James B. Hadaway

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.

Results of the beryllium AMSD mirror cryogenic optical testing

The 1.4-meter semi-rigid, beryllium Advanced Mirror System Demonstrator (AMSD) mirror completed initial cryogenic testing at Marshall’s X-ray Calibration Facility (XRCF) in August of 2003. Results of this testing show the mirror to have very low cryogenic surface deformation and possess exceptional figure stability. Additionally, the mirror substrate exhibits virtually no change in surface figure over the James Webb Space Telescope (JWST) operational temperature range of 30 to 62 Kelvin. The lightweighted, semi-rigid mirror architecture approach demonstrated here is a precursor to the mirror technology being applied to the JWST observatory. Testing at ambient and cryogenic temperatures included the radius of curvature actuation system and the rigid body displacement system. These two systems incorporated the use of 4 actuators to allow the mirror to change piston, tilt, and radius of curvature. Presented here are the results of the figure change, alignment change, and radius change as a function of temperature. Also shown will be the actuator influence functions at both ambient and cryogenic temperatures.

Interferometer for Testing in Vibration Environments

Temporal phase shifting interferometers require a stable environment during the data acquisition, so that well controlled phase steps can be introduced between successively acquired interferograms. In contrast, single-frame interferometers need to acquire only one interferogram to provide a phase map with very good precision at high spatial resolution. Thus these interferometers are well suited for the interferometric testing of large optics with long radius of curvature for which vibration isolation is difficult, e.g. testing astronomical telescope mirrors in a test tower, or testing space optics inside a cryogenic vacuum chamber. This paper describes the Instantaneous Phase Interferometer (IPI) by ADE Phase Shift, together with measurement results at NASA. The IPI consists of a polarization Twyman-Green interferometer operating at 632.8nm, with single-frame phase acquisition based on a spatial carrier technique. The spatial carrier fringes are generated by introducing large amount of tilt between the test beam and the reference beam. The phase information of the optical surface under test is encoded in the straightness of the interference fringes, which can be detected in a single frame with spatial sampling of 1000x1000 pixels. Measurements taken at the NASA Marshall Space Flight Center in support of the characterization of developmental optics for the Next Generation Space Telescope are presented. Such tests consist of a mirror placed inside a cryogenic vacuum chamber, with the IPI placed outside the test chamber without any additional vibration isolation.

Survey of interferometric techniques used to test JWST optical components

JWST optical component in-process optical testing and cryogenic requirement compliance certification, verification & validation is probably the most difficult metrology job of our generation in astronomical optics. But, the challenge has been met: by the hard work of dozens of optical metrologists; the development and qualification of multiple custom test setups; and several new inventions, including 4D PhaseCam and Leica Absolute Distance Meter. This paper summarizes the metrology tools, test setups and processes used to characterize the JWST optical components.

Final results of the Subscale Beryllium Mirror Demonstrator (SBMD) program

The Subscale Beryllium Mirror Demonstrator (SBMD) has been fabricated and tested, successfully demonstrating some of the necessary enabling technologies for the Next Generation Space Telescope (NGST) and other lightweight cryogenic space mirror applications. The SBMD is a 0.532-meter diameter concave spherical mirror with a 20-meter radius of curvature fabricated from a single billet of consolidated spherical powder beryllium. The mirror is lightweighted by 90% through the use of open back triangular cells and a thin facesheet. The mirror is mounted to a rigid backplane with titanium bipod flexures. Surface figure requirements at 35K of 1/4 wave p-v (full aperture) and 1/10 wave p-v (1-10 cm spatial frequency) required initial vacuum cryogenic characterization of the mirror. Cryogenic deformation and repeatability were characterized using the Optical Testing System (OTS) at the X-Ray Calibration Facility (XRCF) at Marshall Space Flight Center (MSFC). The mirror underwent cryofiguring to optimize performance and was subsequently tested to verify final performance requirements of surface figure, radius of curvature, and microroughness. Presented here are the final results of the SBMD program, showing that all requirements have been met.

JWST mirror production status

The James Webb Space Telescope (JWST) is an on axis three mirror anastigmat telescope with a primary mirror, a secondary mirror, and a tertiary mirror. The JWST mirrors are constructed from lightweight beryllium substrates and the primary mirror consists of 18 hexagonal mirror segments each approximately 1.5 meters point to point. Ball Aerospace and Technologies Corporation leads the mirror manufacturing team and the team utilizes facilities at six locations across the United States. The fabrication process for each individual mirror assembly takes approximately six years due to limitations dealing with the number of segments and manufacturing & test facilities. The primary mirror Engineering Development Unit (EDU) recently completed the manufacturing process with the final cryogenic performance test of the mirror segment assembly. The 18 flight primary mirrors segments, the secondary mirror, and the tertiary mirror are all advanced in the mirror production process with many segments through the final polishing process, coating process, final assembly, vibration testing, and final acceptance testing. Presented here is a status of the progress through the manufacturing process for all of the flight mirrors.

Cryogenic performance of lightweight SiC and C/SiC mirrors

The technology associated with the use of silicon carbide (SiC) for high-performance mirrors has matured significantly over the past 10-20 years. More recently, the material has been considered for cryogenic applications such as space-based infrared telescopes. In light of this, NASA has funded several technology development efforts involving SiC mirrors. As part of these efforts, three lightweight SiC mirrors have been optically tested at cryogenic temperatures within the X-Ray Calibration Facility (XRCF) at Marshall Space Flight Center (MSFC). The three mirrors consisted of a 0.50 m diameter carbon fiber-reinforced SiC, or C/SiC, mirror from IABG in Germany, a 0.51 m diameter SiC mirror from Xinetics, Inc., and a 0.25 m diameter SiC mirror from POCO Graphite, Inc. The surface figure error was measured interferometrically from room temperature (~290 K) to ~30 K for each mirror. The radius-of-curvature (RoC) was also measured over this range for the IABG C/SiC & Xinetics SiC mirrors. This paper will describe the test goals, the test instrumentation, and the test results for these cryogenic tests.

All-reflective four-element zoom telescope: design and analysis

The design process for an all-reflective zoom telescope is presented. Special consideration is given to the development of the starting configuration and the subsequent optimization process. The non-traditional optimization route utilized as a result of the unusual pupil characteristics of such an all-reflective zoom system is examined. Results of the design are presented.

The optical metrology system for cryogenic testing of the JWST primary mirror segments

The James Webb Space Telescope (JWST) primary mirror is 6.6 m in diameter and consists of 18 hexagonal mirror segments each approximately 1.5 m point-to-point. Each primary mirror segment assembly (PMSA) is constructed from a lightweight beryllium substrate with both a radius-of-curvature actuation system and a six degree-of-freedom hexapod actuation system. With the JWST being a near to mid-infrared observatory, the nominal operational temperature of a PMSA is 45 K. Each PMSA must be optically tested at 45 K twice, first to measure the change in the surface figure & radius-of-curvature between ambient & cryogenic temperatures and then to verify performance at cryo following final polishing. This testing is conducted at Marshall Space Flight Centers (MSFCs) X-Ray & Cryogenic Facility (XRCF). The chamber & metrology system can accommodate up to six PMSAs per cryo test. This paper will describe the optical metrology system used during PMSA cryogenic testing. This system evolved from systems used during the JWST mirror technology development program. The main components include a high-speed interferometer, a computer-generated holographic null, an absolute distance meter, a tiltable window, and an imaging system for alignment. The optical metrology system is used to measure surface figure error, radius-of-curvature, conic constant, prescription alignment, clear aperture, and the range & resolution of the PMSA actuation systems.

Cryogenic performance of the JWST primary mirror segment engineering development unit

The JWST (James Webb Space Telescope) primary mirror consists of 18 hexagonal mirror segments each approximately 1.5 meters point to point. The mirror segments are constructed from a lightweight beryllium substrate with both a radius-of-curvature actuation system and a six degree-of-freedom hexapod actuation system. The manufacturing process for each individual mirror assembly takes approximately six years due to limitations dealing with the number of segments and manufacturing & test facilities. In order to catch any manufacturing or technology roadblocks, as well as to streamline specific processes, an Engineering Development Unit (EDU) was built to lead the mirror manufacturing flow. This development unit has all of the same requirements as the flight units and is actually considered to be one of the flight spare mirrors. The EDU was manufactured with a lead time of approximately six months over the other mirrors to assure adequate time to optimize each step in the manufacturing process. Manufacturing and tests occurred at six locations across the U.S. with multiple trips between each. The EDU recently completed this arduous process with the final cryogenic performance test of the mirror assembly taking place at Marshall Space Flight Centers (MSFC) X-Ray & Cryogenic Facility (XRCF). Testing included survivability tests to 25 Kelvin, hexapod & radius-of-curvature actuation systems testing, and cryogenic figure & prescription testing. Presented here is a summary of the tests performed along with the results of that testing.