B. Sedghi

E-ELT primary mirror control system

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.

Field stabilization (tip/tilt control) of E-ELT

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.

Acceleration feedback control on an AT

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 are presented.

Perturbation rejection control strategy for OWL

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.

Dynamical aspects in control of E-ELT segmented primary mirror (M1)

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 loops.

Improving E-ELT M1 prototype hard position actuators with active damping

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.

AOF: standalone test results of GALACSI

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.

GALACSI integration and functional tests

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.

E-ELT M1 test facility

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.

E-ELT modeling and simulation toolkits: philosophy and progress status

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.