Bryan H. Kang

An integrated optimal estimation approach to Spitzer Space Telescope focal plane survey

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.

An introduction to the Guide-2 telescope testbed for the SIM Planet Quest Light mission

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.

Focal plane calibration of the Spitzer space telescope

The Spitzer space telescope (Spitzer) is currently NASAs largest and most sensitive infrared (IR) telescope in space. Spitzers focal plane carries detectors from three science instruments, namely, the infrared array camera (IRAC), the infrared spectrograph (IRS), and the multiband imaging photometer for Spitzer (MIPS). In this article we discuss the instrument pointing frame (IPF) Kalman filter, which is used to calibrate Spitzers telescope focal plane. The IPF filter is a high-order square-root iterated linearized Kalman filter that carries 37 states to estimate frame misalignments, while correcting for systematic errors due to optical distortions, scan-mirror errors, thermomechanically induced drift variations, and gyro bias and drift in all axes. The Spitzer application demonstrates that the integrated approach offers significant advantages with respect to optimality, time-efficiency, anomaly detection, and health monitoring compared to existing telescope-calibration approaches, where the parameters are artificially broken into subsets that are estimated by separate teams of analysts. Performance results for the IPF Kalman filter indicate that all Spitzer calibration requirements are satisfied, and are consistent with margins predicted by preflight error analysis. On a final note, after more than five-and-a-half years of probing the cool cosmos, Spitzer entered standby mode on May 15, 2009, as a result of running out of the liquid helium coolant that kept its infrared instruments chilled. This event marks the successful completion of the Spitzers cold mission as originally commissioned by NASA. However, even though the telescope is warming up, the IRAC arrays continue to operate and provide useful scientific data. A new follow-on warm mission based on the IRAC arrays has been defined and initiated, so that Spitzer will remain in commission for several years to come.

Results of the Guide-2 telescope testbed for the SIM Light Astrometric Observatory

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.

SIM PlanetQuest interferometer real-time control system architecture

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.

Instrument pointing control system for the Stellar Interferometry Mission: Planet Quest

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.