mirror requirements, the choices for the manufacturing, at all levels, are presented. A short work progress is presented, with the achieved mirror properties.
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