The SIM Lite Astrometric Observatory is to perform narrow angle astrometry to search for Earth-like planets, and global
astrometry for a broad astrophysics program, for example, mapping the distribution of dark matter in the Galaxy. The
new SIM Lite consists of two Michelson interferometers and one star tracking telescope. The main six-meter baseline
science interferometer observes a target star and a set of reference stars. The four-meter baseline interferometer (guide-1)
monitors the attitude of the instrument in the direction of a target star. The Guide-2 telescope (G2T) tracks a bright star
to monitor the attitude of the instrument in the other two orthogonal directions. A testbed has been built to demonstrate
star-tracking capability of the G2T concept using a new interferometric angle metrology system. In the presence of
simulated 0.2 arcsecond level of expected spacecraft attitude control system perturbations, the measured star-tracking
capability of the G2T testbed system is less than 43 micro-arcsecond during single narrow angle observation.
The Space Interferometry Mission - Planet Quest Lite (SIM-PQL) Guide 2 telescope system is currently being developed
at JPL. The SIM-PQL is a new mission concept to perform micro-arcsecond narrow-angle astrometry to search
approximately 50 nearby stars for Earth-like planets, and to perform global astrometry with an accuracy of six micro-arcseconds
in position and parallax. The novel cost and mass reducing "Lite" concept includes reduction in
interferometer baselines and replacing the second guide interferometer (Guide 2) with a telescopic system. The resulting
simplification still allows meeting most science goals without significant performance degradation. The Guide 2
telescope employs a nulling pointing control system that utilizes a Fast Steering Mirror (FSM) as an actuator and a star
tracking CCD camera as a control sensor. Under the nulling closed loop control, the modulated attitude motion of the
instrument is picked off by a metrology FSM tip-tilt sensor (AMET). The Guide 2 pointing control system requires mili-arcsecond
class fine pointing, maintenance of low jitter and thermal stability and a sub-nanometer class metrology
system that ties the sensor bench to rest of the instrument. This paper presents the Guide 2 telescope pointing control
system design and resulting performance estimates. The pointing control requirements are first stated followed by the
descriptions of the system architecture, algorithm design and simulation results. Concept and algorithm validation is
conducted on a workstation-based simulation testbed, specifically developed to capture critical sensor/actuator behavior
and environmental disturbances.
The Space Interferometry Mission Light (SIM-Lite) is a new mission concept to perform a micro-arcsecond narrow-angle
astrometry to search approximately 50 nearby stars for Earth-like planets, and to perform a global astrometry with
an accuracy of six micro-arcsecond position and parallax measurements. The SIM-Lite consists of two Michelson
interferometers and one telescope. The main six-meter baseline science interferometer observes a target star and a set of
reference stars. The four-meter baseline interferometer (guide-1) monitors the attitude of the instrument in the direction
of a target star. A Guide-2 telescope (G2T) tracks a bright star to monitor the attitude of the instrument in the other two
orthogonal directions. To demonstrate the concept of the G2T, we have developed a testbed using brassboard optics
built for the SIM project. The G2T testbed consists of a 35 cm siderostat, a beam compressor, and a fast steering mirror
(FSM) in closed loop with a CCD based pointing sensor. A heterodyne laser angle metrology system is used to monitor
angular positions of the FSM with required accuracy of 20 micro-arcsecond during SIM-Lite narrow-angle observation
time. We present the concept of the testbed architecture and preliminary test results of the angular metrology (aMet)
system.
This paper presents a top-level architectural overview of the instrument real-time control system currently under
development at JPL for the SIM-Planet Quest interferometer. The control system must meet challenging requirements
for providing milliarcsecond class pointing and nanometer class delay-line control performance while tracking science
stars as dim as 20th visual magnitude. The driving functional requirements call for a three-interferometer system that
also serves as an attitude sensing and tracking system. Due to the dim science requirements and complicated control
initialization processes, the control system is architectured using complex estimators, multiloop feedforward signals, and
distributed computational infrastructure. Control objectives and requirements are presented and the necessary control
sensors and actuators are discussed. Initialization of the interferometer control system is explained, including processes
for target star search, acquisition, and tracking. The nominal tracking control modes are then presented, including
incorporation of pathlength and angle feedforward signals. The estimation architecture is explained next including its
role in generating the necessary feedforward signals. The resulting overall algorithm structure and implementation using
distributed processors on a ring-bus architecture is also briefly discussed.
KEYWORDS: Stars, Cameras, Control systems, Reticles, Imaging systems, Error analysis, Mirrors, Interferometers, Received signal strength, Data modeling
This paper presents a performance analysis of the instrument pointing control system for NASA's Space Interferometer
Mission (SIM). SIM has a complex pointing system that uses a fast steering mirror in combination
with a multirate control architecture to blend feedforward information with feedback information. A pointing
covariance analysis tool (PCAT) is developed specifically to analyze systems with such complexity. The development
of PCAT as a mathematical tool for covariance analysis is outlined in the paper. PCAT is then applied
to studying performance of SIM's science pointing system. The analysis reveals and clearly delineates a fundamental
limit that exists for SIM pointing performance. The limit is especially stringent for dim star targets.
Discussion of the nature of the performance limit is provided, and methods are suggested to potentially improve
pointing performance.
This paper describes the high precision Instrument Pointing Control System (PCS) for the Stellar Interferometry Mission (SIM) - Planet Quest. The PCS system provides front-end pointing, compensation for spacecraft motion, and feedforward stabilization, which are needed for proper interference. Optical interferometric measurements require very precise pointing (0.03 as, 1-σ radial) for maximizing the interference pattern visibility. This requirement is achieved by fine pointing control of articulating pointing mirrors with feedback from angle tracking cameras. The overall pointing system design concept is presented. Functional requirements and an acquisition concept are given. Guide and Science pointing control loops are discussed. Simulation analyses demonstrate the feasibility of the design.
This paper discusses an accurate and efficient method for
focal plane survey that was used for the Spitzer Space Telescope.
The approach is based on using a high-order 37-state Instrument Pointing Frame (IPF) Kalman filter that combines both engineering parameters and science parameters into a single filter formulation. In this approach, engineering parameters such as pointing alignments, thermomechanical drift and gyro drifts are estimated along with
science parameters such as plate scales and optical distortions. This integrated approach has many advantages compared to estimating the engineering and science parameters separately. The resulting focal plane survey approach is applicable to a diverse range of science instruments such as imaging cameras, spectroscopy slits, and scanning-type arrays alike. The paper will summarize results from applying the IPF Kalman filter to calibrating the Spitzer Space Telescope focal plane, containing the MIPS, IRAC, and the IRS science instrument arrays.
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