Erin Sabatke

TRL-6 for JWST wavefront sensing and control

NASAs Technology Readiness Level (TRL)-6 is documented for the James Webb Space Telescope (JWST) Wavefront Sensing and Control (WFSC) subsystem. The WFSC subsystem is needed to align the Optical Telescope Element (OTE) after all deployments have occurred, and achieves that requirement through a robust commissioning sequence consisting of unique commissioning algorithms, all of which are part of the WFSC algorithm suite. This paper identifies the technology need, algorithm heritage, describes the finished TRL-6 design platform, and summarizes the TRL-6 test results and compliance. Additionally, the performance requirements needed to satisfy JWST science goals as well as the criterion that relate to the TRL-6 Testbed Telescope (TBT) performance requirements are discussed.

Analytic diffraction analysis of a 32-m telescope with hexagonal segments for high-contrast imaging

Large segmented telescopes cannot be modeled accurately with fast-Fourier-transform techniques since small features such as gaps between the segments will be inadequately sampled. An analytic Fourier-transform method can be used to model any pupil configuration with straight edges, including tolerance analysis and some types of apodization. We analytically investigated a 32-m segmented primary with 18 hexagonal segments for high-contrast imaging. There are significant regions in the image in which extrasolar planets could be detected. However, the hexagonal profile of the pupil was not as useful as expected. The gaps between the segments, the secondary obscuration, and the secondary spiders must be as small as possible and their edges must be apodized. Apodizing the edges of the individual segments reduced the useful regions in the image since the gaps appeared to be wider.

Cryogenic beam-combiner for very low background 2- to 20-μm interferometry on the 22.8-m Large Binocular Telescope

The 22.8 m Large Binocular Telescope Interferometer will be a uniquely powerful tool for imaging and nulling interferometry at thermal infrared wavelengths (2 - 20 micrometers ) because of the LBTs unusual combination of low emissivity, high spatial resolution, broad (u,v)-plane coverage, and high photometric sensitivity. The gregorian adaptive secondary mirrors permit beam combination after only three warm reflections. They also control the relative pathlength, wavefront tip/tilt, and focus of the two telescope beams, thus greatly simplifying the complexity of the beam-combiner. The resulting four-mirror beam-combiner reimages the original focal plane and also images the telescope pupil onto a cold stop to limit thermal background. At first-light in 2004, an all-reflective, cooled beam-combiner can provide a 2 arcmin diameter field for Fizeau-style imaging as well as the low thermal background and achromaticity required for nulling interferometry.

Optical design of giant telescopes for space

Increased performance for optical telescopes has historically come from larger apertures, from technological advances for the telescope components, such as detectors, and from access to better sites, such as space. Little has changed in the basic telescope design for a century. These conventional designs have served us well and will continue to do so with the Next Generation Space Telescope. There is an upper limit to the size of thsi type of telescope, set by the capacity to launch the required mass. For future space telescopes of 50, 100, 500 meter apertures, we have developed a new type of optical design. We use a primary reflector made from segments of flat and near-flat membranes. The secondary reflector and subsequent optics are supported in separate spacecraft, flying in formation with the primary reflector. In addition, each spacecraft maintains sunshields to keep the optics shaded from the sun. This paper explores the optical design issues for this type of giant space telescope.

Using multifield measurements to eliminate alignment degeneracies in the JWST testbed telescope

The primary mirror of the James Webb Space Telescope (JWST) consists of 18 segments and is 6.6 meters in diameter. A sequence of commissioning steps is carried out at a single field point to align the segments. At that single field point, though, the segmented primary mirror can compensate for aberrations caused by misalignments of the remaining mirrors. The misalignments can be detected in the wavefronts of off-axis field points. The Multifield (MF) step in the commissioning process surveys five field points and uses a simple matrix multiplication to calculate corrected positions for the secondary and primary mirrors. A demonstration of the Multifield process was carried out on the JWST Testbed Telescope (TBT). The results show that the Multifield algorithm is capable of reducing the field dependency of the TBT to about 20 nm RMS, relative to the TBT design nominal field dependency.

Basic principles in the optical design of imaging multiple aperature systems

We discuss the basic concepts that have been useful in our work designing multiple aperture telescopes with wide fields of view. We examine combining errors at zero field and errors that are linear as a function of field. An easy optimization for satisfying the sine condition to eliminate linear piston errors is given. Methods for estimating the RMS wavefront errors for the lower-order combining errors are given.

Phase theory for multiple aperture systems

We establish the groundwork for a phase theory applicable to multiple aperture systems. To do this, we define ideal behavior as the phase behavior of an off-axis piece of a system that has rotational symmetry. Then we examine the phase behavior of a more general system that has only a single plane of symmetry. This system represents a branch of a multiple aperture system. Comparison of the two systems leads to conditions for which the plane symmetric system has ideal behavior. As a result of this comparison, design rules that are commonly applied to multiple aperture systems appear naturally, including the well- known requirement that the exit pupil is a scaled copy of the entrance pupil. The phase theory that we present is cohesive, provides useful design guidelines, and can be considered an addition to wave aberration theory.

Imaging interferometers using flat primary segments

Gossamer mirrors have the potential to reach 100 meter baselines in space because of their very light weight. We explore a type of system that uses an array of flat gossamer mirrors as a primary mirror. Using wavefront reconstruction, we can easily estimate the fields of view for these systems. We report the fields of view as a function of the free parameters for these systems.

A snapshot distortion test using Moiré fringes

We developed a distortion measurement technique that works in snapshot mode. Distortion information across the full field of view can be captured in a single short exposure. To do this, a Ronchi ruling is placed in the object and image planes of the system under test. The undistorted ruling in the image plane interferes with the distorted image of the ruling, producing a Moire fringe pattern that can be analyzed in several ways. Phase shifting can be carried out by shifting the Ronchi ruling in object space. The technique is insensitive to vibration and turbulence. Measurements were routinely made with P-V noise levels of 1 μm on measured chief ray locations in 20 mm image planes (0.01%). Repeated measurements showed disagreements on the 6 μm level across a 20 mm image plane (0.03% repeatability).

Wavefront Control Toolbox for James Webb Space Telescope Testbed

A MATLAB toolbox has been developed for wavefront control of segmented optical systems. The toolbox is applied to the optical models of the James Webb Space Telescope (JWST) in general and to the JWST Testbed Telescope (TBT) in particular, implementing both unconstrained and constrained wavefront optimization to correct for possible misalignments of the segmented primary mirror or the monolithic secondary mirror. The optical models are implemented in the ZEMAX optical design program and information is exchanged between MATLAB and ZEMAX via the Dynamic Data Exchange (DDE) interface. The model configuration is managed using the Extensible Markup Language (XML) protocol. The optimization algorithm uses influence functions for each adjustable degree of freedom of the optical model. Both iterative and non-iterative algorithms have been developed that converge to a local minimum of the root-mean-square (rms) wavefront error using singular value decomposition (SVD) of the control matrix of influence functions. The toolkit is highly modular and allows the user to choose control strategies for the degrees-of-freedom (DOF) on a given iteration and also allows the wavefront convergence criterion to be checked on each iteration. As the influence functions are nonlinear over the full control parameter space, the toolkit also allows for trade-offs between frequency of updating the local influence functions and execution speed. The functionality of the toolbox and the validity of the underlying algorithms have been verified through extensive simulations.