The European Extremely Large Telescope (ELT) is a first-of-its-kind project, putting together a range of unique engineering challenges. The performances of its mirror cells are particularly demanding, and their verification has required the development of innovative methodologies. This paper addresses the developed verification methodologies for the most critical performances of the Secondary and Tertiary Mirror Cells (M2 and M3) and the Fifth Mirror Cell (M5). Concerning the monolithic 4-meter class M2 and M3 Cells, that is the maximum surface error (SFE) allowed in operational conditions. The SFE due to manufacturing and integration tolerances is singularly complex to evaluate, and testing is required to ensure compliance of the Cell supports as built. Moreover, such testing must be performed with a non-reflective Dummy Mirror. Hence, a methodology to obtain the SFE from the forces measured on all Mirror supports was developed. The application of uncertainty reduction techniques was essential to obtain reliable conclusions. The M5 Cell supports the largest fast steering mirror (2.7 by 2.2 metres) ever employed in a telescope. The most challenging performances regarding dynamic response are its control bandwidth above 10 Hz, a phase lag below 30 degrees up to 4 Hz, and a cross-coupling below 1% between degrees of freedom. After investigations with an actuator prototype, a method based on application of optimised frequency sweep commands was selected. The test setup was designed to avoid artificial resonant modes, and dedicated postprocessing methods to extract the frequency response were developed. This paper describes in detail the SFE verification methodology for the M2 and M3 Cells, and the tip-tilt capability verification for the M5 Cell. The obtained test results are presented and discussed.
The ELT Phasing and Diagnostic Station (PDS), is a multi-purpose optomechanical system providing metrology tools to phase the segmented primary mirror of the ELT and hosting the sensors required to verify AO-assisted diffraction limited image quality at the ELT. The purpose of the PDS in the context of the ELT lifecycle is twofold. On one side, during the AIV phase of the ELT, the PDS will be the fundamental tool enabling commissioning of the telescope. On the other hand, during operation, the PDS will provide the essential metrology means to monitor performance and detect and isolate potential failures within the observatory. The project, which is one of the most important internal development endeavors at ESO, passed PDR in mid-2021 and underwent an optical final design review in late 2021, where challenges associated to the schedule were identified and more time was given to optimize the design. The project underwent a restructuring in early 2022 before starting its final design phase which has been successfully completed in 2023. In the same period all procurements concerning critical long lead items have been launched. The present contribution first introduces the project in the context of the ELT construction programme, outlining the project structure and the project management tools employed for planning and progress monitoring. Subsequently, the main system engineering processes used within the project will be described. Finally, we report on the main technical results obtained during the final design phase and the plans for the assembly, integration and test of the system.
The European Extremely Large Telescope (ELT) construction1 is advancing and the M2, M3 and M5 cells are running the same path. The M22 and M3 mirrors are 4-meter class Zerodur® meniscus supported axially and laterally by mechanical whiffletrees. To allow low order corrections, axial support is provided with a warping harness system. Additionally, an electromechanical hexapod allows the movement in 5 degrees of freedom to optically align the telescope. The M2 Cell is ending the integration and test phase and approaching to the verification phase, where the highly demanding requirements will be checked. On the other hand, M3 cell is being assembled, taking advantage of the lessons learned during the M2 cell assembly. The M5 mirror is lightweighted silicon carbide elliptical flat mirror of 2.7 x 2.1 m², having the objective of folding the optical beams towards the Nasmyth platforms. Additionally, stabilizes the image movements induced by the telescope mechanisms and wind shaking vibrations. The M5 Cell is divided in two stages: the Tip-tilt and the Alignment Stage. The former allows to perform a fast-steering control while the latter provides active alignment capabilities to cope with the overall telescope structure misalignments The ELT M5 Cell is currently in the final design review, with qualification models for both stages successfully developed and tested. Preliminary results shown submicrometric resolutions for the Alignment Stage and around 5 nanometres for the Tip-Tilt Stage. This paper summarizes the design of the M2, M3 and M5 cells and presents its current development status.
ESO took a systematic approach at earliest phases of the ELT programme to address different aspects of vibration at the telescope, from modelling, error budgeting, requirement specifications, to envisaging verification and mitigation methods. Recent activities focused on measuring and characterizing the vibrational forces generated by typical equipment in the observatory. In addition, the measurements are performed to design and verify the efficiency of the required isolation systems. In this paper, a complete system analysis using these measurement data as input to the detailed model of the telescope structure combined with hosted units, i.e. mirrors, instruments and other equipment, (all at final design phase) is presented. The analysis serves as a verifying tool to observe the actual state of the performance versus the top-level budget. It is also discussed how the results are used for improving the design and envisaging the potential mitigation strategies.
The European Southern Observatory (ESO) is managing the Extremely Large Telescope (ELT) system performance and budgets. The Telescope Main Structure, Hosted Optical Units and Scientific Instruments are designed and built outside ESO by contractors and consortia. Within this scope, system simulations are performed including the coupling between the Telescope Main Structure and its Hosted Units. Both dynamic couplings relevant for control-structure interaction, seismic loads, and micro-vibrations as well as quasi-static deflections are included in these simulations. This paper explains the modeling approach and the first simulation results.
The ELT is a project led by the European Southern Observatory (ESO) for a 40-m class optical, near- and mid-infrared, ground-based telescope. When it will enter into operation, the ESO ELT will be the largest and most powerful optical telescope ever built. It will not only offer unrivalled light collecting power, but also exceedingly sharp images, thanks to its ability to compensate for the adverse effect of atmospheric turbulence on image sharpness. The basic optical solution for the ESO ELT is a folded three-mirror anastigmat, using a 39-m segmented primary mirror (M1), a 4-m convex secondary mirror (M2), and a 4-m concave tertiary mirror (M3), all active. Folding is provided by two additional flat mirrors sending the beams to either Nasmyth foci along the elevation axis of the telescope. The folding arrangement (flat M4 and M5 mirrors) is conceived to provide conveniently located flat surfaces for an adaptive shell (M4) and field stabilization (M5). The M5 Mirror and M5 Cell contracts started in 2019. Both sub-units are currently designed by the selected contractors. While the cell is still in an early design phase, the mirror design is in the final phase and the manufacturing of the blank has already started. With the focus on the M5 mirror, we flow down the key requirements to the cell and the mirror and highlight the main characteristics of the current design, discussing the challenges of mirror manufacturing. Finally, we conclude with the current status and an overview of the coming milestones.
The ELT M4 is the telescope-facility adaptive unit for the European ELT. Final design and construction were awarded in 2015 to AdOptica, a consortium of Microgate and ADS International; on-site delivery is planned for 2024. The unit is based on a monolithic, structural reference body manufactured by Mersen Boostec. The flat thin mirror, controlled using the contactless voice-coil-motor based technology, is split in 6 segments produced by Safran Reosc. The M4 unit is ready for integration: we report here the results of the construction and component level testing, introducing also the forthcoming integration and system-level tests.
ESO took a systematic approach at earliest phases of the ELT programme to address different aspects of vibration at telescope, from modelling, error budgeting, requirement specifications, to envisaging verification and mitigation methods. Recent activities focused on characterisation of the vibrational forces generated by typical equipment in the observatory. Those measured forces combined with the models of the telescope are used to verify various subsystems specification as well as to verify the overall system performance. In this paper, an approach used for vibration force measurements together with some examples of the characterised sources are discussed. The verification of performance/requirements at telescope system/subsystem is performed using these measured data.
The present paper reports on the results of the validation test campaign of the ELT M1 Position Actuators (M1 PACTs). The main function of these actuators is positioning the 798 segments composing the primary mirror (M1) of the ELT with nanometric tracking error over the relatively large stroke of ten millimetres. In order to achieve this challenging goal the PACTs feature an hybrid actuation concept including a spindle drive for large and coarse movements in series with a piezo actuator for fine position adjustments. Active damping techniques are used to ensure robustness and benign dynamic response to perturbations transmitted through the supporting back structure. The design and manufacturing project of the M1 PACTs has recently entered in the final design phase. In this phase extensive validation testing is planned to verify that the final product is fit for purpose throughout its lifecycle. To this end the M1 PACT is first tested in stand-alone mode, with the objective of verifying its performances in a controlled and stable environment and deriving a reliable model of its dynamic response to be exploited for M1 performance estimation. Then several M1 PACTs are integrated in the M1 Test Stand, a small-scale mock-up of the primary mirror of the ELT. In this configuration multiple actuators are driven together to demonstrate relative position control between two segments. On the basis of the obtained results the paper discusses the expected M1 performance and possible control tuning strategies to be used during the commissioning of the ELT in Chile.
The Prefocal Station (PFS) is the last opto-mechanical unit before the telescope focal plane in the Extremely Large Telescope (ELT) optical train. The PFS distributes the telescope optical beam to the Nasmyth and Coudé instrument focal stations and it contains all of the sky metrology (imaging and wavefront sensing) that will be used by the active optics of the telescope and to support operations such as phasing the primary mirror (phasing and diagnostic station). It also hosts local metrology that will be used for coarse alignment and maintenance. We present the main results of a concept design study for the Nasmyth A prefocal station.
SpicA FAR infrared Instrument, SAFARI, is an imaging spectrometer which is being designed to map large areas of the sky in the far infrared. The SPICA mission, having a large cold telescope cooled to 6K above absolute zero, will provide an optimum environment where instruments are limited only by the cosmic background itself.
PHI is a diffraction limited, wavelength tunable, quasi-monochromatic, and polarization sensitive imager. These capabilities are needed to infer the magnetic field and line-of-sight (LOS) velocity of the region targeted by the spacecraft (spacecraft (S/C)).
PHI will consist of two telescopes: The High Resolution Telescope (HRT)[1] and the Full Disk Telescope (FDT). The HRT and the FDT will view the Sun through entrance windows located in the S/C heat shield. These windows act as heat rejecting filters with a transmission band of about 30 nm width centered on the science wavelength, such that the total transmittance (integral over the filter curve weighted with solar spectrum, including white leakage plus transmission profile of the pass band) does not exceed 4% of the total energy falling onto the window [2][3].
The HREW filter has been designed by SELEX in the framework of an ESA led technology development activity under original ESTEC contract No. 20018/06/NL/CP[4], and extensions thereof. For FDT HREW SLEX will provide the windows and it coatings.
The HREW consists of two parallel-plane substrate plates (window 1 & window 2)[5] made of SUPRASIL 300 with a central thickness of 9 mm and a wedge of 30 arcsec each. These two substrates are each coated on both sides with four different coatings. These coatings and the choice of SUPRASIL help to minimize the optical absorptivity in the substrate and to radiatively decouple the HREW, which is expected to run at high temperatures during perihelion passages, from the PHI instrument cavity.
The temperature distribution of the HREW is driven by two main factors: the mechanical mounting of the substrates to the feedthrough, and the radiative environment within the heat-shield/feedthrough assembly.
The mechanical mount must ensure the correct integration of both suprasil substrates in its correct position and minimize the loads in windows due to thermal induced deformations and launching vibration environment.
All the subsystem must survive to a launching vibration environment and fulfill optical requirements in an environmental conditions according o its position in the external part of the spacecraft with a pressure of 0.0013Pa and a temperature -163°C<T<230°C.
The Polarimetric Helioseismic Imager for Solar Orbiter (SO/PHI) is an instrument on board in the Solar Orbiter mission. The Full Disk Telescope (FDT) will have the capability of providing images of the solar disk in all orbital faces with an image quality diffraction-limited. The Heat Rejection Entrance Window (HREW) is the first optical element of the instrument. Its function is to protect the instrument by filtering most of the Solar Spectrum radiation. The HREW consists of two parallel-plane plates made from Suprasil and each surface has a coating with a different function: an UV shield coating, a low pass band filter coating, a high pass band filter coating and an IR shield coating, respectively.
The temperature gradient on the HREW during the mission produces a distortion of the transmitted wave-front due to the dependence of the refractive index with the temperature (thermo-optic effect) mainly. The purpose of this work is to determine the capability of the PHI/FDT refocusing system to compensate this distortion.
A thermal gradient profile has been considered for each surface of the plates and a thermal-elastic analysis has been done by Finite Element Analysis to determine the deformation of the optical elements. The Optical Path Difference (OPD) between the incident and transmitted wavefronts has been calculated as a function of the ray tracing and the thermo-optic effect on the optical properties of Suprasil (at the work wavelength of PHI) by means of mathematical algorithms based on the 3D Snell Law. The resultant wavefronts have been introduced in the optical design of the FDT to evaluate the performance degradation of the image at the scientific focal plane and to estimate the capability of the PHI refocusing system for maintaining the image quality diffraction-limited. The analysis has been carried out considering two different situations: thermal gradients due to on axis attitude of the instrument and thermal gradients due to 1° off pointing attitude. The effect over the boresight at the instrument focal plane has also been analyzed.
The results show that the effect of the FDT HREW thermal gradients on the FDT performance can be optically corrected. The influence of the thermal gradients on the system is also presented.
SpicA FAR infrared Instrument, SAFARI, is one of the instruments planned for the SPICA mission. The SPICA
mission is the next great leap forward in space-based far-infrared astronomy and will study the evolution of galaxies,
stars and planetary systems. SPICA will utilize a deeply cooled 2.5m-class telescope, provided by European industry, to
realize zodiacal background limited performance, and high spatial resolution. The instrument SAFARI is a cryogenic
grating-based point source spectrometer working in the wavelength domain 34 to 230 μm, providing spectral resolving
power from 300 to at least 2000.
The instrument shall provide low and high resolution spectroscopy in four spectral bands. Low Resolution mode is the
native instrument mode, while the high Resolution mode is achieved by means of a Martin-Pupplet interferometer.
The optical system is all-reflective and consists of three main modules; an input optics module, followed by the Band
and Mode Distributing Optics and the grating Modules. The instrument utilizes Nyquist sampled filled linear arrays of
very sensitive TES detectors.
The work presented in this paper describes the optical design architecture and design concept compatible with the
current instrument performance and volume design drivers.
This paper describes the optical design of the far infrared imaging spectrometer for the JAXA’s SPICA mission. The SAFARI instrument, is a cryogenic imaging Fourier transform spectrometer (iFTS), designed to perform backgroundlimited spectroscopic and photometric imaging in the band 34-210 μm. The all-reflective optical system is highly modular and consists of three main modules; input optics module, interferometer module (FTS) and camera bay optics. A special study has been dedicated to the spectroscopic performance of the instrument, in which the spectral response and interference of the instrument have been modeled, as the FTS mechanism scans over the total desired OPD range.
The Wind Evaluation Breadboard (WEB) for the European Extremely Large Telescope (ELT) is a primary mirror and
telescope simulator formed by seven segments simulators, including position sensors, electromechanical support systems
and support structures. The purpose of the WEB is to evaluate the performance of the control of wind buffeting
disturbance on ELT segmented mirrors using an electro-mechanical set-up which simulates the real operational
constrains applied to large segmented mirrors. The instrument has been designed and developed by IAC, ALTRAN,
JUPASA and ESO, with FOGALE responsible of the Edge Sensors, and TNO of the Position Actuators. This paper
describes the mechanical design and analysis, the control architecture, the dynamic model generated based on the Finite
Element Model and the close loop performance achieved in simulations. A comparison in control performance between
segments modal control and actuators local control is also presented.
WEB, the Wind Evaluation Breadboard, is an Extremely Large Telescope Primary Mirror simulator, developed with the
aim of quantifying the ability of a segmented primary mirror to cope with wind disturbances. This instrument supported
by the European Community (Framework Programme 6, ELT Design Study), is developed by ESO, IAC, MEDIA-ALTRAN,
JUPASA and FOGALE. The WEB is a bench of about 20 tons and 7 meter diameter emulating a segmented
primary mirror and its cell, with 7 hexagonal segments simulators, including electromechanical support systems.
In this paper we present the WEB central control electronics and the software development which has to interface with:
position actuators, auxiliary slave actuators, edge sensors, azimuth ring, elevation actuator, meteorological station and air
balloons enclosure. The set of subsystems to control is a reduced version of a real telescope segmented primary mirror
control system with high real time performance but emphasizing on development time efficiency and flexibility, because
WEB is a test bench. The paper includes a detailed description of hardware and software, paying special attention to real
time performance.
The Hardware is composed of three computers and the Software architecture has been divided in three
intercommunicated applications and they have been implemented using Labview over Windows XP and Pharlap ETS
real time operating system. The edge sensors and position actuators close loop has a sampling and commanding
frequency of 1KHz.
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