Joel Shields

Low-order wavefront sensing and control for WFIRST-AFTA coronagraph

Abstract. To maintain the required Wide-Field Infrared Survey Telescope (WFIRST) coronagraph performance in a realistic space environment, a low-order wavefront sensing and control (LOWFS/C) subsystem is necessary. The LOWFS/C uses the rejected stellar light from the coronagraph to sense and suppress the telescope pointing errors as well as low-order wavefront errors (WFEs) due to changes in thermal loading of the telescope and the rest of the observatory. We will present a conceptual design of a LOWFS/C subsystem for the WFIRST-AFTA coronagraph. This LOWFS/C uses a Zernike phase contrast wavefront sensor (ZWFS) with a phase shifting disk combined with the stellar light rejecting occulting masks, a key concept to minimize the noncommon path error. We will present our analysis of the sensor performance and evaluate the performance of the line-of-sight jitter suppression loop, as well as the low-order WFE correction loop with a deformable mirror on the coronagraph. We will also report the LOWFS/C testbed design and the preliminary in-air test results, which show a very promising performance of the ZWFS.

Spacecraft inertia estimation via constrained least squares

This paper presents a new formulation for spacecraft inertia estimation from flight data. Specifically, the inertia estimation problem is formulated as a constrained least squares minimization problem with explicit bounds on the inertia matrix incorporated as LMIs (linear matrix inequalities). The resulting minimization problem is a semidefinite optimization problem that can be solved efficiently with guaranteed convergence to the global optimum by readily available algorithms. This method is applied to test data collected from a robotic testbed consisting of a free rotating body. The results show that the constrained least squares approach produces more accurate estimates of the inertia matrix than standard unconstrained least squares estimation methods

Metrology sensor characterization and pointing control for the formation interferometer testbed (FIT)

StarLight is a NASA/JPL sponsored mission that will demonstrate formation flying and interferometric technologies needed for future missions such as Terrestrial Planet Finder (TPF). The StarLight stellar interferometer is composed of two spacecraft, a combiner spacecraft and a collector spacecraft, forming separated apertures. In this paper, the metrology pointing sensor of the StarLight ground instrument prototype is characterized and a siderostat pointing control loop is closed between two optical benches with a 10 meter separation. The Formation Interferometer Testbed (FIT) metrology pointing sensor (MPS) uses four photodiodes to detect gradients in the metrology laser caused by relative motion of the two optical benches. By differencing across the photodiodes, a measurement of the laser beam centroid can be obtained. In the flight design, this signal is passed to the combiner spacecraft, via an interspacecraft communication link, and used as a feedback signal to point a siderostat on the combiner spacecraft.

Starlight pointing subsystem for the formation interferometer testbed (FIT)

StarLight is a NASA/JPL sponsored mission that will be the first spaceborne stellar interferometer. The objective of the mission is to develop and test technologies for performing interferometric observations in space and for the extremely precise formation flying required to make the instrument work. The Formation Interferometer Testbed (FIT) is a ground based prototype of the two spacecraft StarLight instrument. In this paper a complete model of the FIT pointing system is characterized that includes backlash in the siderostat (SD), hysteresis in the PZT actuators of the fast steering mirrors (FSM), time delays, and air path disturbances. Based on this model, a dual stage pointing controller is designed that achieves a bandwidth sufficient to satisfy the pointing requirement. Simulated and experimental results are given that validate the performance of the pointing system.

Dynamic testbed demonstration of WFIRST coronagraph low order wavefront sensing and control (LOWFS/C)

To maintain the required performance of WFIRST Coronagraph in a realistic space environment, a Low Order Wavefront Sensing and Control (LOWFS/C) subsystem is necessary. The LOWFS/C uses a Zernike wavefront sensor (ZWFS) with the phase shifting disk combined with the starlight rejecting occulting mask. For wavefront error corrections, WFIRST LOWFS/C uses a fast steering mirror (FSM) for line-of-sight (LoS) correction, a focusing mirror for focus drift correction, and one of the two deformable mirrors (DM) for other low order wavefront error (WFE) correction. As a part of technology development and demonstration for WFIRST Coronagraph, a dedicated Occulting Mask Coronagraph (OMC) testbed has been built and commissioned. With its configuration similar to the WFIRST flight coronagraph instrument the OMC testbed consists of two coronagraph modes, Shaped Pupil Coronagraph (SPC) and Hybrid Lyot Coronagraph (HLC), a low order wavefront sensor (LOWFS), and an optical telescope assembly (OTA) simulator which can generate realistic LoS drift and jitter as well as low order wavefront error that would be induced by the WFIRST telescope’s vibration and thermal changes. In this paper, we will introduce the concept of WFIRST LOWFS/C, describe the OMC testbed, and present the testbed results of LOWFS sensor performance. We will also present our recent results from the dynamic coronagraph tests in which we have demonstrated of using LOWFS/C to maintain the coronagraph contrast with the presence of WFIRST-like line-of-sight and low order wavefront disturbances.

Terrestrial Attitude Estimation for the Formation Control Testbed (FCT)

In this paper we look at the problem of terrestrial (earth based) attitude estimation of a unique robotic vehicle using full three axis attitude measurements and three axis inertial rate sensors (gyros). The vehicle is completely autonomous and uses air bearings to simulate the drag free dynamic environment of space. An onboard infrared camera system is used to provide quaternion measurements representing the attitude of the robot relative to the room frame of the test facility. Fiber optic gyros are used to sense the inertial angular rates. To simulate the performance of the system, a stochastic model of the gyros was developed based on long term rate table data. The angle random walk, bias, and bias stability were determined to agree with the data provided in the manufactures specification sheet. We show that a 3times reduction in the standard deviation of the attitude estimates can be achieved by proper mixing of the two sensor measurements. The attitude estimation algorithm used in this paper also provides bias free estimates of the angular rate which can be used for control or other purposes. These results are established in both high fidelity simulations and experimentally using data taken during real time operation of the robot.

WFIRST low order wavefront sensing and control dynamic testbed performance under the flight like photon flux

To maintain the required performance for the WFIRST Coronagraph Instrument (CGI) in a realistic space environment, a Low Order Wavefront Sensing and Control (LOWFS/C) subsystem is necessary. The WFIRST CGI LOWFS/C subsystem will use the Zernike wavefront sensor, which has a phase-shifting disk combined with the coronagraph’s focal plane mask, to sense the low-order wavefront drift and line-of-sight (LoS) error using the rejected starlight. The dynamic tests on JPL’s Occulting Mask Coronagraph (OMC) Testbed have demonstrated that LOWFS/C can maintain coronagraph contrast to better than 10-8 in presence of WFIRST-like line of sight and low order wavefront disturbances in both Shaped Pupil Coronagraph (SPC) and Hybrid Lyot Coronagraph (HLC) modes. However, the previous dynamic tests have been done using a bright source with photon flux equivalent to stellar magnitude of MV = -3.5. The LOWFS/C technology development on the OMC testbed has since then concentrated in evaluating and improving the LOWFS/C performance under the realistic photon flux that is equivalent to WFIRST Coronagraph target stars. Our recent testbed tests have demonstrated that the LOWFS/C can work cohesively with the stellar light suppression wavefront control, which brings broad band coronagraph contrast from ~1x10-6 to 6x10-9, while LOWF/C is simultaneously suppressing the WFIRST like LoS and low order wavefront drift disturbances on a source that photon flux is equivalent to a MV = 2 star. This lab demonstration mimics the CGI initial dark hole establish process on a bright reference star. We have also demonstrated on the testbed that LOWFS/C can maintain the coronagraph contrast by suppressing the WFIRST like line-of-sight disturbances on a fainter MV = 5 star. This mimics scenario of CGI science target observations. In this paper we will present the recent dynamic testbed performance results of LOWFS/C LoS loops and low order wavefront error correction loop on the flight like photon flux.

Real-time Interferometer Control System Toolbox evolutionary improvements

The Real-time Interferometer Control System Toolbox (RICST) software components have been evolving for the last ten years and have been used by many optical interferometry testbeds. Most of our work in the last year has been in support of the Space Interferometry Mission. A number of improvements have been added to our toolkit in order to bring the elements of the Instrument to improved state of technological readiness.

Design, Modeling and Control of an Optical Pointing Sensor for the Formation Control Testbed (FCT)

In this paper the design and modeling of a sensor system that gives relative position measurements is described. The position is provided in the form of bearing and range to a retro target placed on a far field target.

Some Observations on Dynamics and Control of Spacecraft Formations

Abstract We investigate the simplest dynamical models of a formation of spacecraft in orbit, and develop the stability conditions for the equivalent virtual structure. These stability conditions represent a first step towards an understanding of how to control these formations. The concept of generalized formation joint is also introduced, using multibody dynamics concepts, and its use for formation reconfigurations is also demonstrated. Finally, the concept of the virtual structure is extended to swarms with stochastic distributions.