Wavefront sensing and control software for a segmented space telescope

The next generation of large space telescopes, including ATLAST, SAFIR, IXO and Generation-X will require the development of key technologies to enable their development at an affordable cost. This includes technologies for the rapid, low cost fabrication of ultra-light weight primary mirror segments, active figure control of primary mirror segments, high speed wavefront sensing and control, highly-packageable and scalable deployment techniques, and active vibration and thermal control for light weight structural elements to supply good pointing stability. In this paper we discuss the current state-of-the-art for these technologies and roadmaps for future development in these areas.

Vibration isolation and precision pointing control will be essential to the success of future space science missions such as Next Generation Space Telescope (NGST). This research investigated the use of actively controlled piezoelectric stack actuators to reduce the transmission of mechanical disturbances to the optics and to coarsely point the telescope. Initial actuator requirements were based on approximate expected motions on the spacecraft side (due to unbalanced reaction wheel assemblies), the structural dynamic properties of the flexible, deployable truss structure that supports the active optics, and the allowable motions on the telescope side. Piezoelectric actuator design was based on flextensional motion amplification, with sizing details determined using results from finite element analysis. A compressive pre-load was maintained on the fragile piezoceramic stacks via a mechanical interference (center block) fit into the actuator frame. Device performance characteristics, including blocked force, passive stiffness, free displacement, motion amplification factor, natural vibration frequencies, and electromechanical coupling coefficient were determined numerically and experimentally, with good agreement. An initial controlled isolation test of the entire system reduced the transmissibility by more than 80% over the frequency range from 4-20 Hz. Finally, an active isolation system with dual sensor feedback was implemented and shown to successfully reduce telescope motion by a factor of 10 (from 4-40 Hz) and to provide lower-frequency pointing capability. Introduction As space systems continue to grow in complexity and require greater precision, piezoelectric materials will be investigated for an increasing number of actuator applications, including precision pointing and vibration isolation and control. There are several approaches to vibration isolation, which have been successful in the past. Vibration isolation can be achieved through passive or active techniques or a combination of the two. The chosen method of isolation depends primarily on the application and requirements of the system. Precision pointing, on the other hand, is achieved solely by active control systems, which combine the necessary structural and mechanical components with a control design. Pointing systems are primarily limited by the quality of the mechanical system as well as the drive system, which is actuating movement. The structure (holding the pay load) to be controlled can be a rigid body, quasi-static flexible body, or a dynamic flexible body. With the classical arrangements of sensors and actuators in these control systems, only the rigid body configuration is commonly controlled. NGST (Next Generation Space Telescope) is the successor to the Hubble Space Telescope (HST) mission. NGST is a deployable 8-meter diameter truss structure holding the telescope and instruments such as cameras and spectrometers that are required by the scientific goals of the mission. In order to meet the scientific goals or projects such as the Origins of Galaxies studies [Mather, et al] along with independent science projects dealing with current scientific interests, positioning the optics and minimizing vibratory loads are very important. Positioning control for the HST was done as a rigid structure pointed as a unit to a precision of a small fraction of an arcsecond, but NGST is a dynamic flexible structure with sub-micron positioning performance. For NGST, the isolation system is to be used to reduce the ultrafine pointing requirements on the fine pointing system (fast steering mirror and adjusters distributed across the mirror) by as much as 1 2 orders of magnitude. This isolation system is aimed to reduce the estimated 30-nm peak disturbances at the mast to 6-nm or less in a range of 4 4 0 Hz, with most concern on the 15 20 Hz region. By substantially reducing these axial disturbances, the dynamic, wavefront error correction requirements will also be reduced, and the fine pointing system will have less of a burden. In order to achieve these requirements, piezoelectric stack actuators are ideal because they can provide sufficiently high output forces with large bandwidths. Individually, piezoelectric stack displacements are extremely small, on the order of approximately 2000 jiie; therefore, multiple stacks can be used in series to produce the desired output. In addition to this, mechanical amplification devices can be used to increase actuation displacement by reducing the actuation force. Compliant mechanisms, flexurehinged mechanisms and flextensional mechanisms Copyright © 2001 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. (c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization. are commonly used for motion amplification, depending on the application. The flextensional actuator used in this research is shown in Figure I and consists of four stacks of piezoceramics connected in parallel to a flextensional mechanism that amplifies and converts the output

A series of developmental as well as flight mirrors have been in process over the last few years for IR cryogenic telescope applications such as the Space Infrared Telescope Facility (SIRTF) and the Next Generation Space Telescope (NGST) and for visible ambient systems such as Space Based Laser (SBL). We will discuss the performance of the 0.85-m SIRTF primary mirror (26.6 kg/m 2 areal density) and the 0.5-m Subscale Beryllium Mirror Demonstrator (SBMD) beryllium mirror (9.8 kg/m 2 areal density) as well as the current status of the 1.4-m Ball semi-rigid, beryllium Advanced Mirror System Demonstrator (AMSD). The AMSD mirror itself has an areal density of 10.4 kg/m 2 and is currently in polishing. The entire AMSD assembly including composite reaction structure, flexures, and actuators, has an areal density less than 15 kb/m 2 . Cyrogenic test results of the SIRTF and SBMD mirrors will be presented along with test data on the AMSD actuators. The SBMD mirror wsa cryofigured based on ambient and cryo testing to achieve a wavefront quality of 19 nm rms at 35 K. In addition, the effects of optically coating SBMD with a protected gold multi-layer system will be shown - demonstrating that a lightweight mirror can be coated without adverse print-through due to coating stress at ambient or cryo operating temperatures.

Some proposed architectures for large telescopes in space such as the Next Generation Space Telescope (NGST) utilize a segmented primary mirror with active control. The image quality for those systems can be characterized by the quality of the point spread function (PSF). We will present results from our simulations that demonstrate the quality of the PSF at various wavelengths, as characterized by the metrics of Strehl Ratio and encircled energy fraction. These metrics capture the critical features of the PSF that ensure good spatial resolution of the core at the nominal diffraction limited wavelength without having excessive scattering into the nearby diffraction pattern for observations at shorter wavelengths. The relationship of the image quality to the spatial frequency content of the system wave front error and to the image motion that occurs during the exposure (line-of-sight jitter) are explored.

The Lockheed Martin/Advanced Technology Center (LM/ATC) developed a lightweight, compact, high-load capable and yet high precision latch for use on deployable optical systems such as the Next Generation Space Telescope (NGST). The design allows precise self-centering and control of the stiffness at the latch interface. It also incorporates unique capabilities to evaluate the effects of gravity loads, latch preload level, creep, and very low vibration loads on the dynamics and microdynamics of the deployed instrument. The stiffness, nonlinearity and hysteresis characteristics of the latch and its catch flexure assembly were thoroughly tested in 6 axes down to the nanometer level at room temperature using the LM/ATC Compliance Measurement Device. The latch is stiff enough to hold an NGST-size mirror segment cantilevered against gravity allowing only small gravity sag when the primary mirror is horizontal, thus enabling end-to-end performance verification in 1-G in that orientation. The latch hysteresis is less than 1.0 nm/N under mechanical loads less than 25 N, which meets the NGST stability requirements with significant margin (20 nm at the tip of the petal in space environment). Several of these latches were integrated and demonstrated at the petal assembly level on a Single Petal Test-bed and the experimental results obtained on that test-bed are consistent with the component level results described in this report. We experimentally demonstrated that the latch engagement performance is not affected by exposure to cryogenic temperatures down to 20K, as required for use of the device on cryogenic infrared optical instruments such as NGST. A structural model of the latch was developed using Finite Element Analysis. Good correlation was obtained between the linear components of the analytical and of the experimental results: the model can therefore reliably be used in future NGST or other mission design efforts. This paper includes a brief description of the LM/ATC latch hardware and its principle of operation as well as the results of the modeling and the experimental characterization work performed on that hardware in the NGST Phase I formulation.

The Next Generation Space Telescope (NGST) will be a segmented, deployable, infrared-optimized 6.5m space telescope. Its active primary segments will be aligned, co-phased, and then fine-tuned in order to deliver image quality sufficient for the telescope's intended scientific goals. Wavefront sensing used to drive this tuning will come from the analysis of focussed and defocussed images taken with its near-IR science camera, NIRCAM. There is a pressing need to verify that this will be possible with the near-IR detectors that are still under development for NGST. We create simulated NIRCAM images to test the maintenance phase of this plan. Our simulations incorporate Poisson and electronics read noise, and are designed to be able to include various detector and electronics non-linearities. We present our first such simulation, using known or predicted properties of HAWAII HgCdTe focal plane array detectors. Detector effects characterized by the Independent Detector Testing Laboratory will be included as they become available. Simulating InSb detectors can also be done within this framework in future. We generate Point-Spread Functions (PSF's) for a segmented aperture geometry with various wavefront aberrations, and convolve this with typical galaxy backgrounds and stellar foregrounds. We then simulate up-the-ramp (MULTIACCUM in HST parlance) exposures with cosmic ray hits. We pass these images through the HST NICMOS `CALNICA' calibration task to filter out cosmic ray hits. The final images are to be fed to wavefront sensing software, in order to find the ranges of exposure times, filter bandpass, defocus, and calibration star magnitude required to keep the NGST image within its specifications.

The Blocked Impurity Band (BIB) detector was invented in the early 1980's and subsequently developed by our team. The original arsenic-doped silicon (Si:As) detectors addressed the need for low-noise, radiation-tolerant, mid-IR detectors for defense surveillance from space. We have since developed large-format BIB focal plane arrays to address high-background requirements of ground-based telescopes and missile interceptors, low-background requirements of the Space Infrared Telescope Facility (SIRTF), and very low background requirements of the mid-IR instruments for the Next Generation Space Telescope (NGST) and Terrestrial Planet Finder. Most of these applications employ Si:As BIB detectors, but antimony-doped silicon (Si:Sb) BIB detectors are used for some SIRTF bands. Other demonstrated types including phosphorus (Si:P) and gallium-doped (Si:Ga) BIB detectors may have application niches. We have proposed development of a BIB detector type utilizing both Si:As and Si:P layers to optimize dark current vs. wavelength performance. Wavelength response for silicon BIB detectors extend to a maximum of ~40 microns (Si:Sb), but we have also demonstrated germanium BIB detectors for wavelengths extending to several hundred microns. We are currently developing germanium BIB detector arrays for astrophysics applications, including space telescopes beyond NGST.

We discuss dynamical mass measurements of high-z galaxies with the Next Generation Space Telescope (NGST). In particular, we review some of the observational limits with the current instrument/telescope generation, we discuss the redshift limits and caveats for absorption and emission lines studies with NGST, and the existence of suitable targets at high redshift. We also briefly summarize strengths and weaknesses of proposed NGST instruments for dynamical studies.

The James Webb Space Telescope (JWST) is the successor to the Hubble Space Telescope. JWST will be an infrared-optimized telescope, with an approximately 6.5 m diameter primary mirror, that is located at the Sun-Earth L2 Lagrange point. Three of JWST’s four science instruments use Teledyne HgCdTe HAWAII-2RG (H2RG) near infrared detector arrays. During 2010, the JWST Project noticed that a few of its 5 μm cutoff H2RG detectors were degrading during room temperature storage, and NASA chartered a “Detector Degradation Failure Review Board” (DD-FRB) to investigate. The DD-FRB determined that the root cause was a design flaw that allowed indium to interdiffuse with the gold contacts and migrate into the HgCdTe detector layer. Fortunately, Teledyne already had an improved design that eliminated this degradation mechanism. During early 2012, the improved H2RG design was qualified for flight and JWST began making additional H2RGs. In this article, we present the two public DD-FRB “Executive Summaries” that: (1) determined the root cause of the detector degradation and (2) defined tests to determine whether the existing detectors are qualified for flight. We supplement these with a brief introduction to H2RG detector arrays, some recent measurements showing that the performance of the improved design meets JWST requirements, and a discussion of how the JWST Project is using cryogenic storage to retard the degradation rate of the existing flight spare H2RGs.

The James Webb Space Telescope (JWST) Optical Simulation Testbed (JOST) is a tabletop workbench to study aspects of wavefront sensing and control for a segmented space telescope, including both commissioning and maintenance activities. JOST is complementary to existing optomechanical testbeds for JWST (e.g. the Ball Aerospace Testbed Telescope, TBT) given its compact scale and flexibility, ease of use, and colocation at the JWST Science & Operations Center. We have developed an optical design that reproduces the physics of JWST’s three-mirror anastigmat using three aspheric lenses; it provides similar image quality as JWST (80% Strehl ratio) over a field equivalent to a NIRCam module, but at HeNe wavelength. A segmented deformable mirror stands in for the segmented primary mirror and allows control of the 18 segments in piston, tip, and tilt, while the secondary can be controlled in tip, tilt and x, y, z position. This will be sufficient to model many commissioning activities, to investigate field dependence and multiple field point sensing & control, to evaluate alternate sensing algorithms, and develop contingency plans. Testbed data will also be usable for cross-checking of the WFS&C Software Subsystem, and for staff training and development during JWST’s five- to ten-year mission.

The James Webb Space Telescope (JWST) is a space-based, infrared observatory designed to study the early stages of galaxy formation in the Universe. The telescope will be launched into an elliptical orbit about the second Lagrange point and passively cooled to 30-50 K to enable astronomical observations from 0.6 to 28 microns. A group from the NASA Goddard Space Flight Center and the Northrop Grumman Space Technology prime contractor team has developed an optical and mechanical layout for the science instruments within the JWST field of view that satisfies the telescope s high-level performance requirements. Four instruments required accommodation within the telescope's field of view: a Near-Infrared Camera (NIRCam) provided by the University of Arizona; a Near-Mared Spectrometer (NIRSpec) provided by the European Space Agency; a Mid-Infrared Instrument (MIRI) provided by the Jet Propulsion Laboratory and a European consortium; and a Fine Guidance Sensor (FGS) with a tunable filter module provided by the Canadian Space Agency. The size and position of each instrument's field of view allocation were developed through an iterative, concurrent engineering process involving the key observatory stakeholders. While some of the system design considerations were those typically encountered during the development of an infrared observatory, others were unique to the deployable and controllable nature of JWST. This paper describes the optical and mechanical issues considered during the field of view layout development, as well as the supporting modeling and analysis activities.

The next generation of space telescopes will be designed to meet increasingly challenging science goals. The operating environment and required precision of these telescopes will make complete verification via ground tests impossible, and will place a greater reliance on numerical simulation. The current state of the art in thermal, mechanical and optical modeling involves three disparate computational models, several analysis codes and tools to transition results between these models. However, the active controls necessary to meet the next generation of requirements for space telescopes will require integrated thermal, structural, optical and controls analysis. To meet these challenges, JPL has developed Cielo, an in-house finite element tool capable of multi-physics simulations using a common finite element model, for thermal, structural and optical aberration analysis. In this paper, we will discuss the use of Cielo for analysis of a coronagraph and an occulter designed to observe Earth-like planets around nearby stars. We will compare thermal and structural results from Cielo with results from commercial off the shelf (COTS) tools to verify the new approach. We will perform variations of key parameters to demonstrate how margins and uncertainties can be quantified using the new approach.

to develop high-energy petawatt laser science and technology for the Department of Energy (DOE), (c) to develop advanced solid-state laser systems and optical components for the Department of Defense (DoD), and (d) to invent, develop, and deliver improved concepts and hardware for other government agencies and industry. LS&T activities during 2002 focused on seven major areas: (1) NIF Project-LS&T led major advances in the deployment of NIF Final Optics Assembly (FOA) and the development of 30.1 optics processing and treatment technologies to enhance NIF's operations and performance capabilities. (2) Stockpile Stewardship Program (SSP)-LS&T personnel continued development of ultrashort-pulse lasers and high-power, large-aperture optics for applications in SSP, extreme-field science and national defense. To enhance the high-energy petawatt (HEPW) capability in NIF, LS&T continued development of advanced compressor-grating and front-end laser technologies utilizing optical-parametric chirped-pulse amplification (OPCPA). (3) High-energy-density physics and inertial fusion energy-LS&T continued development of kW- to MW-class, diode-pumped, solid-state laser (DPSSL). (4) Department of Defense (DoD)-LS&T continued development of a 100 kw-class solid-state heat-capacity laser (SSHCL) for missile defense. (5) Nuclear energy applications-LS&T continued to develop laser-shock peening technology to improve the service lifetime of metal nuclear waste containment canisters designed for DOES Yucca Mountain Project. (6) Materials processing-Under cooperative research and development agreements (CRADA) with U.S. industry, LS&T developed and delivered kW-class solid-state lasers for shock peening and hole drilling of metals. (7) Diffractive optics for space telescopes and petawatt lasers-LS&T continued fabrication of large-aperture beam sampling gratings (BSGs) for NIF, and development of large-scale, lightweight diffractive optics for the next generation of space telescope (Eyeglass).

A multidisciplinary analysis is demonstrated for the NEXUS space telescope precursor mission. This mission was originally designed as an in-space technology testbed for the Next Generation Space Telescope (NGST). One of the main challenges is to achieve a very tight pointing accuracy with a sub-pixel line-of-sight (LOS) jitter budget and a root-mean-square (RMS) wavefront error smaller than λ/50 despite the presence of electronic and mechanical disturbances sources. The analysis starts with the assessment of the performance for an initial design, which turns out not to meet the requirements. Twentyfive design parameters from structures, optics, dynamics and controls are then computed in a sensitivity and isoperformance analysis, in search of better designs. Isoperformance allows finding an acceptable design that is well “balanced” and does not place undue burden on a single subsystem. An error budget analysis shows the contributions of individual disturbance sources. This paper might be helpful in analyzing similar, innovative space telescope systems in the future.

This paper summarizes the findings of the Next Generation Space Telescope (NGST) Detector Requirements Review Panel. This panel was comprised of NGST Integrated Science Instrument Module study representatives, detector specialists, and members of the NGST project science team. It has produced a report that recommends detector performance levels, and has provided rationale for deriving these levels from basic, anticipated NGST science goals and programs. Key parameters such as detector array format, quantum efficiency, and noise are discussed and prioritized.