The positioning requirements for secondary mirrors and instruments for large ground-based telescopes are becoming
increasingly challenging. Modern telescope designs, such as LSST and TMT, are specifying repeatability and/or absolute
accuracy limits below 10 μm and 10 μrad for the hexapod positioning systems generally used for these applications.
Hexapod error sources, including lead screw pitch variations, windup, backlash, friction, thermal expansion, compliance,
sensing, and joint node location uncertainties, are examined along with methods for reducing or eliminating these errors
by mechanical means or through calibration. Alternative sensing approaches are discussed and their relative benefits are
evaluated. Finally, a model-based design approach is presented for conducting initial design trade studies, assessing
technical risk, predicting achievable performance, establishing subsystem and component requirements, and tracking
positioning error budgets through the entire development process. A parametric actuator model and its initial results are
described, and testing approaches are outlined to identify key model parameters and verify subsystem and component
performance.
KEYWORDS: Telescopes, Actuators, Control systems, Amplifiers, Space telescopes, Observatories, Control systems design, Operating systems, Analog electronics, Human-machine interfaces
The NASA/DLR Stratospheric Observatory for Infrared Astronomy (SOFIA) employs a 2.5-meter reflector telescope in a Boeing 747SP. The image stability goal for SOFIA is 0.2 arc-seconds. An active damping control system is being developed for SOFIA to reduce image jitter and degradation due to resonance of the telescope assembly. We describe the vibration control system design and implementation in hardware and software. The system’s unique features enabling system testing, control system design, and online health monitoring will also be presented.
The NASA/DLR Stratospheric Observatory for Infrared Astronomy (SOFIA) employs a 2.5-meter reflector telescope in
a Boeing 747SP. The telescope is housed in an open cavity and is subjected to aeroacoustic and inertial disturbances in
flight. To meet pointing requirements, SOFIA must achieve a pointing stability of approximately 0.5 arcseconds RMS.
An active damping control system is being developed for SOFIA to reduce image jitter and image degradation due to
resonance of the telescope assembly. Our paper discusses the history of the active damping design for SOFIA, from
early concepts to the current implementation which has recently completed a ground and flight testing for proof-of-concept.
We describe some milestones in the analysis and testing of the telescope assembly which guided the
development of the vibration control system. The control synthesis approach and current implementation of the active
damping control system is presented. Finally, we summarize the performance observed in early flight tests and the steps
that are currently foreseen to completing the development of this system.
KEYWORDS: Telescopes, Image quality, Mirrors, Space telescopes, Motion measurement, Observatories, Control systems design, Control systems, Gyroscopes, Motion estimation
The Stratospheric Observatory for Infrared Astronomy (SOFIA) is an airborne observatory for astronomical observations at wavelengths ranging from 0.3-1600 µm. It consists of a telescope with an effective aperture of 2.5 m, which is mounted in a heavily modified Boeing 747SP. The aircraft features an open port cavity that gives the telescope an unobstructed view of the sky. Hence the optical system is subject to both aerodynamic loads from airflow entering the cavity, and to inertial loads introduced by motion of the airborne platform. A complex suspension assembly was designed to stabilize the telescope. Detailed end-to-end simulations were performed to estimate image stability based on the mechatronic design, the expected loads, and optical influence parameters. In December 2010 SOFIA entered its operational phase with a series of Early Science flights, which have relaxed image quality requirements compared to the full operations capability. At the same time, those flights are used to characterize image quality and image stability in order to validate models and to optimize systems. Optimization of systems is not based on analytical models, but on models derived from system identification measurements that are performed on the actual hardware both under controlled conditions and operational conditions. This paper discusses recent results from system identification measurements, improvements to image stability, and plans for the further enhancement of the system.
The NASA/DLR Stratospheric Observatory for Infrared Astronomy (SOFIA) employs a 2.5-meter reflector telescope in
a Boeing 747SP. The telescope is housed in an open cavity and will be subjected to aeroacoustic and inertial
disturbances. The image stability goal for SOFIA is 0.2 arc-seconds (RMS). Throughout the development phase of the
project, analytical models were employed to predict the image stability performance of the telescope, and to evaluate
pointing performance improvement measures. These analyses clearly demonstrated that key aspects which determined
performance were:
1) Disturbance environment and relevant load-paths
2) Telescope modal behavior
3) Sensor and actuator placement
4) Control algorithm design
The SOFIA program is now entering an exciting phase in which the characteristics of the telescope and the cavity
environment are being verified through ground and airborne testing. A modal survey test (MST) was conducted in early
2008 to quantify the telescope modal behavior. We will give a brief overview of analytical methods which have been
employed to assess/improve the pointing stability performance of the SOFIA telescope. In this context, we will describe
the motivation for the MST, and the pre-test analysis which determined the modes of interest and the required MST
sensor/shaker placement. A summary will then be given of the FEM-test correlation effort, updated end-to-end
simulation results, and actual data coming from telescope activation test flights.
KEYWORDS: Telescopes, Control systems, Space telescopes, Motion measurement, Control systems design, Mirrors, Observatories, Space operations, Aerodynamics, Actuators
During observation flights the telescope structure of the Stratospheric Observatory for Infrared Astronomy (SOFIA) is
subject to disturbance excitations over a wide frequency band. The sources can be separated into two groups: inertial
excitation caused by vibration of the airborne platform, and aerodynamic excitation that acts on the telescope assembly
(TA) through an open port cavity. These disturbance sources constitute a major difference of SOFIA to other ground
based and space observatories and achieving the required pointing accuracy of 1 arcsecond cumulative rms or better
below 70 Hz in this environment is driving the design of the TA pointing and control system. In the current design it
consists of two parts, the rigid body attitude control system and a feed forward based compensator of flexible TA
deformation. This paper discusses the characterization and control system tuning of the as-built system. It is a process
that integrates the study of the structural dynamic behavior of the TA, the resulting image motion in the focal plane, and
the design and implementation of active control systems. Ground tests, which are performed under controlled
experimental conditions, and in-flight characterization tests, both leading up to the early science performance capabilities
of the observatory, are addressed.
KEYWORDS: Telescopes, Kinematics, Spherical lenses, Space telescopes, Large telescopes, Electroluminescence, Actuators, Data modeling, Interfaces, Safety
Secondary mirrors and lenses in several planned ground-based telescopes have masses on the order of 5000 kg and
require a positioning system that is repeatable to one-tenth the pixel size of the optical sensors, nominally 10
micrometers or less. Hexapods, or Stewart Platforms, are frequently integrated into the support structure as six degree of
freedom parallel positioning and alignment systems. These systems are limited in resolution by friction in the 36
kinematic degrees of freedom (DOF) necessary for properly constrained motion of the platform. The 30 passive DOF,
typically implemented with one 3-DOF and one 2-DOF joint on each hexapod leg, introduce unwanted friction and/or
backlash into the positioning system. Backlash is generally unacceptable and elimination requires significant preloading
of the joints, which in turn increases joint friction. This paper will: review various joint types including rolling element,
plain bearing (sliding), and flexure; examine the backlash and friction tradeoffs involved in selecting joint type including
unwanted deflections due to joint moments, static position resolution limitations, dynamic positioning settling time
effects, self-locking mechanisms, and power dissipation; compare with experimental data and previously published
results; present methods for modeling both static and dynamic effects of friction; and suggest recommendations for
general positioning system design. Considerations for both equatorial and altitude-azimuth telescopes will be discussed,
along with variation of effects due to telescope positioning. Analyses will be reinforced with friction and backlash
measurements made on several physical joints.
The Stratospheric Observatory For Infrared Astronomy, SOFIA is being developed by NASA and the German space agency, Deutschen Zentrum fur Luft- und Raumfahrt (DLR), with an international contractor team. The 2.5-meter reflecting telescope of SOFIA will be the world's largest airborne telescope. Flying in an open cavity on a modified 747 aircraft, SOFIA will perform infrared astronomy while cruising at 41,000 feet and while being buffeted by a 550- mile-per-hour slipstream. A primary system requirement of SOFIA is tracking stability of 0.2 arc-seconds, and a 3-axis pointing control model has been used to evaluate the feasibility of achieving this kind of stability. The pointing control model shows that increased levels of damping in certain elastic modes of the telescope assembly will help achieve the tracking stability goal and also expand the bandwidth of the attitude controller. This paper describes the preliminary work that has been done to approximate the reduction in image motion yielded by various structure configurations that use reaction masses to attenuate the flexible motions of the telescope structure. Three approaches are considered: passive tuned-mass dampers, active-mass dampers, and attitude control with reaction-mass actuators. Expected performance improvements for each approach, and practical advantages and disadvantages associated with each are presented.
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