Scott A. Basinger

Wavefront sensing and control for a Next-Generation Space Telescope

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.

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.

Extreme wave front sensing accuracy for the Eclipse coronagraphic space telescope

The Eclipse coronagraphic telescope will allow for high contrast imaging near a target star to facilitate planet finding. One key element will be its high accuracy, high authority deformable mirror (DM) that controls the wave front error (WFE) down to an acceptable level. In fact, to achieve the desired contrast ratio of nine orders of magnitude (in intensity) to within 0.35 arcseconds of the target star, the WFE in the telescope must be controlled to level below 1Å rms within the controllable bandwidth of the DM. To achieve this extreme wave front sensing (WFS) accuracy, we employ a focus-diverse phase retrieval method extended from the Next Generation Space Telescope baseline approach. This method processes a collection defocused point-spread functions, measured at the occulting position in the Eclipse optical system, into a high accuracy estimate of the exit-pupil WFE. Through both simulation and hardware experiments, we examine and establish the key data requirements, such as the defocus levels and imaging signal-to noise level, that are necessary to obtain the desired WFS accuracy and bandwidth.

Performance of TPF's high-contrast imaging testbed: modeling and simulations

The performance of the high-contrast imaging testbed (HCIT) at JPL is investigated through optical modeling and simulations. The analytical tool is an optical simulation algorithm developed by combining the HCITs optical model with a speckle-nulling algorithm that operates directly on coronagraphic images, an algorithm identical to the one currently being used on the HCIT to actively suppress scattered light via a deformable mirror. It is capable of performing full three-dimensional end-to-end near-field diffraction analysis on the HCITs optical system. By conducting extensive speckle-nulling optimization, we clarify the HCITs capability and limitations in terms of its contrast performance under various realistic conditions. Considered cases include non-ideal occulting masks, such as a mask with parasitic phase-delay errors (i.e., a not band-limited occulting mask) and one with damped ripples in its transmittance profiles, as well as the phase errors of all optics. Most of the information gathered on the HCITs optical components through measurement and characterization over the last several years at JPL has been used in this analysis to make the predictions as accurate as possible. Our simulations predict that the contrast values obtainable on the HCIT with narrow-band (monochromatic) illumination at 785nm wavelength are Cm=1.58x10-11 (mean) and C4=5.11x10-11(at 4λ/D), in contrast to the measured results of Cm~6×10-10 and C4~8×10-10, respectively. In this paper we report our findings about the monochromatic light performance of the HCIT. We will describe the results of our investigation about the HCITs broad-band performance in an upcoming paper.

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.

DCATT dispersed fringe sensor: modeling and experimenting with the transmissive phase plates

Control algorithms developed for coarse phasing the segmented mirrors of the Next Generation Space Telescope (NGST) are being tested in realistic modeling and on the NGST wavefront control testbed, also known as DCATT. A dispersed fringe sensor (DFS) is used to detect piston errors between mirror segments during the initial coarse phasing. Both experiments and modeling have shown that the DFS provides an accurate measurement of piston errors over a range from just under a millimeter to well under a micron.

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.