Mark J. Sirota

Analysis of TMT Primary Mirror Control-Structure Interaction

The primary mirror control system (M1CS) keeps the 492 segments of the Thirty Meter Telescope primary mirror aligned in the presence of disturbances. A global position control loop uses feedback from inter-segment edge sensors to three actuators behind each segment that control segment piston, tip and tilt. If soft force actuators are used (e.g. voice-coil), then in addition to the global position loop there will be a local servo loop to provide stiffness. While the M1 control system at Keck compensates only for slow disturbances such as gravity and thermal variations, the M1CS for TMT will need to provide some compensation for higher frequency wind disturbances in order to meet stringent error budget targets. An analysis of expected high-wavenumber wind forces on M1 suggests that a 1Hz control bandwidth is required for the global feedback of segment edge-sensorbased position information in order to minimize high spatial frequency segment response for both seeing-limited and adaptive optics performance. A much higher bandwidth is required from the local servo loop to provide adequate stiffness to wind or acoustic disturbances. A related paper presents the control designs for the local actuator servo loops. The disturbance rejection requirements would not be difficult to achieve for a single segment, but the structural coupling between segments mounted on a flexible mirror cell results in controlstructure interaction (CSI) that limits the achievable bandwidth. Using a combination of simplified modeling to build intuition and the full telescope finite element model for verification, we present designs and analysis for both the local servo loop and global loop demonstrating sufficient bandwidth and resulting wind-disturbance rejection despite the presence of CSI.

TMT telescope structure system: design and development progress report

The Thirty Meter Telescope (TMT) project has revised the reference optical configuration from an Aplanatic Gregorian to a Ritchey-Chrétien design. This paper describes the revised telescope structural design and outlines the design methodology for achieving the dynamic performance requirements derived from the image jitter error budget. The usage of transfer function tools which incorporate the telescope structure system dynamic characteristics and the control system properties is described along with the optimization process for the integrated system. Progress on the structural design for seismic considerations is presented. Moreover, mechanical design progress on the mount control system hardware such as the hydrostatic bearings and drive motors, cable wraps and safety system hardware such as brakes and absorbers are also presented.

Control analysis of the TMT primary segment assembly

The primary mirror control system (M1CS) stabilizes the 492 segments of the Thirty Meter Telescope primary mirror in the presence of disturbances. Each Primary Segment Assembly (PSA) has three actuators and position sensors that control the piston, tip, and tilt of the mirror segment. Requirements for the PSA position controller are presented, with the main requirements being 10 Newton per micron stiffness below one Hertz, where wind is the primary disturbance. Bandwidths of the PSA position controller of about twenty Hertz, assuming a soft actuator, are needed to meet this requirement. A finite element model of the PSA was developed and used for a preliminary control design. PSA structural modes at 40, 90, and 120 impact the control design. We have studied control designs with different actuators, sensors, and structural filters in order to assess disturbance rejection properties and interactions with the PSA structural modes. The performance requirements are achieved using voice coil actuators with modal control architecture for piston, tip, and tilt. Force interactions with the underlying mirror cell are important, and we present the status of our studies of the control structure interaction effect (CSIE). A related paper presents further analysis of the CSIE and MICS global position control loop.

Analysis of the TMT mount control system

The TMT mount control system provides telescope pointing and tracking. Requirements include wind disturbance rejection, offsetting time and accuracy, control system robustness, and the magnitude of response at structural resonances. A finite element model of the complete telescope has been developed and the transfer functions used for the control designs are presented. Wind disturbance, encoder, and wave-front-sensor models are presented that are used for the control design. A performance analysis translates the requirements to a required bandwidth. Achieving this bandwidth is important for reducing telescope image motion due to wind-buffeting. A mount control design is presented that meets the demanding requirements by maximizing low frequency gain and using structural filters to roll-off structural modes. The control system analysis includes an outer guide loop using a wave front sensor. Offsetting time and accuracy requirements are satisfied using feed-forward control architecture.

Control of many coupled oscillators and application to segmented-mirror telescopes

The largest optical telescopes today use a segmented primary mirror, with the outof-plane position of each segment actively controlled. The segments are supported by a flexible structure that introduces dynamic coupling. This coupling leads to controlstructure-interaction (CSI), which limits the achievable bandwidth of the primary mirror control system. As telescopes increase in size, the number of mirror segments increase, and the potential for CSI increases. The dynamics can be approximated by n identical oscillators coupled through the support structure. These dynamics are explored herein. First, in the special case where the support-structure modes provide an orthonormal basis for the oscillator dynamics, the problem can be transformed into n separate coupled oscillator problems. The application to more realistic support structures is then investigated using the dynamic model of the Thirty Meter Telescope. The achievable bandwidth is estimated by projecting the dynamics onto Zernike basis functions, with higher bandwidth possible for higher spatial frequency.

Advances in edge sensors for the Thirty Meter Telescope primary mirror

The out-of-plane degrees of freedom (piston, tip, and tilt) of each of the 492 segments in the Thirty Meter Telescope primary mirror will be actively controlled using three actuators per segment and two edge sensors along each intersegment gap. We address two important topics for this system: edge sensor design, and the correction of fabrication and installation errors. The primary mirror segments are passively constrained in the three lateral degrees of freedom. We evaluate the segment lateral motions due to the changing gravity vector and temperature, using site temperature and wind data, thermal modeling, and finite-element analysis. Sensor fabrication and installation errors combined with these lateral motions will induce errors in the sensor readings. We evaluate these errors for a capacitive sensor design as a function of dihedral angle sensitivity. We also describe operational scenarios for using the Alignment and Phasing System to correct the sensor readings for errors associated with fabrication and installation.

Development of the Primary Mirror Segment Support Assemblies for the Thirty Meter Telescope

This paper describes the studies performed to establish a baseline conceptual design of the Segment Support Assembly (SSA) for the Thirty Meter Telescope (TMT) primary mirror. The SSA uses a combination of mechanical whiffletrees for axial support, a central diaphragm for lateral support, and a whiffletree-based remote-controlled warping harness for surface figure corrections. Axial support whiffletrees are numerically optimized to minimize the resulting gravityinduced deformation. Although a classical central diaphragm solution was eventually adopted, several lateral support concepts are considered. Warping harness systems are analyzed and optimized for their effectiveness at correcting second and third order optical aberrations. Thermal deformations of the optical surface are systematically analyzed using finite element analysis. Worst-case performance of the complete system as a result of gravity loading and temperature variations is analyzed as a function of zenith angle using an integrated finite element model.

Dynamic characterization of a prototype of the Thirty Meter Telescope primary segment assembly

Finite element models (FEMs) are being used extensively in the design of the Thirty Meter Telescope (TMT). One such use is in the design and analysis of the Primary Segment Assembly (PSA). Each PSA supports one primary mirror segment on the mirror cell, as well as three actuators, which are used to control three degrees of freedom - tip, tilt, and piston - of the mirror segment. The dynamic response of the PSA is important for two reasons: it affects the response of the mirror to fluctuating wind forces, and high-Q modes limit the bandwidth of the control loops which drive the actuators, and impact vibration transmissivity, thereby degrading image quality. We have completed a series of tests on a prototype PSA, in which the dynamic response was tested. We report on the test methods used to measure the dynamic response of the PSA alone and with candidate actuators installed, and we present comparisons between the measured response and FEM predictions. There is good agreement between FEM predictions and measured response over the frequency range within which the dynamic response is critical to control system design.

Control System Modeling for the Thirty Meter Telescope Primary Mirror

The Thirty Meter Telescope primary mirror is composed of 492 segments that are controlled to high precision in the presence of wind and vibration disturbances, despite the interaction with structural dynamics. The higher bandwidth and larger number of segments compared with the Keck telescopes requires greater attention to modeling to ensure success. We focus here on the development and validation of a suite of quasi-static and dynamic modeling tools required to support the design process, including robustness verification, performance estimation, and requirements flowdown. Models are used to predict the dynamic response due to wind and vibration disturbances, estimate achievable bandwidth in the presence of control-structure-interaction (CSI) and uncertainty in the interaction matrix, and simulate and analyze control algorithms and strategies, e.g. for control of focus-mode, and sensor calibration. Representative results illustrate TMT performance scaling with parameters, but the emphasis is on the modeling framework itself.

Investigation of Thirty Meter Telescope wavefront maintenance using low-order Shack-Hartmann wavefront sensors to correct for thermally-induced misalignments

We evaluate how well the performance of the Thirty Meter Telescope (TMT) can be maintained against thermally induced errors during a night of observation. We first demonstrate that using look-up-table style correction for TMT thermal errors is unlikely to meet the required optical performance specifications. Therefore, we primarily investigate the use of a Shack-Hartmann Wavefront Sensor (SH WFS) to sense and correct the low spatial frequency errors induced by the dynamic thermal environment. Given a basic SH WFS design, we position single or multiple sensors within the telescope field of view and assess telescope performance using the JPL optical ray tracing tool MACOS for wavefront simulation. Performance for each error source, wavefront sensing configuration, and control scheme is evaluated using wavefront error, plate scale, pupil motion, pointing error, and the Point Source Sensitivity (PSSN) as metrics. This study provides insight into optimizing the active optics control methodology for TMT in conjunction with the Alignment and Phasing System (APS) and primary mirror control system (M1CS).