Vibration from equipment mounted on the telescope and in summit support buildings has been a source of performance degradation at existing astronomical observatories, particularly for adaptive optics performance. Rather than relying only on best practices to minimize vibration, we present here a vibration budget that specifies allowable force levels from each source of vibration in the observatory (e.g., pumps, chillers, cryocoolers, etc.). This design tool helps ensure that the total optical performance degradation due to vibration is less than the corresponding error budget allocation and is also useful in design trade-offs, specifying isolation requirements for equipment, and tightening or widening individual equipment vibration specifications as necessary. The vibration budget relies on model-based analysis of the optical consequences that result from forces applied at different locations and frequencies, including both image jitter and primary mirror segment motion. We develop this tool here for the Thirty Meter Telescope but hope that this approach will be broadly useful to other observatories, not only in the design phase, but for verification and operations as well.
The LSST is an integrated, ground based survey system designed to conduct a decade-long time domain survey of the
optical sky. It consists of an 8-meter class wide-field telescope, a 3.2 Gpixel camera, and an automated data processing
system. In order to realize the scientific potential of the LSST, its optical system has to provide excellent and consistent
image quality across the entire 3.5 degree Field of View. The purpose of the Active Optics System (AOS) is to optimize
the image quality by controlling the surface figures of the telescope mirrors and maintaining the relative positions of the
optical elements. The basic challenge of the wavefront sensor feedback loop for an LSST type 3-mirror telescope is the
near degeneracy of the influence function linking optical degrees of freedom to the measured wavefront errors. Our
approach to mitigate this problem is modal control, where a limited number of modes (combinations of optical degrees
of freedom) are operated at the sampling rate of the wavefront sensing, while the control bandwidth for the barely
observable modes is significantly lower. The paper presents a control strategy based on linear approximations to the
system, and the verification of this strategy against system requirements by simulations using more complete, non-linear
models for LSST optics and the curvature wavefront sensors.
This paper provides an overview of the system design, architecture, and construction phase system engineering processes of the Thirty Meter Telescope project. We summarize the key challenges and our solutions for managing TMT systems engineering during the construction phase. We provide an overview of system budgets, requirements and interfaces, and the management thereof. The requirements engineering processes, including verification and plans for collection of technical data and testing during the assembly and integration phases, are described. We present configuration, change control and technical review processes, covering all aspects of the system design including performance models, requirements, and CAD databases.
Unsteady wind loads due to turbulence inside the telescope enclosure result in image jitter and higher-order image degradation due to M1 segment motion. Advances in computational fluid dynamics (CFD) allow unsteady simulations of the flow around realistic telescope geometry, in order to compute the unsteady forces due to wind turbulence. These simulations can then be used to understand the characteristics of the wind loads. Previous estimates used a parametric model based on a number of assumptions about the wind characteristics, such as a von Karman spectrum and frozen-flow turbulence across M1, and relied on CFD only to estimate parameters such as mean wind speed and turbulent kinetic energy. Using the CFD-computed forces avoids the need for assumptions regarding the flow. We discuss here both the loads on the telescope that lead to image jitter, and the spatially-varying force distribution across the primary mirror, using simulations with the Thirty Meter Telescope (TMT) geometry. The amplitude, temporal spectrum, and spatial distribution of wind disturbances are all estimated; these are then used to compute the resulting image motion and degradation. There are several key differences relative to our earlier parametric model. First, the TMT enclosure provides sufficient wind reduction at the top end (near M2) to render the larger cross-sectional structural areas further inside the enclosure (including M1) significant in determining the overall image jitter. Second, the temporal spectrum is not von Karman as the turbulence is not fully developed; this applies both in predicting image jitter and M1 segment motion. And third, for loads on M1, the spatial characteristics are not consistent with propagating a frozen-flow turbulence screen across the mirror: Frozen flow would result in a relationship between temporal frequency content and spatial frequency content that does not hold in the CFD predictions. Incorporating the new estimates of wind load characteristics into TMT response predictions leads to revised estimates of the response of TMT to wind turbulence, and validates the aerodynamic design of the enclosure.
We describe measurements of both the vibration forces imparted by various types of observatory equipment, and the transmission of these forces through the soil, foundations and telescope pier. These are key uncertainties both in understanding how to mitigate vibration at existing observatories and for developing a vibration budget in the design of future observatories such as the Thirty Meter Telescope. Typical vibration surveys have measured only the resulting motion (acceleration); however, this depends on both the source and the system being excited (for example, isolating equipment results in less force being transmitted, but greater motion of the equipment itself). Instead, here we (a) apply a known force input to the pier from a shaker and measure the response at different locations, and (b) use isolator properties combined with measured acceleration to infer the forces applied by various equipment directly. The soil foundation and pier transmission can then be combined with a finite element model based vibration transmission analysis to estimate the optical consequences. Estimates of plausible source levels supports the development of a vibration budget for TMT that allocates allowable forces to the sources of vibration; this is described in a companion paper.
Vibration from equipment mounted on the telescope and in summit support buildings has been a source of performance degradation at existing observatories, for adaptive optics performance in particular. To ensure that that the total optical performance degradation due to vibration is less than the corresponding optical error budget allocation, a vibration budget has been created that specifies allowable force levels from each source of vibration in the observatory (e.g., pumps, chillers, cryocoolers, etc.). In addition to its primary purpose, the vibration budget allows us to make design trade-offs, specify isolation requirements for equipment, and tighten or widen individual equipment vibration specifications as necessary. Defining this budget relies on two types of information: (i) vibration transmission analysis that determines the optical consequences that result from forces applied at different locations in the Observatory and at different frequencies; and (ii) initial estimates for plausible source amplitudes in order to allocate force budgets to different sources in the most realistic and cost-effective manner. The transmission of vibration from sources through to their optical consequences uses the finite element model of the telescope structure, including primary mirror seg- ment models and control loops. Both the image jitter and higher-order deformations due to M1 segment motion are included, along with the spatial- and temporal-correctability by the adaptive optics system. Measurements to support estimates of plausible soil transmissibility are described in a companion paper. As the detailed design progresses and more information is available regarding what is achievable at realistic cost, the vibration budget will be refined.
While it is attractive to integrate a deformable mirror (DM) for adaptive optics (AO) into the telescope itself
rather than using relay optics within an instrument, the resulting large DM can be expensive, particularly for
extremely large telescopes. A low-cost approach for building a large DM is to use voice-coil actuators, and rely
on feedback from mechanical sensors to improve the dynamic response of the mirror sufficiently so that it can
be used in a standard AO control system. The use of inexpensive voice-coil actuators results in many lightly-
damped structural resonances within the desired control bandwidth. We present a robust control approach for
this problem, and demonstrate performance in a closed-loop AO simulation, incorporating realistic models of
low-cost actuators and sensors. The first contribution is to demonstrate that high-bandwidth active damping
can be robustly implemented even with non-collocated sensors, by relying on the "acoustic limit" of the structure
where the modal bandwidth exceeds the modal spacing. Next we introduce a novel local control approach, which
significantly improves the high spatial frequency performance relative to collocated position control, but without
the robustness challenges associated with a global control approach. The combination of these "inner" control
loops results in DM command response that is demonstrated to be sufficient for integration within an AO system.
To improve the mechanical characteristics of actively controlled continuous faceplate deformable mirrors in adaptive optics, a strategy for reducing crosstalk between adjacent actuators and for suppressing low-order eigenmodes is proposed. The strategy can be seen as extending Saint-Venant's principle beyond the static case, for small local families of actuators. An analytic model is presented, from which we show the feasibility of the local control. Also, we demonstrate how eigenmodes and eigenfrequencies are affected by mirror parameters, such as thickness, diameter, Young's modulus, Poisson's ratio, and density. This analysis is used to evaluate the design strategy for a large deformable mirror, and how many actuators are needed within a family.
We study a concept for a low-cost, large deformable mirror for an Extremely Large Telescope. The use of inexpensive voice-coil actuators leads to a poorly damped faceplate, with many modes within the desired control bandwidth. A control architecture, including rate and position feedback to add damping and stiness, for the faceplate has been presented in our previous papers. An innovative local control scheme which decouples adjacent actuators and suppresses low-order eigenmodes is a key feature in our controller. Here, we present an integrated
model of a partially illuminated large deformable mirror in an experimental laboratory setup with a limited amount of actuators. From the model, conclusions are drawn regarding the number of actuators needed to identify the key features, such as local control performance, dynamic range, and controllability and robustness of the deformable 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.
Modeling is an integral part of systems engineering. It is utilized in requirement validation, system verification, as well as for supporting design trade studies. Modeling highly complex systems poses particular challenges, including the definition and interpretation of system performance, and the combined evaluation of physical processes spanning a wide range of time frames. Our solution is based on statistical interpretation of system performance and a unique image quality metric developed by TMT. The Stochastic Framework and Point Source Sensitivity allow us to properly estimate and combine the optical effects of various disturbances and telescope imperfections.
The principal dynamic disturbances acting on a telescope segmented primary mirror are unsteady wind pressure
(turbulence) and narrowband vibration from rotating equipment. Understanding these disturbances is essential
for the design of the segment support assembly (SSA), segment actuators, and primary mirror control system
(M1CS). The wind disturbance is relatively low frequency, and is partially compensated by M1CS; the response
depends on the control bandwidth and the quasi-static stiffness of the actuator and SSA. Equipment vibration is
at frequencies higher than the M1CS bandwidth; the response depends on segment damping, and the proximity
of segment support resonances to dominant vibration tones. We present here both disturbance models and
parametric response. Wind modeling is informed by CFD and based on propagation of a von Karman pressure
screen. The vibration model is informed by analysis of accelerometer and adaptive optics data from Keck. This
information is extrapolated to TMT and applied to the telescope structural model to understand the response
dependence on actuator design parameters in particular. Whether the vibration response or the wind response
is larger depends on these design choices; "soft" (e.g. voice-coil) actuators provide better vibration reduction
but require high servo bandwidth for wind rejection, while "hard" (e.g. piezo-electric) actuators provide good
wind rejection but require damping to avoid excessive vibration transmission to the primary mirror segments.
The results for both nominal and worst-case disturbances and design parameters are incorporated into the TMT
actuator performance assessment.
The primary mirror control system for the Thirty Meter Telescope (TMT) maintains the alignment of the 492
segments in the presence of both quasi-static (gravity and thermal) and dynamic disturbances due to unsteady
wind loads. The latter results in a desired control bandwidth of 1Hz at high spatial frequencies. The achievable
bandwidth is limited by robustness to (i) uncertain telescope structural dynamics (control-structure interaction)
and (ii) small perturbations in the ill-conditioned influence matrix that relates segment edge sensor response
to actuator commands. Both of these effects are considered herein using models of TMT. The former is explored
through multivariable sensitivity analysis on a reduced-order Zernike-basis representation of the structural
dynamics. The interaction matrix ("A-matrix") uncertainty has been analyzed theoretically elsewhere, and is
examined here for realistic amplitude perturbations due to segment and sensor installation errors, and gravity
and thermal induced segment motion. The primary influence of A-matrix uncertainty is on the control of "focusmode";
this is the least observable mode, measurable only through the edge-sensor (gap-dependent) sensitivity
to the dihedral angle between segments. Accurately estimating focus-mode will require updating the A-matrix
as a function of the measured gap. A-matrix uncertainty also results in a higher gain-margin requirement for
focus-mode, and hence the A-matrix and CSI robustness need to be understood simultaneously. Based on the
robustness analysis, the desired 1 Hz bandwidth is achievable in the presence of uncertainty for all except the
lowest spatial-frequency response patterns of the primary mirror.
The Thirty Meter Telescope has 492 primary mirror segments, each incorporated into a Primary Segment Assembly
(PSA), each of which in turn has three actuators that control piston, tip, and tilt, for a total of 1476 actuators. Each
actuator has a servo loop that controls small motions (nanometers) and large motions (millimeters). Candidate actuators
were designed and tested that fall into the categories of "hard" and "soft," depending on the offload spring stiffness
relative to the PSA structural stiffness. Dynamics models for each type of actuator are presented, which respectively use
piezo-electric transducers and voice coils. Servo design and analysis are presented that include assessments of stability,
performance, robustness, and control structure interaction. The analysis is presented for a single PSA on a rigid base, and
then using Zernike approximations the analysis is repeated for 492 mirror segments on a flexible mirror cell. Servo
requirements include low-frequency stiffness, needed for wind rejection; reduced control structure interaction, specified
by a bound on the sensitivity function; and mid-frequency damping, needed to reduce vibration transmission. The last of
these requirements, vibration reduction, was found to be an important distinguishing characteristic for actuator selection.
Hard actuators have little inherent damping, which is improved using PZT shunt circuits and force feedback, but still
these improvements were found to result in less damping than is provided by the soft actuator. Results of the servo
analysis were used for an actuator down-select study.
Large (>1m) deformable mirrors with hundreds or thousands of actuators are attractive for extremely large
telescopes. Use of force actuators coupled to the mirror via suction cups, and electret microphones for position
sensing, has the potential of substantially reducing costs. However, a mirror controlled with force actuators
will have many structural resonances within the desired system bandwidth, shifting the emphasis somewhat
of the control aspects. Local velocity and position loop for each actuator can add significant damping, but
gives poor performance at high spatial frequencies. We therefore introduce a novel control strategy with many
parallel "actuator families", each controlled by
single-input-single-output controllers. This family approach
provides performance close to that of global control, but without the accompanying robustness challenges. Using
a complete simulation model of a representative large deformable mirror, we demonstrate feasibility of the
This paper describes the challenges of non-ideal actuators and sensors. The results presented give an understanding
of the required actuator bandwidth and the effects of the sensors dynamics. The conclusion is that the
introduction of actuator and sensor dynamics does not limit the control system of the deformable mirror.
The primary mirror segment aberrations after shape corrections with warping harness have been identified as
the single largest error term in the Thirty Meter Telescope (TMT) image quality error budget. In order to better
understand the likely errors and how they will impact the telescope performance we have performed detailed
simulations. We first generated unwarped primary mirror segment surface shapes that met TMT specifications.
Then we used the predicted warping harness influence functions and a Shack-Hartmann wavefront sensor model
to determine estimates for the 492 corrected segment surfaces that make up the TMT primary mirror. Surface
and control parameters, as well as the number of subapertures were varied to explore the parameter space. The
corrected segment shapes were then passed to an optical TMT model built using the Jet Propulsion Laboratory
(JPL) developed Modeling and Analysis for Controlled Optical Systems (MACOS) ray-trace simulator. The
generated exit pupil wavefront error maps provided RMS wavefront error and image-plane characteristics like
the Normalized Point Source Sensitivity (PSSN). The results have been used to optimize the segment shape
correction and wavefront sensor designs as well as provide input to the TMT systems engineering error budgets.
Telescope-Enclosure-Soil Interaction could result in additional telescope movement due to two main sources: (i)
enclosure windshake and (ii) vibrations of machinery located at enclosure, summit and utility facilities. To analyze and
minimize these vibrations, a novel FE model was developed based on existing FE models for the TMT enclosure and
telescope structures. This integrated structural model adequately represents propagation of vibrations from the source to
the telescope structure through surrounding soil/rock region. The model employs 3-D linear-elastic harmonic analysis
using commercial FE code ANSYS. Special attention was devoted to adequate modeling of reflecting and non-reflecting
boundary conditions. Based on the FE model developed, we examined the effects of soil/rock stiffness and damping
upon telescope vibrations and, ultimately, seeing quality. The effects of location, intensity and spectral content of main
sources of machinery vibrations were also investigated.
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.
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.
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.
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.
Dynamic disturbance sources affecting the optical performance of the Thirty Meter Telescope (TMT) include
unsteady wind forces inside the observatory enclosure acting directly on the telescope structure, unsteady wind
forces acting on the enclosure itself and transmitted through the soil and pier to the telescope, equipment
vibration either on the telescope itself (e.g. cooling of instruments) or transmitted through the soil and pier, and
potentially acoustic forces. We estimate the characteristics of these disturbance sources using modeling anchored
through data from existing observatories. Propagation of forces on the enclosure or in support buildings through
the soil and pier to the telescope base are modeled separately, resulting in force estimates at the telescope pier.
We analyze the resulting optical consequences using integrated modeling that includes the telescope structural
dynamics, control systems, and a linear optical model. The dynamic performance is given as a probability
distribution that includes the variation of the external wind speed and observing orientation with respect to the
wind, which can then be combined with dome seeing and other time- or orientation-dependent components of
the overall error budget. The modeling predicts acceptable dynamic performance of TMT.
The Thirty Meter Telescope project is designing a 30m diameter ground-based optical telescope. Unsteady wind loads on the telescope structure due to turbulence inside the telescope enclosure impact the delivered image quality. A parametric model is described that predicts the optical performance due to wind with sufficient accuracy to inform relevant design decisions, including control bandwidths. The model is designed to be sufficiently computationally efficient to allow rapid exploration of the impact of design parameters or uncertain/variable input parameters, and includes (i) a parametric wind model, (ii) a detailed structural dynamic model derived from a finite element model, (iii) a linear optical response model, and (iv) a control model. Model predictions with the TMT structural design are presented, including the parametric variation of performance with external wind speed, desired wind speed across the primary mirror, and optical guide loop bandwidth. For the median
mountaintop wind speed of 5.5 m/s, the combination of dome shielding, minimized cross-sectional area, and control results in acceptable image degradation.
The design of future large optical telescopes must take into account the wind-induced buffeting of the telescope structure caused by large-scale flow structures and turbulence inside the dome. However, estimating the resulting degradation in image quality is difficult due to our relatively poor understanding of the flow inside the dome. Data has been collected in a scaled wind-tunnel test of a telescope enclosure to understand the flow-field around the region near the dome opening where the secondary mirror and supporting structure would be subjected to wind loads. Digital particle image velocimetry (DPIV) data was collected in a vertical plane near the dome opening to obtain mean velocity and fluctuation kinetic energy. In addition, hotwire data was collected along the telescope axis to obtain temporal spectra of the velocity, and flow visualization was used to determine the general flow patterns. In addition to its direct use in telescope modeling and design, this data is of particular value in validation of computational fluid dynamic (CFD) analyses, so that CFD can be used with confidence in future design work.
A sound system engineering approach and the appropriate tools to support it are essential in achieving the scientific and financial objectives of the Thirty Meter Telescope project. Major elements of the required tool set are those providing estimates for the performance of the telescope. During the last couple of years, the partners in the consortium developed a wide range of modeling and simulation tools with various levels of fidelity and flexibility. There are models available for time domain and frequency domain simulations and analysis, as well as for lower fidelity, parametric investigations of design trade-offs and for high fidelity, integrated modeling of structure, optics and control. Presented are characteristic simulation results using the existing preliminary point designs of the TMT, with emphasis on the telescope performance degradation due to wind buffeting. Under the conditions modeled, the wind induced image jitter and image quality degradation was found comparable to good atmospheric seeing.
A parametric model of the dynamic performance of an optical telescope
due to wind-buffeting is presented. The model is being developed to
support the design of next generation segmented-mirror optical telescopes through enabling rapid design iterations and allowing a more thorough exploration of the design space. A realistic performance assessment requires parametric descriptions of the wind, the structural dynamics, active control of the structure, and the optical response. The current model and its assumptions are presented, with the primary emphasis being on the parameterization of the wind forces. Understanding the temporal spectrum and spatial distribution of wind disturbances inside the telescope enclosure is one of the most challenging aspects in developing the overall parametric model. This involves integrating information from wind tunnel tests, computational fluid dynamics, and measurements at existing observatories. The potential and limitations of control to mitigate the response are also discussed, with realistic constraints on the control bandwidth obtained from the detailed structural model of a particular point design. Finally, initial results are presented on performance trends with a few key parameter variations.
The next generation of large ground-based optical telescopes are
likely to involve a highly segmented primary mirror that must be
controlled in the presence of wind and other disturbances, resulting in a new set of challenges for control. The current design concept for
the California Extremely Large Telescope (CELT) includes 1080 segments in the primary mirror, with the out-of-plane degrees of freedom actively controlled. In addition to the 3240 primary mirror actuators,the secondary mirror of the telescope will also require at least 5 degree of freedom control. The bandwidth of both control systems will be limited by coupling to structural modes. I discuss three control issues for extremely large telescopes in the context of the CELT design, describing both the status and remaining challenges. First, with many actuators and sensors, the cost and reliability of the control hardware is critical; the hardware requirements and current actuator design are discussed. Second, wind buffeting due to turbulence inside the telescope enclosure is likely to drive the control bandwidth higher, and hence limitations resulting from control-structure-interaction must be understood. Finally, the impact on the control architecture is briefly discussed.
Adaptive optics systems with Shack-Hartmann wavefront sensors require reconstruction of the atmospheric phase error from subaperture slope measurements, with every sensor in the array being used in the computation of each actuator command. This fully populated reconstruction matrix can result in a significant computational burden for adaptive optics systems with large numbers of actuators. A method for generating sparse wavefront reconstruction matrices for adaptive optics is proposed. The method exploits the relevance of nearby subaperture slope measurements for control of an individual actuator, and relies upon the limited extent of the influence function for a zonal deformable mirror. Relying only on nearby sensor information can significantly reduce the calculation time for wavefront reconstruction. In addition, a hierarchic controller is proposed to recover some of the global wavefront information. The performance of these sparse wavefront reconstruction matrices was evaluated in simulation, and tested on the Palomar Adaptive Optics System. This paper presents some initial results from the simulations and experiments.
The current design concept for the California Extremely Large Telescope (CELT) includes 1080 segments in the primary mirror, with the out-of-plane degrees of freedom actively controlled. We construct the control matrix for this active control system, and describe its singular modes and sensor noise propagation. Data from the Keck telescopes are used to generate realistic estimates of the control system contributions to the CELT wavefront error and wavefront gradient error. Based on these estimates, control system noise will not significantly degrade either seeing-limited or diffraction-limited observations. The use of supplemental wavefront information for real-time control is therefore not necessary. We also comment briefly on control system bandwidth requirements and limitations.
We propose thin silicon wafers as the building blocks of highly segmented space telescope primary mirrors. Using embedded MEMS actuators operating at high bandwidth control, this technology can achieve diffraction-limited image quality in the 3-300 micron wavelength range. The use of silicon wafers as cryogenic mirror segments is carried forward considering a point design of a future FAIR-class NASA ORIGINS mission.
We recognize four major economic factors that justify a massive paradigm shift in the fabrication of ultralightweight space telescopes:
The precise process control and repeatability of silicon wafer manufacturing dramatically reduces the huge labor investment in mirror figuring experienced with Hubble Space Telescope.
Once developed, the incremental cost of additional space telescopes based upon proven silicon manufacturing techniques can be very small. We estimate the marginal cost of a 30m mirror when deploying a constellation can be as low as $36 million (Year 2002 dollars).
Federal R&D funding in the area of microelectromechanical devices and advanced 3-D silicon processing is certain to have far greater economic return than similar investments in other technologies, such as optical membrane technology.
The $300B per year silicon processing industry will continue to drive increased MEMS functionality, higher product yields, and lower cost. These advances will continue for decades.
The intention here is to present the case for the economic advantage of silicon as a highly functional optical substrate that can be fabricated using unparalleled industry experience with precision process control. We maintain that many architectures superior to the ASSiST concept presented here are possible, and hope that this effort prompts future thinking of the silicon wafer telescope paradigm
The Middeck Active Control Experiment is a space shuttle flight experiment intended to demonstrate high authority active structural control in zero gravity conditions. The prediction of on-orbit closed-loop dynamics is based on analysis and ground testing. The MACE test article is representative of multiple payload platforms, and includes two 2-axis gimballing payloads connected by a flexible bus. The goal of active control is to maintain pointing accuracy of one payload, while the remaining payload is moving independently. Current control results on the ground test article are presented. Multiple input, multiple output controllers are designed based on high order measurement based models. Linear Quadratic Gaussian controllers yield reasonable performance. At high authority, however, these controllers destabilize the actual structure, due to parametric errors in the control design model. A robust control design procedure is required to yield high performance in the presence of these errors.