Catherine M. Ohara

Segmented mirror coarse phasing with a dispersed fringe sensor: experiments on NGST's wavefront control testbed

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 NGSTs 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.

Phase retrieval camera for testing NGST optics

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 PRCs 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 NGSTs Wavefront Control Testbed (WCT), while the PRC software was derived from WCTs 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.

Phase Retrieval Camera optical testing of the Advanced Mirror System Demonstrator (AMSD)

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.

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.

Wavefront sensing and control software for a segmented space telescope

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 contractors 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.

Interferometric validation of image-based wavefront sensing for NGST

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.

Autonomous phase retrieval control for calibration of the Palomar Adaptive Optics system

An autonomous wavefront sensing and control software suite (APRC) has been developed as a method to calibrate the internal static errors in the Palomar Adaptive Optics system. An image-based wavefront sensing algorithm, Adaptive Modified Gerchberg-Saxton Phase Retrieval (MGS), provides wavefront error knowledge upon which actuator command voltages are calculated for iterative wavefront control corrections. This automated, precise calibration eliminates non-common path error to significantly reduce AO system internal error to the controllable limit of existing hardware, or can be commanded to prescribed polynomials to facilitate high contrast astronomy. System diagnostics may be performed through analysis of the wavefront result generated by the phase retrieval software.

Target selection and imaging requirements for JWST fine phasing

To achieve and maintain the fine alignment of its segmented primary mirror the James Webb Space Telescope (JWST) plans to use focus-diverse wavefront sensing (WFS) techniques with science camera imagery. The optical requirements for JWST are such that the error contribution from the WFS itself must be limited tp 10nm rms over the controllable degrees of freedom of the telescope. In this paper, we will explore the requirements on the target selection and imaging requirements necessary to achieve the desired level of WFS accuracy. Using Monte Carlo simulations we explore the WFS error as a function of wavefront aberrations level, defocus-diversity level, optical bandwidth and imaging signal-to-noise ratio to establish the key imaging requirements. By taking into account practical integration time limits along with the distribution of the defocused point-spread functions, we establish the bright and faint star magnitude limits suitable for WFS target selection.

Performance of segmented mirror coarse phasing with a dispersed fringe sensor: modeling and simulations

Dispersed Fringe Sensing (DFS) is an efficient and robust method for coarse phasing of a segmented primary mirror such as the James Webb Space Telescope (JWST). Results from testbed experiments and modeling have shown that among the many factors that affect the performance of DFS, the diffraction from segment aperture and the interference between the segment wavefronts have the most intrinsic influence on the DFS performance. In this paper, modeling and simulations based on diffraction are used to study the formation of DFS fringe and fringe properties such as visibility. We examine the DFS piston detection process and explore the limitation of DFS wavefront piston detection accuracy and the DFS dynamic range under different segment aperture geometries, aperture orientations, and image samplings.

Phase retrieval methods for wavefront sensing

Phase retrieval is an image-based wavefront sensing process, used to recover phase information from defocused stellar images. Phase retrieval has proven to be useful for diagnosis of optical aberrations in space telescopes, calibration of adaptive optics systems, and is intended for use in aligning and phasing the James Webb Space Telescope. This paper describes a robust and accurate phase retrieval algorithm for wavefront sensing, which has been successfully demonstrated on a variety of testbeds and telescopes. Key features, such as image preprocessing, diversity adaptation, and prior phase nulling, are described and compared to other methods. Results demonstrate high accuracy and high dynamic range wavefront sensing.