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 Extremely Large Telescope (ELT) is a 39 meters optical telescope under construction at an altitude of about 3000m in the Chilean Atacama desert. The optical design is based on a novel five-mirror scheme and incorporates adaptive optics mirrors. The primary mirror consists of 798 segments, each 1.4 meters wide. The control of this telescope and of the instruments that will be mounted on it is very challenging, because of its size, the number of sensors and actuators, the computing performance required for the phasing of the primary mirror, the adaptive optics and the correlation between all the elements in the optical path. In this paper we describe the control system architecture, emerging from scientific and technical requirements. We also describe how the procurement strategy (centered on industrial contracts at subsystem level) affects the definition of the architecture and the technological choices. We first introduce the global architecture of the system, with Local Control Systems and a Supervisory Control layer. The Local Control Systems is astronomy-agnostic and isolate the control of the subsystems procured through industrial contracts. The Supervisory Control layer is instead responsible for coordinating the operation of the different subsystems to realize the observation cases identified for the operation of the telescope. The control systems of the instruments interface with the telescope using a well-defined and standardized interface. To facilitate the work of the Consortia responsible for the construction of the instruments, we provide an Instrumentation Control Software Framework. This will ensure uniformity in the design of the control systems across instruments, making maintenance easier. This approach was successfully adopted for the instrumentation of the Very Large Telescope facility. We will analyze the process that was followed for defining the architecture from the requirements and use cases and to produce a design that addresses the technical challenges.
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.
GALACSI is the Adaptive Optics (AO) module that will serve the MUSE Integral Field Spectrograph. In Wide Field Mode it will enhance the collected energy in a 0.2”×0.2” pixel by a factor 2 at 750 nm over a Field of View (FoV) of 1’×1’ using the Ground Layer AO (GLAO) technique. In Narrow Field Mode, it will provide a Strehl Ratio of 5% (goal 10%) at 650 nm, but in a smaller FoV (7.5”×7.5” FoV), using Laser Tomography AO (LTAO). Before being ready for shipping to Paranal, the system has gone through an extensive testing phase in Europe, first in standalone mode and then in closed loop with the DSM in Europe. After outlining the technical features of the system, we describe here the first part of that testing phase and the integration with the AOF ASSIST (Adaptive Secondary Setup and Instrument Stimulator) testbench, including a specific adapter for the IRLOS truth sensor. The procedures for the standalone verification of the main system performances are outlined, and the results of the internal functional tests of GALACSI after full integration and alignment on ASSIST are presented.
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.
During the last 2 years ESO has operated the “M1 Test Facility”, a test stand consisting of a representative section of the E-ELT primary mirror equipped with 4 complete prototype segment subunits including sensors, actuators and control system. The purpose of the test facility is twofold: it serves to study and get familiar with component and system aspects like calibration, alignment and handling procedures and suitable control strategies on real hardware long before the primary mirror (hereafter M1) components are commissioned. Secondly, and of major benefit to the project, it offered the possibility to evaluate component and subsystem performance and interface issues in a system context in such detail, that issues could be identified early enough to feed back into the subsystem and component specifications. This considerably reduces risk and cost of the production units and allows refocusing the project team on important issues for the follow-up of the production contracts. Experiences are presented in which areas the results of the M1 Test Facility particularly helped to improve subsystem specifications and areas, where additional tests were adopted independent of the main test facility. Presented are the key experiences of the M1 Test Facility which lead to improved specifications or identified the need for additional testing outside of the M1 Test Facility.
The fifth mirror of the European Extremely Large Telescope optical train is a field stabilization tip/tilt unit responsible for correcting the dynamical tip and tilt caused mainly by wind load on the telescope. A scale-one prototype including the inclined support, the fixed frame and a basic control system was designed and manufactured by NTE-SENER (Spain) and CSEM (Switzerland) as part of the prototyping and design activities. All interfaces to the mirror have been reproduced on a dummy structure reproducing the inertial characteristics of the optical element. The M5 unit is required to have sufficient bandwidth for tip/tilt reference commands coming from the wavefront control system. Such a bandwidth can be achieved using local active damping loop to damp the low frequency mechanical modes before closing a position loop. Prototyping on the M5 unit has been undertaken in order to demonstrate the E-ELT control system architecture, concepts and development standards and to further study active damping strategies. The control system consists of two nested loops: a local damping loop and a position loop. The development of this control system was undertaken following the E-ELT control system development standards in order to determine their applicability and performance and includes hardware selection, communication, synchronization, configuration, and data logging. In this paper we present the current status of the prototype M5 control system and the latest results on the active damping control strategy, in particular the promising results obtained with the method of positive position feedback.
GALACSI is the Adaptive Optics (AO) modules of the ESO Adaptive Optics Facility (AOF) that will correct the wavefront delivered to the MUSE Integral Field Spectrograph. It will sense with four 40×40 subapertures Shack-Hartmann wavefront sensors the AOF 4 Laser Guide Stars (LGS), acting on the 1170 voice-coils actuators of the Deformable Secondary Mirror (DSM). GALACSI has two operating modes: in Wide Field Mode (WFM), with the four LGS at 64” off axis, the collected energy in a 0.2”×0.2” pixel will be enhanced by a factor 2 at 750 nm over a Field of View (FoV) of 1’×1’ using the Ground Layer AO (GLAO) technique. The other mode, the Narrow Field Mode (NFM), provides an enhanced wavefront correction (Strehl Ratio (SR) of 5% (goal 10%) at 650 nm) but in a smaller FoV (7.5”×7.5”), using Laser Tomography AO (LTAO), with the 4 LGS located closer, at 10” off axis. Before being shipped to Paranal, GALACSI will be first integrated and fully tested in stand-alone, and then moved to a dedicated AOF facility to be tested with the DSM in Europe. At present the module is fully assembled, its main functionalities have been implemented and verified, and AO system tests with the DSM are starting. We present here the main system features and the results of the internal functional tests of GALACSI.
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.
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.
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.
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.
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 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
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 drive and bearing technologies have a major impact on the static and dynamic performance of steerable
structures such as telescope and dome. Merging drive and bearing system into friction drive mechanical devices
(bogie) can reduce the complexity and cost of the design. In the framework of ELT design study (European
FP6) a breadboard test setup was realized to test and evaluate the static and dynamic behavior of such bogies.
In this paper some of the characterization test results are presented. Characterization of the bogies and the
setup structure in the frequency domain, quantification and measure of the most important parameters of the
friction forces, the control of the bogies and the tracking performance of the test setup are among the main
results discussed in this paper.
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.
he VLT observatory operated by ESO is located on Cerro Paranal in
Chile and consists of four identical 8-m telescopes and four 1.8-m
VLTI Auxiliary telescopes (ATs). In order to further improve the
tracking axes performance of telescopes regarding wind rejection,
different control techniques have been evaluated. Ongoing investigation and studies show that by measuring the
acceleration and using that in appropriate control strategy the
performance of telescope tracking in face of external perturbation
can be improved. The acceleration signal contains the non filtered
information (advanced phase compared to velocity and position) of
the perturbation load, e.g. wind load. As a result the reaction of
the control is faster and hence the perturbation rejection is more
efficient. In this paper, two acceleration feedback techniques are
discussed and the results of the measurement test on an AT telescope
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.
The Active Segmented Mirror is a key subsystem of the Active Phasing Experiment. The size of the ASM is 154 mm in diameter. It will be used to test new types of phasing sensors recently developed within the ELT design study supported by the European Union. To our knowledge it is the first time that such miniature active optics composed of hexagonal segments having 3 degrees of freedom with a resolution of the order of a few nanometers and a range of several micrometers is manufactured. The ASM is composed of 61 hexagonal segments called "modules". Each module is assembled, glued and integrated from standard (piezo-actuators) and custom-made (mirrors, mechanics) parts procured from industries. The ASM has been designed and integrated at the European Southern Observatory. Specifications, designs, assembly tools, hand work skills, electronics, software, control algorithms and test procedures are the field of competences required to obtain in the end a "plug and play" product. The concept of the ASM is tested and validated by a prototype version composed of 7 modules equivalent of the central area of the ASM itself. The design, integration and results of the ASM tests are presented.
The purpose of the Active Phasing Experiment, designed under the lead of ESO, is to validate wavefront control concepts for ELT class telescopes. This instrument includes an Active Segmented Mirror, located in a pupil image. It will be mounted at a Nasmyth focus of one of the Unit Telescopes of the ESO VLT. APE contains four different types of phasing sensors, which are developed by Istituto Nazionale di Astrofisica in Arcetri, Instituto Astrofisica Canarias, Laboratoire d'Astrophysique de Marseille and ESO. These phasing sensors can be compared simultaneously under identical optical and environmental conditions. All sensors receive telecentric F/15 beams with identical optical quality and intensity. Each phasing sensor can measure segmentation errors of the active segmented mirror and correct them in closed loop. The phasing process is supervised by an Internal Metrology system developed by FOGALE Nanotech and capable of measuring piston steps with an accuracy of a few nanometers. The Active Phasing Experiment is equipped with a turbulence generator to simulate atmospheric seeing between 0.45 and 0.85 arcsec in the laboratory. In addition, the Active Phasing Experiment is designed to control simultaneously with the phasing corrections the guiding and the active optics of one of the VLT Unit Telescopes. This activity is supported by the European Community (Framework Programme 6, ELT Design Study, contract No 011863).
The control system of the ESO 100m telescope (OWL) has to reject slow and fast perturbations in several subsystems. In this paper we focus on the wind rejection control strategies for two subsystems: the main axes and the segmented mirror. It is shown that facing the same disturbance the 2 control designs have to deal with completely different problems: control of a flexible SISO (Single input-Single output) system for the altitude axis versus a dynamically coupled MIMO (Multi input-Multi output) system for the segmented mirror. For both subsystems feasible solutions are given.
The "phase A" of the opto-mechanical design, which started in 1997, is now basically completed. It provides a clean, symmetrical geometry of the pupil, with a near-circular outer edge. The modular design of the mechanical structure is built on the size of the hexagonal segments, provides a perfect match with the optical elements and allows production at reasonable costs. This paper is a summary of the various design iterations. A discussion is devoted to the evaluation of the design assumptions and principles which have been set at the beginning of the study, and to their validity after the completion of this first phase. This includes a discussion about specific aspects whose criticality had been under- or overestimated, and the methodology applied to define system and sub-system requirements. Finally, we present a summary of the present and future activities, which are mainly devoted to sub-systems definition.