William L. Hayden

TRL-6 for JWST wavefront sensing and control

NASAs Technology Readiness Level (TRL)-6 is documented for the James Webb Space Telescope (JWST) Wavefront Sensing and Control (WFSC) subsystem. The WFSC subsystem is needed to align the Optical Telescope Element (OTE) after all deployments have occurred, and achieves that requirement through a robust commissioning sequence consisting of unique commissioning algorithms, all of which are part of the WFSC algorithm suite. This paper identifies the technology need, algorithm heritage, describes the finished TRL-6 design platform, and summarizes the TRL-6 test results and compliance. Additionally, the performance requirements needed to satisfy JWST science goals as well as the criterion that relate to the TRL-6 Testbed Telescope (TBT) performance requirements are discussed.

Wavefront Sensing and Controls for the James Webb Space Telescope

The James Webb Space Telescope (JWST) is a segmented deployable telescope, utilizing 6 degrees of freedom for adjustment of the Secondary Mirror (SM) and 7 degrees of freedom for adjustment of each of its 18 segments in the Primary Mirror (PM). When deployed, the PM segments and the SM will be placed in their correct optical positions to within a few mm, with accordingly large wavefront errors. The challenge, therefore, is to position each of these optical elements in order to correct the deployment errors and produce a diffraction-limited telescope, at λ=2μm, across the entire science field. This paper describes a suite of processes, algorithms, and software that has been developed to achieve this precise alignment, using images taken from JWST’s science instruments during commissioning. The results of flight-like end-to-end simulations showing the commissioning process are also presented.

Space telescope design considerations

The design considerations for astronomical space telescopes cover many disciplines but can be simplified into two overarching constraints: the desire to maximize science while adhering to budgetary constraints. More than ever, understanding the cost implications up front will be critical to success. Science performance can be translated into a set of simple performance metrics that set the requirements for design options. Cost is typically estimated by considering mass, complexity, technology maturity, and heritage. With this in mind, we survey the many diverse design considerations for a space telescope and, where appropriate, relate them to these basic performance metrics. In so doing, we hope to provide a roadmap for future space telescope designers on how best to optimize the design to maximize science and minimize total cost.

Optical design and performance of the NGST wavefront control testbed

The NGST Wavefront Control Testbed (WCT) is a joint technology program managed by the Goddard Space Flight Center (GSFC) and the Jet Propulsion Laboratory (JPL) for the purpose of developing technologies relevant to the NGST optical system. The WCT provides a flexible testing environment that supports the development of wavefront sensing and control algorithms that may be used to align and control a segmented optical system. WCT is a modular system consisting of a Source Module (SM), Telescope Simulator Module (TSM) and an Aft-Optics (AO) bench. The SM incorporates multiple sources, neutral density filters and bandpass filters to provide a customized point source for the TSM. The telescope simulator module contains a flip-in mirror that selects between a small deformable mirror and three actively controlled spherical mirror segments. The TSM is capable of delivering a wide range of aberrated, unaberrated, continuous and segmented wavefronts to the AO optical bench for analysis. The AO bench consists of a series of reflective and transmissive optics that images the exit pupil of the TSM onto a 349 actuator deformable mirror that is used for wavefront correction. A Fast Steering Mirror (FSM) may be inserted into the system (AO bench) to investigate image stability and to compensate for systematic jitter when operated in a closed loop mode. We will describe the optical design and performance of the WCT hardware and discuss the impact of environmental factors on system performance.

Optical state estimation using wavefront data

The use of wavefront measurements to deduce the state of multiple optics in a telescope beam train - their misalignments and figure errors - can be confused by the fact that there are multiple potential sources for the same measured error. This talk applies Kalman filtering techniques as a tool for separating true telescope errors from artifactual testing errors in the alignment and testing of NASAs James Webb Space Telescope, a large segmented-aperture cryogenic telescope to be launched after 2010.

Estimation of the on-orbit distortion of the Mars Orbiter Laser Altimeter (MOLA II) primary mirror

This paper describes the analyses performed to estimate the on-orbit distortion of the Mars Orbiter Laser Altimeter (MOLA II) primary mirror. MOLA II is one of five scientific instruments that will be flown on the Mars Global Surveyor. The MOLA II instrument will map the surface profile of Mars for a full Marian year to a resolution of 2 meters vertical and 160 meters horizontal. The MOLA II telescope is an f/6 Cassegrain telescope with a 0.85 milliradian (mrad) field of view. The telescope is made entirely of Brush Wellman S200F vacuum hot pressed beryllium. The primary mirror diameter is 508 mm with a base radius of curvature of 711.2 mm. This mirror is plated first with electroless nickel and then with electrolytic gold. The purpose of these analyses was (1) to estimate the on-orbit distortion of the large primary mirror due to thermal loading, interface stresses, and gravity release and (2) to calculate the expected damage to the mirror surface due to micrometeroid impacts. A detailed NASA structural analysis program finite element model was used as a tool for evaluating the mirror performance. The results of the analyses indicate that a stability error of 2.4 microns peak-to-valley and 0.6 microns root mean square is expected for the on-orbit distortion of the primary mirror surface. The estimated surface damage due to micrometeoroids is 0.03 cm2, which is 0.002 percent of the total surface area. Both of these results are within mission acceptance parameters.

Overview of laser communication technology at NASA Goddard Space Flight Center

A geosynchronous free space optical communications crosslink system is described. System analysis and technology development for a direct detection baseband digital optical crosslink at 650 Mbps is presented.

Preparing for JWST wavefront sensing and control operations

The James Webb Space Telescopes segmented primary and deployable secondary mirrors will be actively con- trolled to achieve optical alignment through a complex series of steps that will extend across several months during the observatorys commissioning. This process will require an intricate interplay between individual wavefront sensing and control tasks, instrument-level checkout and commissioning, and observatory-level calibrations, which involves many subsystems across both the observatory and the ground system. Furthermore, commissioning will often exercise observatory capabilities under atypical circumstances, such as fine guiding with unstacked or defocused images, or planning targeted observations in the presence of substantial time-variable offsets to the telescope line of sight. Coordination for this process across the JWST partnership has been conducted through the Wavefront Sensing and Control Operations Working Group. We describe at a high level the activities of this group and the resulting detailed commissioning operations plans, supporting software tools development, and ongoing preparations activities at the Science and Operations Center. For each major step in JWSTs wavefront sensing and control, we also explain the changes and additions that were needed to turn an initial operations concept into a flight-ready plan with proven tools. These efforts are leading to a robust and well-tested process and preparing the team for an efficient and successful commissioning of JWSTs active telescope.

Space station laser communication transceiver

NASA/Goddard Space Flight Center (GSFC) has initiated the development of experimental optical communication system which will be installed on Space Station Freedom. This system is part of the Space Station Attached a Payloads Program and is currently scheduled for a 1997 launch. The system is being designed to carry out comprehensive set of tests to evaluate and demonstrate the capabilities of this relatively new technology. Communication tests at rates up to 1200 mbps will be conducted over the space-to-ground link using an existing tracking facility at the GSFC. GaAlAs semiconductor lasers will be intensity modulated using 4 slot pulse position modulation format. Direct detection receivers using silicon avalanche photodiodes will be utilized, and 1 microradian accuracy pointing will be achieved with 2 cascaded pointing stages. Successful completion of this in-orbit test program should demonstrate both the technical maturity and readiness of this technology for follow-up operational missions.

The large UV/optical/infrared surveyor (LUVOIR): decadal mission study update

NASA commissioned the study of four large mission concepts, including the Large Ultraviolet / Optical / Infrared (LUVOIR) Surveyor, to be evaluated by the 2020 Decadal Survey in Astrophysics. In response, the Science and Technology Definition Team (STDT) identified a broad range of science objectives for LUVOIR that include the direct imaging and spectral characterization of habitable exoplanets around sun-like stars, the study of galaxy formation and evolution, the exchange of matter between galaxies, star and planet formation, and the remote sensing of Solar System objects. To meet these objectives, the LUVOIR Study Office, located at NASA’s Goddard Space Flight Center (GSFC), completed the first design iteration of a 15-m segmented-aperture observatory that would be launched by the Space Launch System (SLS) Block 2 configuration. The observatory includes four serviceable instruments: the Extreme Coronagraph for Living Planetary Systems (ECLIPS), an optical / near-infrared coronagraph capable of delivering 10−10 contrast at inner working angles as small as 2 λ/D; the LUVOIR UV Multi-object Spectrograph (LUMOS), which will provide low- and medium-resolution UV (100 – 400 nm) multi-object imaging spectroscopy in addition to far-UV imaging; the High Definition Imager (HDI), a high-resolution wide-field-of-view NUV-Optical-NIR imager; and Pollux, a high-resolution UV spectro-polarimeter being contributed by Centre National d’Etudes Spatiales (CNES). The study team has executed a second design iteration to further improve upon the 15-m concept, while simultaneously studying an 8-m concept. In these proceedings, we provide an update on these two architectures.