Today several methods are developed and used to integrate structural FEA and optical analyses results, enabling detailed prediction of our instrument performance. This publication gives an overview of some of these developments and the choices and challenges that currently exist without claiming to be complete. The focus is on the combination of Structural, Thermal and Optical Performance analyses, also known as ‘STOP’. These analysis solutions are available for projects large or small and can be combined with existing analysis packages.
Challenges identified related to start using STOP analysis are related to knowing which S/W solutions are available, and secondly how to define the integrated analysis process that fits with existing ‘design and analyses’ experience.
These challenges and implementation options are listed in this publication with the aim to encourage the integration of engineering analysis. By applying this integrated analysis process, the risk of finding mistakes and design flaws late in the project are reduced, avoiding delays and additional costs.
The European Extremely Large Telescope (ELT) is a 39-m Class telescope with active and adaptive capability included into the telescope being developed by the European Southern Observatory (ESO).
The Telescope Secondary (M2) and Tertiary (M3) Mirrors are 4-metre class Zerodur mirrors close to 3.2 Tons that are passively supported by Cells with an 18 points axial whiffletree and a warping harness system that allows to correct low order deformations of the Mirrors. Laterally the Mirrors are supported on 12+2 points by Lateral Supports. In addition, the Cells have alignment capabilities by means of a high precision hexapod.
SENER has been contracted by ESO for the design, construction, validation and delivery of the ELT Secondary Mirror (M2) and Tertiary Mirror (M3) Cells.
The Cells’ mechanisms guarantee the alignment of the Telescope during observation while correcting optics deformations. In this process, a high precision hexapod will be responsible for aligning and tracking the mirrors and an active structure will be used to compensate errors in the mirrors’ surface. These are large-size critical elements of the Telescope that require extremely high precision levels to give the Telescope optimal image quality.
This manuscript describes the preliminary design and key aspects of the ELT M2 and M3 Mirrors Cells mechanisms, in particular the Mirror Suppor.
The construction of a diffraction limitable telescope as large as the ESO’s ELT is enabled by its embedded deformable quaternary mirror. Besides its essential function in the telescope control, M4 also contributes to compensating the free atmosphere aberrations for all post-focal AO applications. The paper presents how the telescope manages M4 to maintain its optical performance while offering to the instruments a clean wavefront interface, supporting the desired AO functionalities. The paper reviews the telescope strategy to derive its wavefront dynamic properties directly from the analysis of the control data collected in science mode, with the goal to minimize the observatory time spent on dedicated wavefront calibration tasks.
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). This paper provides an update of the specifications, design, and manufacturing of the ESO ELT optical systems
The E-ELT primary mirror is 39m in diameter composed of 798 segments. It is exposed to external large but slow amplitude perturbations, mostly gravity, thermal and wind. These perturbations are efficiently rejected by a combination of edge sensor loop and adaptive optics (AO) in order to leave a small residual wavefront error (WFE). Vibrations induced by various equipment in the observatory are typically smaller amplitude but higher frequency perturbations exceeding the rejection capabilities of these control loops. They generate both, low spatial frequency and high spatial frequency WFE. Especially segment phasing errors, i.e. high spatial frequency errors, cannot be compensated by AO. The effect of vibrations is characterized by excitation sources and transmission of the telescope structure and segment support. They all together define the WFE caused by M1 due to vibrations. It is important to build a proper vibration error budget and specification requirements from an early stage of the project. This paper presents the vibration analysis and budgeting approach developed for E-ELT M1 and addresses the impact of vibrations onto WFE.
In this paper we will briefly revisit the optical vibration measurement system (OVMS) at the Large Binocular Telescope (LBT) and how these values are used for disturbance compensation and particularly for the LBT Interferometer (LBTI) and the LBT Interferometric Camera for Near-Infrared and Visible Adaptive Interferometry for Astronomy (LINC-NIRVANA). We present the now centralized software architecture, called OVMS+, on which our approach is based and illustrate several challenges faced during the implementation phase. Finally, we will present measurement results from LBTI proving the effectiveness of the approach and the ability to compensate for a large fraction of the telescope induced vibrations.
The E-ELT dynamical modeling toolkit is used extensively to understand the effect of vibrations from observatory equipments on the final performance of the telescope. The dynamical and control modeling toolkit uses the finite element model of the telescope structure and mirror units, the optical sensitivity and knowledge of the wavefront control correction capability to estimate the transmission of vibration from potential vibrational sources to the wavefront error. In addition, it helps i) to identify the sensitive optical units and sensitive vibrational sources and the frequency intervals they might affect most the wavefront error, ii) to perform design trade-offs, and iii) to derive subsystem specification requirements. In this paper, a vibration budgeting approach for the E-ELT using the modeling toolkit is presented.
The E-ELT is an active and adaptive 39-m telescope, with an anastigmat optical solution (5 mirrors including two flats), currently being developed by the European Southern Observatory (ESO). The convex 4-metre-class secondary mirror (M2) is a thin Zerodur meniscus passively supported by an 18 point axial whiffletree. A warping harness system allows to correct low order deformations of the M2 Mirror. Laterally the mirror is supported on 12 points along the periphery by pneumatic jacks. Due to its high optical sensitivity and the telescope gravity deflections, the M2 unit needs to allow repositioning the mirror during observation. Considering its exposed position 30m above the primary, the M2 unit has to provide good wind rejection. The M2 concept is described and major performance characteristics are presented.
As part of the preparation for the arrival of the MUSE instrument to the VLT, it was required to adapt the hosting
telescope (UT4) guide probe, to increase its back focal length. This is to allow enough space for the later deployment of
the MUSE Adaptive Optics module GALACSI, in-between the telescope adapter rotator and the instrument itself. The
UT guide probe is a critical component for the successful operation of the telescope, so its modification to increase the
telescope’s back focal length, while maintaining full compatibility with the existing operation model and other hardware,
was rather demanding.
The design, manufacture, assembly and test for the new supporting arm in the UT guiding probe is presented. It mixes
the use of novel materials (HB-CESIC® for the mirrors substrates) and state of the art manufacturing techniques (3D
printing mould production and rapid casting for the support structure), which allow producing easily a high performance
subsystem. Characterization of the system prior delivery to the telescope, its integration in the UT and results after
commissioning is presented. Its successful implementation has validated new manufacturing techniques that may prove
very useful for future instruments development.
The Enhanced Resolution Imager and Spectrograph (ERIS) is the next-generation adaptive optics near-IR imager and
spectrograph for the Cassegrain focus of the Very Large Telescope (VLT) Unit Telescope 4, which will soon make full
use of the Adaptive Optics Facility (AOF). It is a high-Strehl AO-assisted instrument that will use the Deformable
Secondary Mirror (DSM) and the new Laser Guide Star Facility (4LGSF). The project has been approved for
construction and has entered its preliminary design phase. ERIS will be constructed in a collaboration including the Max-
Planck Institut für Extraterrestrische Physik, the Eidgenössische Technische Hochschule Zürich and the Osservatorio
Astrofisico di Arcetri and will offer 1 - 5 μm imaging and 1 - 2.5 μm integral field spectroscopic capabilities with a high
Strehl performance. Wavefront sensing can be carried out with an optical high-order NGS Pyramid wavefront sensor, or
with a single laser in either an optical low-order NGS mode, or with a near-IR low-order mode sensor. Due to its highly
sensitive visible wavefront sensor, and separate near-IR low-order mode, ERIS provides a large sky coverage with its 1’
patrol field radius that can even include AO stars embedded in dust-enshrouded environments. As such it will replace,
with a much improved single conjugated AO correction, the most scientifically important imaging modes offered by
NACO (diffraction limited imaging in the J to M bands, Sparse Aperture Masking and Apodizing Phase Plate (APP)
coronagraphy) and the integral field spectroscopy modes of SINFONI, whose instrumental module, SPIFFI, will be
upgraded and re-used in ERIS. As part of the SPIFFI upgrade a new higher resolution grating and a science detector
replacement are envisaged, as well as PLC driven motors. To accommodate ERIS at the Cassegrain focus, an extension
of the telescope back focal length is required, with modifications of the guider arm assembly. In this paper we report on
the status of the baseline design. We will also report on the main science goals of the instrument, ranging from exoplanet
detection and characterization to high redshift galaxy observations. We will also briefly describe the SINFONI-SPIFFI
upgrade strategy, which is part of the ERIS development plan and the overall project timeline.
The fifth mirror unit (M5) of the E-ELT is a field stabilization unit responsible to correct for the dynamical tip
and tilt caused mainly due to the wind load on the telescope. The unit is composed of: i) an electromechanical
subunit, and ii) an elliptical mirror with a size of approximately 2.4 by 3-m. The M5 unit has been designed
and prototyped using a three point support for the mirror actuated by piezo actuators without the need of a
counter weight system. To be able to meet the requirements of the telescope, i.e. sufficient wavefront rejection
capability, the unit shall exhibit a sufficient bandwidth for tip/tilt reference commands. In the presence of the
low damped mechanical resonant modes, such a bandwidth can be guaranteed thanks to an active damping loop.
In this paper, different active damping strategies for the M5 unit are presented. The efficiency of the approaches
are analyzed using a detailed model of the unit. On a scale-one prototype active damping was implemented and
the efficiency was demonstrated.
During the advanced design phase of the European Extremely Large Telescope (E-ELT) several critical components
have been prototyped. During the last year some of them have been tested in dedicated test stands. In particular, a
representative section of the E-ELT primary mirror has been assembled with 2 active and 2 passive segments. This test
stand is equipped with complete prototype segment subunits, i.e. including support mechanisms, glass segments, edge sensors, position actuators as well as additional metrology for monitoring. The purpose is to test various procedures such as calibration, alignment and handling and to study control strategies. In addition the achievable component and subsystem performances are evaluated, and interface issues are identified. In this paper an overview of the activities related to the E-ELT M1 Test Facility will be given. Experiences and test results are presented.
A 40 meters class telescope does require adaptive optics to provide few milli arcseconds resolution images. In the current
design of the E-ELT, M4 provides adaptive correction and has also to cancel part of telescope wind shaking and static
aberrations. The 2.4 meters adaptive mirror will provide as well Nasmyth focus selection.
We will present the main design drivers and the main specifications quaternary mirror will have to meet. We will discuss
what the challenges are in term of stability and performance of the associated key technologies. We will finally describe
the current baseline design and the required schedule and work plan to adequately manufacture the E-ELT quartenary
To predict the performance of the E-ELT three sets of toolkits are developed at ESO: i) The main structure and associated optical unit dynamical and feedback control toolkit, ii) Active optics and phasing toolkit, and iii) adaptive optics simulation toolkit. There was a deliberate policy not to integrate all of the systems into a massive model and tool. The dynamical and control time scale differences are used to separate the simulation environments and tools. Therefore, each toolkit contains an appropriate detail of the problem and holds sufficient overlap with the others to ensure the consistency of the results. In this paper, these toolkits together with some examples are presented.
The dynamical behavior of the primary mirror (M1) has an important impact on the control of the segments and the performance of the telescope. Control of large segmented mirrors with a large number of actuators and sensors and multiple control loops in real life is a challenging problem. In virtual life, modeling, simulation and analysis of the M1 bears similar difficulties and challenges. In order to capture the dynamics of the segment subunits (high frequency modes) and the telescope back structure (low frequency modes), high order dynamical models with a very large number of inputs and outputs need to be simulated. In this paper, different approaches for dynamical modeling and simulation of the M1 segmented mirror subject to various perturbations, e.g. sensor noise, wind load, vibrations, earthquake are presented.
In this paper two case studies, highlighting the role of the integrated modeling in the performance prediction and requirement verification of the E-ELT are presented. First example discusses the effect of the local wind load on the secondary mirror and the global wavefront error. Analysis and simulations showed that such local effect is one of the main contributors to the telescope error budget. A detailed modeling and simulation approach with the possibility to modify some important mechanical and control parameters led to a better understanding of the problem and searching for appropriate mitigation strategies. In the second example the effect of friction devices of the lateral support of the azimuth structure is presented. Such devices were originally proposed by the telescope structure contractor to reduce the induced stresses and force due to the temperature variations. A detailed modeling of the friction devices and its inclusion into the telescope dynamical model revealed that some performance and interface requirements are violated.
In order to evaluate the telescope performance and to derive error budget and stroke allocations a ray-tracing and performance analysis toolkit was developed at the European Southern Observatory (ESO) during project Phase B. Performance estimates and individual error and stroke budget allocations are derived by analyzing the impact of a defined set of perturbations at sub - system level after propagation through a model of the wavefront control. The ray-tracing and performance analysis toolkit is used in parallel to other modeling activities such as structural, control and detailed adaptive optics modeling and provides interfaces to any of those.
The ray-tracing and performance analysis toolkit consists of a set of models each suitable for analyzing sub-systems at a specific temporal and spatial frequency. The impact of quasi - static and dynamic loads is computed by implementing finite element model (FEM) and control model analysis results in optical models which either use linear optical sensitivities or ray - tracing at different levels of resolution.
In the recent past, SCHOTT has demonstrated its ability to manufacture large light weighted ZERODUR® mirror blanks
for telescope projects like the GREGOR solar-telescope, for example. In 2010, SCHOTT was commissioned with a
study aimed at developing a design for the M5 mirror blank of the ESO E-ELT.
The tip and tilt M5 mirror of the European Extremely Large Telescope (E-ELT) requires a demanding approach in light
weighting. The approximately 3.1 m x 2.5 m elliptical plano mirror is specified to a weight of less than 500 kg with high
Eigenfrequencies and low deformation under different inclination angles.
The study was divided into two parts. The first part focused on coming up with an optimized light weighted design with
respect to performance and processability with finite element modeling. In the second part of the study, a concept for the
processing sequence including melting, cold-processing, acid etching and handling of the M5 blank was developed. By
producing a prototype section, SCHOTT demonstrated its ability to manufacture the demanding features, including
pockets 350 mm in depth, thin walls and sloped pocket bottoms.
This paper outlines the results of the design work, processing concept and demonstrator fabrication.
The image motion (tip/tilt) of the telescope is dominated by two types of perturbations: a) atmospheric b)
wind load. The wind load effect on E-ELT can be an order of magnitude higher than the atmospheric effect.
Part of the image motion due to the wind load on the telescope structure is corrected by the main axis control
system (mainly large amplitude, low frequency errors). The residual tip/tilt is reduced by M5 and M4 mirror
units. M5 with its large stroke and relative low bandwidth (higher than main axes) corrects for large amplitude
and low frequency part of the image motion and M4 unit takes the higher frequency parts with smaller stroke
availability. In this paper the two stage control strategy of the E-ELT field stabilization is introduced. The
performance of the telescope due to the wind load and in the presence of the major imperfections in the control
system is presented.
Control of primary segmented mirror of an extremely large telescope with large number of actuators and sensors
and multiple control loops is a complex problem. The designer of the M1 unit is confronted to the dilemma of
trade-off between the relatively though performance requirements and the robust stability of the control loops.
Another difficulty arises from the contradictory requirements of the stiffness of the segment support system and
position actuators for wind rejection on one hand and vibration mitigation on other hand. The presence of low
frequency mechanical modes of the back structure and possible interaction of the large number of control loops
through such structure could be a limiting factor for achieving the required control bandwidths. To address these
issues a better understanding of dynamical behavior of segmented mirror is necessary. This paper addresses the
trade-offs on dynamical aspects of the M1 segmented mirror and the robust stability conditions of various control
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
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
Associated to tracking capabilities, the main axes control system of the E-ELT is the first correcting system in the
chain of control loops for reducing the image motion (tip/tilt) caused by perturbations on the telescope. The main
objective of the closed-loop performance analysis of the axes is to evaluate the trade offs for the choice of control
system hardware, i.e. specification and location of the motors and sensors (encoders/tachometers). In addition,
it defines the design constraints and requirements (actuator stroke and bandwidth) of other correcting systems
in the chain: the field stabilization (M5 unit) and adaptive deformable mirror (M4 unit). In this paper the main
axes control analysis of E-ELT is presented and the performance of telescope in face of external perturbations
such as wind and imperfections of the drive (cogging/ripple) and sensing (noise) systems is evaluated. The
performance metric is the wavefront error at the focal plane which is derived from the mechanical motion of the
telescope's optical elements together with their respective optical sensitivities.
The M5 Field stabilization Unit (M5FU) for European Extremely Large Telescope (E-ELT) is a fast correcting optical
system that shall provide tip-tilt corrections for the telescope dynamic pointing errors and the effect of atmospheric tiptilt
and wind disturbances.
A M5FU scale 1 demonstrator (M5FU1D) is being built to assess the feasibility of the key elements (actuators, sensors,
mirror, mirror interfaces) and the real-time control algorithm. The strict constraints (e.g. tip-tilt control frequency range
100Hz, 3m ellipse mirror size, mirror first Eigen frequency 300Hz, maximum tip/tilt range ± 30 arcsec, maximum tiptilt
error < 40 marcsec) have been a big challenge for developing the M5FU Conceptual Design and its scale 1
The paper summarises the proposed design for the final unit and demonstrator and the measured performances
compared to the applicable specifications.
During the past year the control of the 42m segmented primary mirror of the E-ELT has been studied.
This paper presents the progress in the areas of M1 figure control and control hardware implementation. The critical
issue of coupling through the supporting structure has been considered in the controller design. Different control
strategies have been investigated and from a tradeoff analysis modal control is proposed as a solution addressing the
topics of wind rejection as well as sensor noise in the presence of cross-coupling through the supporting structure.
Various implementations of the M1 Control System have been studied and a centralized architecture has been selected as
baseline. This approach offers maximum flexibility for further iterations. The controller design and main parts of the
control system are described.
Integrated models including optics, structures, control systems, and disturbances are important design tools
for Extremely Large Telescopes (ELTs). An integrated model has been formulated for the European ELT
and it includes telescope structure, main servos, primary mirror segment control system, wind, optics, wavefront
sensors, deformable mirror, and an AO reconstructor and controller. There are three model phases: Initialization,
execution of a solver to determine time responses, and post-processing. In near future, the model will be applied
for performance studies and design trade-offs for the European ELT.
Engineering of complex, large scale systems like the ELT designs currently investigated and developed in Europe and Northern America require powerful and sophisticated tools within specific technical disciplines such as mechanics, optics and control engineering. However, even analyzing a certain component of the telescope like the
telescope structure necessitates a system approach to evaluate the structural effects onto the optical performance.
This paper shows several software tools developed by the European Southern Observatory (ESO) which focus onto the system approach in the analyses: Using modal results of a finite element analysis the SMI-toolbox allows an easy generation of structural models with different sizes and levels of accuracy for the control design and
closed-loop simulations. The optical modeling code BeamWarrior was developed by ESO and Astrium GmbH (Germany) especially for integrated modeling and interfering with a structural model. Within BeamWarrior displacements and deformations can be applied in an arbitrary coordinate system, and hence also in the global coordinates of the FE model avoiding error prone transformations. In addition to this, a sparse state space model object was developed for Matlab to gain in computational efficiency and reduced memory requirements due to the sparsity pattern of both the structural models and the control architecture. As one result these tools allow
building an integrated model in order to reliably simulate interactions, cross-coupling effects, system responses, and to evaluate global performance. In order to evaluate disturbance effects on the optical performance in openloop more efficiently, an optical evaluation toolbox was built in the FE software ANSYS which performs Zernike decomposition and best-fit computation of the deformations directly in the FE analysis.
The European Southern Observatory (ESO) has started to develop the Integrated Model (IM) for OWL by applying software tools, which were already used for an IM of the Very Large Telescope Interferometer. The OWL IM focuses onto model reliability and computational efficiency. The former is taken into consideration by a step-wise implementation and the usage of both tested tools and validated sub-models. The validation methods contain plausibility checks with hand calculations, experiments, measurements or comparison with other
computational models. The latter is addressed by considering the sparsity pattern of the equations governing the structural dynamics and the control loops and by exerting advanced model reduction techniques for the dynamic model of the telescope structure, which especially try to avoid the loss of local structural flexibility due to a modal truncation. Different condensation methods are shown and their performance is evaluated.
The paper shows the architecture of the integrated model with its components such as telescope structures, optics, control loops and disturbance models for wind load, seismic ground acceleration and atmospheric turbulence. Results illustrating the capability of the model approach are presented.