Kenneth R. Lorell

Performance analysis of the segment alignment control system for the ten-meter telescope

Abstract The W. M. Keck Observatory Ten-Meter Telescope (TMT) will be a high-performance astronomical telescope capable of unprecedented ground-based observations. One of the unique features of the TMT is its primary mirror which is composed of 36 hexagonal segments. A segment alignment control system will be used to achieve the optical quality of a glass monolith in the segmented primary mirror. The problems associated with the accurate pointing and imaging of a large flexible telescope such as the TMT are quite different from those posed by traditional rigid telescopes. These problems arise from two main sources: structural dynamic effects, such as enhanced sensitivity to external disturbances, and interactions between the segment alignment control system and the telescope structure. This study consists of a combined structural and control system analysis based upon mathematical models of the TMT structure, the segment alignment control system, and the aerodynamic loads on the primary mirror that are induced by the wind. Data for the structural model were derived from the most recent design iteration available during the time frame of the study. Actual wind velocities measured at three observatory sites were used to develop a generic model of the power spectrum of forces generated by wind loads. Information about the r.m.s. mirror figure errors and limits on the bandwidth of the control system as it is currently implemented are presented.

A Computer-Aided Telescope Pointing System Utilizing a Video Star Tracker

Summary The Video Inertial Pointing (VIP) System is being developed to satisfy the acquisition and pointing requirements of astronomical telescopes. VIP employs a single video sensor to generate three-axis pointing error signals and to provide inputs for a cathode ray tube (CRT) display of the star field. The pointing error signals update the telescope’s gyro stabilization system. The CRT display facilitates target acquisition and positioning of the telescope by a remote operator. The present paper describes the analysis, simulation, and hardware development of a prototype, advanced VIP system. An early model of the system utilizing a silicon-intensified target vidicon camera has flown on a balloon-borne telescope and is briefly described. The advanced system, which can employ either a vidicon camera or a charge-coupled device video sensor, has been tested using an analog/digital hybrid simulation. The advanced VIP hardware is described, and the simulation results presented.

Design of a prototype primary mirror segment positioning actuator for the Thirty Meter Telescope

The Thirty Meter Telescope (TMT) is a collaborative project between the California Institute of Technology (CIT), the University of California (UC), the Association of Universities for Research in Astronomy (AURA), and the Association of Canadian Universities for Research in Astronomy (ACURA). In order for the Thirty Meter Telescope (TMT) to achieve the required optical performance, each of its 738 primary mirror segments must be positioned relative to adjacent segments with nanometer-level accuracy. Three in plane degrees of freedom are controlled via a passive Segment Support Assembly which is described in another paper presented at this conference (paper 6273-45). The remaining three out of plane degrees of freedom, tip, tilt, and piston, are controlled via three actuators for each segment. Because of its size and the shear number of actuators, TMT will require an actuator design, departing from that used on the Keck telescopes, its successful predecessor. Sensitivity to wind loads and structural vibrations, the large dynamic range, low operating power, and extremely reliable operation, all achieved at an affordable unit cost, are the most demanding design requirements. This paper describes a concept that successfully meets the TMT requirements, along with analysis and performance predictions. The actuator concept is based on a prototype actuator developed for the California Extremely Large Telescope (CELT) project. It relies on techniques that achieve the required accuracy while providing a substantial amount of vibration attenuation and damping. A development plan consisting of a series of prototype actuators is envisioned to verify cost, reliability, and performance before mass production is initiated. The first prototype (P1) of this development plan is now being built and should complete initial testing by the end of 2nd QTR 06.

Paper: A microprocessor-based position control system for a telescope secondary mirror

The shuttle infrared telescope facility (SIRTF) is being designed as a 0.85-m cryogenically cooled i.r. telescope to be flown as a shuttle-attached payload in the late-1980s. Pointing requirements for SIRTF dictate image stability of 0.25 arcsec. In addition, enhancement of weak source signal-to-noise ratio is accomplished by chopping the incoming beam. The articulated secondary mirror in SIRTFs Cassegrain optical train provides image-motion compensation, in order to achieve the desired stability, as well as simultaneously chopping the beam. This paper describes a unique, microprocessor-based position control system developed to control the SIRTF secondary mirror. The system utilizes a special control law to minimize energy dissipation, a precision capacitive position sensor, and a specially designed power-amplifier/actuator combination to achieve the desired performance. The microprocessor generates the commanded angular position and rate waveforms to maintain a 90%-dwell-time-to-10%-transition-time ratio independent of selected chop frequency or amplitude. Additionally, the microprocessor supervises system start-up and shutdown to eliminate unnecessary transients to the sensor and actuator, and provides for control-system gain scheduling to compensate for nonlinearities as a function of frequency and amplitude. Performance and test results of a prototype system designed for use with a demonstration model of SIRTFs focal plane fine-guidance sensor are presented.

Paper: Image motion compensation system for the shuttle infrared telescope facility

The shuttle infrared telescope facility (SIRTF) is mounted in the orbiter on the instrument pointing system (IPS) and requires that the image at the focal plane be stabilized to better than 0.25 arcsec in order to achieve the scientific goals of the device. Current estimates of IPS performance accuracy indicate that additional stabilization will be necessary to meet the SIRTF requirements. An image motion compensation system, utilizing a charge coupled device (CCD) star tracker located at the focal plane and a steerable mirror in the SIRTF optical path, has been designed to work in conjunction with the IPS. The CCD tracks multiple stars simultaneously and through the use of a high-performance microcomputer generates three-axis attitude errors and gyro drift estimates to correct disturbances not compensated for the IPS control system. The SIRTF control system is described in detail and performance is analyzed with respect to system noise inputs, environmental disturbances, and error sources such as bending and feed-forward scale factor. It is concluded that the SIRTF accuracy and stability requirements can be met with this design.

Development and demonstration of a precision latch for deployable optical systems

The Lockheed Martin/Advanced Technology Center (LM/ATC) developed a lightweight, compact, high-load capable and yet high precision latch for use on deployable optical systems such as the Next Generation Space Telescope (NGST). The design allows precise self-centering and control of the stiffness at the latch interface. It also incorporates unique capabilities to evaluate the effects of gravity loads, latch preload level, creep, and very low vibration loads on the dynamics and microdynamics of the deployed instrument. The stiffness, nonlinearity and hysteresis characteristics of the latch and its catch flexure assembly were thoroughly tested in 6 axes down to the nanometer level at room temperature using the LM/ATC Compliance Measurement Device. The latch is stiff enough to hold an NGST-size mirror segment cantilevered against gravity allowing only small gravity sag when the primary mirror is horizontal, thus enabling end-to-end performance verification in 1-G in that orientation. The latch hysteresis is less than 1.0 nm/N under mechanical loads less than 25 N, which meets the NGST stability requirements with significant margin (20 nm at the tip of the petal in space environment). Several of these latches were integrated and demonstrated at the petal assembly level on a Single Petal Test-bed and the experimental results obtained on that test-bed are consistent with the component level results described in this report. We experimentally demonstrated that the latch engagement performance is not affected by exposure to cryogenic temperatures down to 20K, as required for use of the device on cryogenic infrared optical instruments such as NGST. A structural model of the latch was developed using Finite Element Analysis. Good correlation was obtained between the linear components of the analytical and of the experimental results: the model can therefore reliably be used in future NGST or other mission design efforts. This paper includes a brief description of the LM/ATC latch hardware and its principle of operation as well as the results of the modeling and the experimental characterization work performed on that hardware in the NGST Phase I formulation.

Test and performance evaluation of the Gemini secondary mirror chopper and position control system

This paper describes the testing and performance evaluation of the Gemini secondary mirror control system. The mechanism and control system have demanding requirements for dynamic performance, precision position control throughout the mirror chop cycle, and the virtual elimination of residual forces and torques. The test procedures and hardware required to measure and verify the system performance were specifically developed for this application. The tests utilize a special computer-controlled laser interferometer to calibrate the mirror position sensors. Dynamic chopping performance of the system is also tested and verified. A range of chop waveform parameters; amplitude, frequency, and duty cycle, is employed to fully exercise the control system and electromechanical hardware. Measurements of angular stability and repeatability under dynamic chop conditions are made to verify performance. Effectiveness of the active force cancellation system is evaluated using a six-axis digital dynamometer.

Design of the infrared fast steering mirror chopping control system for the Keck II Telescope

The Keck 2 ten meter telescope will utilize an advanced chopping secondary mirror in order to enhance observations in the infrared. The Infrared Fast Steering Mirror (IFSM) can execute a square-wave chop at frequencies as high as 25 Hz with an accuracy of +/- 0.1 arcsec. Chopping can be synchronized by focal plane instruments, and the system can simultaneously perform high-performance chopping as well as beam-steering (for atmospheric correction), providing the Keck telescope with greatly enhanced capability. Details of design, testing, and performance of the Keck 2 IFSM are presented in this paper. The mirror is controlled by three voice coil actuators. Reaction forces generated by the actuators are absorbed by a reaction mass suspended from the main IFSM structure. Motor driven springs are used to minimize power dissipation in the actuators. The IFSM all- digital control system uses a unique adaptive algorithm that forces the mirror to precisely follow the commanded chop waveform. Tests use various computerized instruments: a three-axis laser interferometer for calibration and stability, a 6-axis dynamometer to evaluate reaction forces transmitted to the telescope. In addition to specifics of the design, performance, and testing, a video illustrating details of the IFSM hardware and showing it in operation will be presented.

The Advanced Structures/Controls Integrated Experiment (ASCIE): A Control Technology Test Bed For Large Segmented Reflectors

A description is given of an experimental test apparatus for active control of a seven-segment primary reflector (mirror). Segmented reflectors require an active segment-alignment control system to give the reflecting surface the optical performance of a single-piece reflector. The apparatus, called the Advanced Structures/Controls Integrated Experiment, consists of a Cassegrain optical configuration with a 2-m, seven-segment, actively controlled primary mirror supported by a light, flexible truss structure. The testbed is a response to the need for experiments that can simulate the complex dynamic behavior of a large structure and address the myriad problems associated with precision control of optical surfaces. The testbed is described, details of the control and optical measurement systems are presented, and preliminary performance results are reported.<<ETX>>

Development and Microdynamics Characterization of a Deployable Petal Assembly at Full Scale

As part of its risk mitigation efforts related to large, future space-based deployable optics such as NGST, Lockheed Martin developed, implemented, and evolved a full-scale, lightweight, deployable petal structure and associated deployment mechanisms for cryogenic and microdynamic stability testing. The test-bed features a single petal assembly for an 8-meter diameter telescope, including a flight-like mirror support structure and full-size hinges and latches. The work completed on this test-bed include: 1) Characterization of the dynamics and microdynamics response of the full-scale petal and its hinge/latch interface to low-level vibration sources down to 0.1 nanometer, 2) Evaluation of petal deployment repeatability, 3) Evaluation of the performance of simple passive damping strategies for petal vibration control at cryogenic temperatures. In all respects, including microdynamics, deployment repeatability and stability, the hardware demonstrated performance well in excess of the NGST requirements. In this paper, we summarize the development and the results of the performance testing completed during the NGST Phase I formulation, including testing of hysteresis and deployment repeatability at room temperature.