Laura A. Burns

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.

Performance of wavefront sensing and control algorithms on a segmented telescope testbed

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.

Wavefront Control for a Segmented Deployable Space Telescope

By segmenting and folding the primary mirror, quite large telescopes can be packed into the nose cone of a rocket. Deployed after launch, initial optical performance can be quite poor, due to deployment errors, thermal deformation, fabrication errors and other causes. We describe an automatic control system for capturing, aligning, phasing, and deforming the optics of such a telescope, going from initial cm-level wavefront errors to diffraction-limited observatory operations. This system was developed for the Next Generation Space Telescope and is being tested on the NGST Wavefront Control Testbed.

Demonstration of the James Webb Space Telescope commissioning on the JWST testbed telescope

The one-meter Testbed Telescope (TBT) has been developed at Ball Aerospace to facilitate the design and implementation of the wavefront sensing and control (WFS&C) capabilities of the James Webb Space Telescope (JWST). The TBT is used to develop and verify the WFS&C algorithms, check the communication interfaces, validate the WFS&C optical components and actuators, and provide risk reduction opportunities for test approaches for later full-scale cryogenic vacuum testing of the observatory. In addition, the TBT provides a vital opportunity to demonstrate the entire WFS&C commissioning process. This paper describes recent WFS&C commissioning experiments that have been performed on the TBT.

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.

Initial test results from the Next Generation Space Telescope (NGST) wavefront sensing and control testbed (WCT)

This paper describes the results of a few of the initial series of tests being conducted with the first configuration of the Next Generation Space Telescope Wavefront sensing and Control Testbed (WCT1). WCT1 is a 1:1, f/16.6 re-imaging system, incorporating two deformable mirrors located at pupil conjugate positions with 6 actuators/diameter (SM/DM) and 20 actuators/diameter (AO/DM). A CCD on a precision stage is used for obtaining defocused images providing phase diversity for wavefront determination using phase retrieval. In a typical experiment, wavefront error is injected into the optical path with the SM/DM and then corrected using the more densely actuated AO/DM. Wavefront analysis is provided via a phase retrieval algorithm, and control software is used to reshape the AO/DM and correct the wavefront. A summary of the results of some initial tests are presented.

Segmented mirror coarse phasing with white light interferometry: modeling and experimenting on NGST's Wavefront Control Testbed

A method of coarse phasing segmented mirrors using white light 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 NGSTs Wavefront Control Testbed (WCT).

Wavefront control testbed 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.

Wavefront control testbed integrated software system

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.