One of the architectures under consideration for Terrestrial Planet Finder (TPF) is a visible coronagraph. To achieve TPF science goals, the coronagraph must have extreme levels of wavefront correction (less than 1 /spl Aring/ rms over controllable spatial frequencies) and stability to get the necessary suppression of diffracted starlight (10/sup -10/ contrast). The High Contrast Imaging Testbed is the TPF platform for laboratory validation of key coronagraph technologies, as well as demonstration of a flight-traceable approach to coronagraph implementation. Various wavefront sensing approaches are under investigation on the testbed, with wavefront control provided by a precision high actuator density deformable mirror. Diffracted light control is achieved through a combination of an occulting or apodizing mask and stop; many concepts exist for these components and are explored. Contrast measurements on the testbed establishes the technical feasibility of TPF requirements, while model and error budget validation are demonstrate implementation viability. This paper describes the current testbed design and preliminary experimental results.

The Wavefront Control Testbed (WCT) was created to develop and test wavefront sensing and control algorithms and software for the segmented James Webb Space Telescope (JWST). Last year, we changed the system configuration from three sparse aperture segments to a filled aperture with three pie shaped segments. With this upgrade we have performed experiments on fine phasing with line-of-sight and segment-to-segment jitter, dispersed fringe visibility and grism angle; high dynamic range tilt sensing; coarse phasing with large aberrations, and sampled sub-aperture testing. This paper reviews the results of these experiments.

The Optical Navigation Camera (ONC) is a technical demonstration slated to fly on NASA's Mars Reconnaissance Orbiter in 2005. Conventional navigation methods have reduced accuracy in the days immediately preceding Mars orbit insertion. The resulting uncertainty in spacecraft location limits rover landing sites to relatively safe areas, away from interesting features that may harbor clues to past life on the planet. The ONC will provide accurate navigation on approach for future missions by measuring the locations of the satellites of Mars relative to background stars.Because Mars will be a bright extended object just outside the camera's field of view, stray light control at small angles is essential. The ONC optomechanical design was analyzed by stray light experts and appropriate baffles were implemented. However, stray light testing revealed significantly higher levels of light than expected at the most critical angles. The primary error source proved to be the interface between ground glass surfaces (and the paint that had been applied to them) and the polished surfaces of the lenses. This paper will describe troubleshooting and correction of the problem, as well as other lessons learned that affected stray light performance.

The James Webb Space Telescope (JWST) will use image-based wavefront sensing to align the telescope optics and achieve diffraction-limited performance at 2 µm. The Phase Retrieval Camera (PRC) is a high-accuracy, image-based wavefront sensor that was built for the optical characterization of JWST technology-demonstrator mirrors. Recently, experiments with the PRC were performed at the NASA Marshall Space Flight Center to measure the cryogenic surface figure of the beryllium Advanced Mirror System Demonstrator (AMSD). This paper describes the results of these experiments. Using the Modified Gerchberg-Saxton phase retrieval algorithm (JWST’s baseline method for fine-phasing), the PRC measured wavefront aberrations that were as large as 10 waves peak-to-valley (wavefront) in the optical system. A comparison between the PRC results and measurements acquired with an Instantaneous Phase Interferometer will also be presented.

Large space optical systems will require on-orbit wavefront sensing and control systems to correct misalignments and figure errors incurred during manufacture, launch and deployment.

The Wavefront Control Testbed (WCT) is used to demonstrate the wavefront sensing and control algorithms and procedures that will be used on the Next Generation Space Telescope (NGST). The Segmented Telescope Control Software, written in MATLAB®, is the primary development and operational tool used. The software has an extensive graphical user interface that allows the user to interact with the hardware and algorithms. A variety of additional software programs support the Segmented Telescope Control Software (STCS). Various hardware control software interacts with MATLAB via TCP/IP connections. When access to the hardware is unnecessary or undesirable, we can access the model server that simulates the system. A stand-alone safety monitoring LabVIEW program alerts technicians if a hardware failure occurs. A C program gives the operator a graphical way of monitoring the network connections to the various systems. An Interactive Data Language (IDL) data archiving routine creates a database to monitor and maintain the testbed data and executes the MATLAB to Flexible Image Transport System (FITS) translator. Additionally we have implemented a web-based bug tracking and plan to add experiment scheduling and a document archive. Due to the nature of the testbed, these software programs are constantly evolving, causing a variety of challenges over the years. This poster will describe these software elements and the issues that have arisen trying to use them together.

The NGST Phase Retrieval Camera (PRC) is a portable wavefront sensor useful for optical testing in high-vibration environments. The PRC uses focus-diverse phase retrieval to measure the wavefront propagating from the optical component or system under test. Phase retrieval from focal plane images is less sensitive to jitter than standard pupil plane interferometric measurements; the PRC's performance is further enhanced by using a high-speed shutter to freeze out seeing and jitter along with a reference camera to maintain the correct boresight in defocused images. The PRC hardware was developed using components similar to those in NGST's Wavefront Control Testbed (WCT), while the PRC software was derived from WCT's extensive software infrastructure. Primary applications of the PRC are testing and experimenting with NGST technology demonstrator mirrors, along with exploring other wavefront sensing and control problems not easily studied using WCT. An overview of the hardware and testing results will be presented.

To achieve and maintain excellent imaging performance, the Next Generation Space Telescope (NGST) will employ image-based phase retrieval methods to control its segmented primary mirror. In this paper, we present the experimental validation of a focus-diverse wave front sensing (WFS) algorithm with comparative interferometric measurements of a perturbed test mirror. Using sets of defocused point-spread functions measured with the NGST phase retrieval camera, we estimate the aberrations of the test optic in a perturbed and unperturbed state. Interleaved with the focus-diverse sets, we measure the surface figure of the mirror using a ZYGO interferometer. After briefly reviewing the basic WFS algorithm and describing the experimental setup, we show that we can obtain agreement that is better than 1/100th of a wave rms in the difference of the wave front estimates obtained in the perturbed and unperturbed states. Although this experiment does not establish the errors that are solely attributable to our WFS approach, it nevertheless validates the accuracy of our image-based methods for NGST, demonstrating that they are generally competitive with standard industrial optical metrology instruments.

Experience and infrastructure from NGST's Wavefront Control Testbed (WCT) were utilized to develop a portable Wavefront sensor, the Phase Retrieval Camera (PRC). The PRC is useful for the testing of optics in high-jitter environments. The principal uses of the PRC will be testing and experimenting with NGST technology demonstration mirrors as well as exploring other issues of wavefront sensing and control not easily studied using the WCT. This presentation will detail the packaging and hardware chosen for the PRC, the PRC software, and calibration of the instrument.

A piston sensing and control algorithm for segmented mirror coarse phasing using a dispersed fringe sensor (DFS) has been developed for the Next Generation Space Telescope (NGST) wavefront sensing and control. The DFS can detect residual piston errors as large as the order of a depth-of-focus and can phase the segment mirrors with accuracy better than 0.1 microns, which is well within the capture range of fine phasing for NGST. A series of experiments have been carried out on the NGST's Wavefront Control Testbed (WCT) to validate the modeling results, evaluate the DFS performance, and systematically explore the factors that affect the DFS performance. This paper reports the testbed results for several critical issues of DFS performance, including DFS dynamic range, accuracy, fringe visibility, and the effects of segment mirror aberrations.

A method of coarse phasing segmented mirrors using white fight interferometry (WLI) has been developed for the Next Generation Space Telescope (NGST) wavefront sensing and control. Using the broadband point PSF of the segmented mirrors taken during a segment piston scan, the WLI can accurately detect small residual piston errors. WLI does not rely on extra optics and uses only the final imaging camera. With its high sensitivity to small segment piston error WLI can be used as a complementary phasing algorithm to the dispersed fringe sensor (DFS) for NGST. This paper will present the results from modeling and experiment on the NGST's Wavefront Control Testbed (WCT).

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.

The Segmented Telescope Control Software (STCS) uses science camera information to align and phase a deployable segmented optical telescope. It was developed the for the Next Generation Space Telescope (NGST) and has been successfully utilized on the Wavefront Control Testbed (WCT) for NGST and a portable phase retrieval camera (PPRC) system. The software provides an operating environment that will be used for the prime contractor's testbeds for NGST, and will eventually evolve into the Wavefront Sensing and Control (WFS&C) ground support software for NGST. This paper describes the engineering version of the STCS, the algorithms it incorporates, and methods of communicating with the testbed hardware.

A telescope simulator was built as part of the Nexus wavefront control testbed, an NGST technology experiment at NASA's Goddard Space Flight Center. This testbed was designed to demonstrate complete control of a segmented telescope, from initial capture of light, through coarse alignment and phasing, to fine phasing and wavefront control. The existing telescope simulator allows testing of the fine phasing and wavefront control steps. A small deformable mirror in the simulator allows generation of an unobscured aberrated wavefront, for use in exploring the range of measurement and correction using the testbed's image-based wavefront sensor and larger deformable mirror. An alternate path under development for the simulator will create a segmented wavefront using three spherical mirrors; three-degree-of-freedom mounts under each mirror enable alignment and phasing experiments that will cover most of the operation sequence. Details of the hardware design and performance will be presented.

We have developed a focus-diverse phase retrieval algorithm to measure and correct wavefront errors in segmented telescopes, such as the Next Generation Space Telescope. These algorithms incorporate new phase unwrapping techniques imbedded in the phase retrieval algorithms to measure aberrations larger than one wave. Through control of a deformable mirror and other actuators, these aberrations are successfully removed from the system to make the system diffraction limited. Results exceed requirements for the Wavefront Control Testbed. An overview of these techniques and performance results on the Wavefront Control Testbed are presented.

In the summer of 1996, three study teams developed conceptual designs and mission architectures for the Next Generation Space Telescope (NGST)(1,2). All three conceptual designs provided scientific capabilities that met or surpassed those envisioned by the Hubble Space Telescope (HST) and Beyond Committee(3). Each group highlighted areas of technology development that need to be further advanced to meet the goals of the NGST mission. The most important areas for future study included: deployable structures, lightweight optics, cryogenic optics and mechanisms, passive cooling, and on-orbit closed loop wavefront sensing and control. NASA and industry are currently planning to develop a series of ground testbeds and validation nights to demonstrate many of these technologies. The Developmental Cryogenic Active Telescope Testbed (DCATT) is a system level testbed to be developed at Goddard Space Flight Center in three phases over an extended period of time. This testbed will combine an actively controlled telescope with the hardware and software elements of a closed loop wavefront sensing and control system to achieve diffraction limited imaging at 2 microns. We will present an overview of the system level requirements, a discussion of the optical design, and results of performance analyses for the Phase 1 ambient concept for DCATT.

An 85 cm aperture beryllium mirror was fabricated as part of the IR Telescope Technology Testbed (ITTT), a facility to which the SIRTF flight telescope will be traceable. The ITTT was developed to demonstrate that diffraction-limited performance at a wavelength of 6.5 micrometers is attainable from an ultra-lightweight meter-class beryllium telescope operating at a temperature of 5.5K. Cryo-null figuring was employed to meet the requirements for the shape of the primary mirror at its operating temperature over an aperture of 79cm. The results of this process will be presented, including the repeatability of the surface through cryogenic temperature cycling. Modeling of system performance using the residual figure error will be described. Image-based methods were used to characterize a turned up edge that is too steep to be measured with an interferometer.

As part of the technology validation strategy of the next generation space telescope (NGST), a system testbed is being developed at GSFC, in partnership with JPL and Marshall Space Flight Center, which will include al of the component functions envisioned in an NGST active optical system. The system will include an actively controlled, segmented primary mirror, actively controlled secondary, deformable, and fast steering mirrors, wavefront sensing optics, wavefront control algorithms, a telescope simulator module, and an interferometric wavefront sensor for use in comparing final obtained wavefronts from different tests. The developmental cryogenic active telescope testbed will be implemented in three phase. Phase 1 will focus on operating the testbed at ambient temperature. During Phase 2, a cryocapable segmented telescope will be developed and cooled to cryogenic temperature to investigate the impact on the ability to correct the wavefront and stabilize the image. In Phase 3, it is planned to incorporate industry developed flight-like components, such as figure controlled mirror segments, cryogenic, low hold power actuators, or different wavefront sensing and control hardware or software. A very important element of the program is the development and subsequent validation of the integrated multidisciplinary models. The phase 1 testbed objectives, plans, configuration, and design will be discussed.

Various efforts are underway to demonstrate hardware for the Next Generation Space Telescope (NGST). One such effort is the development of the DCATT testbed. This testbed is an NGST effort geared to demonstrating in hardware the end-to-end system functionality. The system includes a segmented telescope, active optics subsystem with a deformable mirror (DM), and a wavefront sensor. The degree to which the DCATT can demonstrate this functionality depends crucially on its performance. A system performance analysis of this testbed is presented. The analysis is based on the design of the DCATT developed by Goddard and JPL. In the analysis, the performance of the system as a function of key system factors is calculated. These factors include the following: control, environmental, fabrication, alignment, and design. The performance after correction by the DM is required to be diffraction-limited at a wavelength of 2.0 microns. The flowdown of this performance is called the corrected error budget. To fully characterize the testbed performance as a system, one must develop a budget for the performance of the system before action of the DM. The flowdown of this performance is called the uncorrected error budget. The top line of this budget is related to the correction capability of the DM.

The Next Generation Space Telescope will depart from the traditional means of providing high optical quality and stability, namely use of massive structures. Instead, a benign orbital environment will provide stability for a large, flexible, lightweight deployed structure, and active wavefront controls will compensate misalignments and figure errors induced during launch and cool-down on orbit. This paper presents a baseline architecture for NGST wavefront controls, including initial capture and alignment, segment phasing, wavefront sensing and deformable mirror control. Simulations and analyses illustrate expected scientific performance with respect to figure error, misalignments, and thermal deformation.