Sensitivity analysis of test methods for aspheric off-axis mirrors

The James Webb Space Telescope (JWST) Optical Simulation Testbed (JOST) is a tabletop workbench to study aspects of wavefront sensing and control for a segmented space telescope, including both commissioning and maintenance activities. JOST is complementary to existing optomechanical testbeds for JWST (e.g. the Ball Aerospace Testbed Telescope, TBT) given its compact scale and flexibility, ease of use, and colocation at the JWST Science & Operations Center. We have developed an optical design that reproduces the physics of JWST’s three-mirror anastigmat using three aspheric lenses; it provides similar image quality as JWST (80% Strehl ratio) over a field equivalent to a NIRCam module, but at HeNe wavelength. A segmented deformable mirror stands in for the segmented primary mirror and allows control of the 18 segments in piston, tip, and tilt, while the secondary can be controlled in tip, tilt and x, y, z position. This will be sufficient to model many commissioning activities, to investigate field dependence and multiple field point sensing & control, to evaluate alternate sensing algorithms, and develop contingency plans. Testbed data will also be usable for cross-checking of the WFS&C Software Subsystem, and for staff training and development during JWST’s five- to ten-year mission.

Future large aperture space telescopes may use lightweight correctable active mirrors. The Naval Postgraduate School's Segmented Mirror Telescope (SMT) test bed uses 1-meter silicon carbide (SiC) active mirror segments to form a sixsegment deployable 3-meter telescope. The active segments suffer from residual surface errors after a correction is applied. A deformable mirror is added at the SMT pupil plane to improve this residual error. The large active SMT segment represents the woofer, and a small continuous micro-electro-mechanical system (MEMS) deformable mirror represents the tweeter. A global influence matrix and closed loop constrained least squares controller command the active segment and additional deformable mirror as a single device. An interferometer measures the surface error and provides feedback to the controller. Simulation and experimental results demonstrate a significant improvement in wavefront error compared to a 2-step sequential woofer-tweeter constrained least squares control approach.

The James Webb Space Telescope (JWST) Primary Mirror Segment Assembly (PMSA) was required to meet NASA Technology Readiness Level (TRL) 06 requirements in the summer of 2006. These TRL06 requirements included verifying all mirror technology systems level readiness in simulated end-to-end operating conditions. In order to support the aggressive development and technology readiness schedule for the JWST Primary Mirror Segment Assembly (PMSA), a novel approach was implemented to verify the nanometer surface figure distortion effects on an in-process non-polished beryllium mirror surface. At the time that the TRL06 requirements needed to be met, a polished mirror segment had not yet been produced that could have utilized the baselined interferometric optical test station. The only JWST mirror segment available was a finished machined segment with an acid-etched optical surface. Therefore an Electronic Speckle Pattern Interferometer (ESPI) was used in coordination with additional metrology techniques to perform interferometric level optical testing on a non-optical surface. An accelerated, rigorous certification program was quickly developed for the ESPI to be used with the unfinished optical surface of the primary mirror segment. The ESPI was quickly implemented into the PMSA test program and optical testing was very successful in quantifying the nanometer level surface figure deformation changes in the PMSA due to assembly, thermal cycling, vibration, and acoustic testing. As a result of the successful testing, the PMSA passed all NASA TRL06 readiness requirements.

Segmented deployable space imaging systems represent a promising solution for future ultra-large aperture space optical systems. With the proposal of larger aperture segmented telescope designs, ultra-lightweight primary mirror (PM) segments with lower areal density have become a research hotspot. However, the lightweight structure poses more stringent challenges in maintaining a high-precision surface figure of PM segments. Active deformable PM segments with surface shape adjustment capability serve as an effective approach to address these challenges. To achieve surface figure control at the PM segments scale, accurate determination of the PM segments' surface figure error magnitude is fundamentally required. After in-orbit deployment of segmented mirror systems, deviations between the actual position and ideal position of PM segments will induce optical pupil localization errors in the system's overall pupil plane. The optical pupil localization error (OPLE) of PM segments leads to an inaccurate decoupling of wavefront errors into individual PM segments, thereby compromising the accuracy of surface figure correction. This paper proposes an active segmented mirror optical pupil localization method based on local wavefront sensitivity analysis. By utilizing the deformation capability of active optics, we establish sensitivity equations through wavefront variations within the PM segments' pupil region before and after surface shape adjustments, enabling the inverse solution for the OPLE of PM segments. The proposed method achieves micrometer-level positioning accuracy of PM segments' positional errors solely relying on focal plane information, while avoiding the introduction of additional mechanisms. Experimental validations were designed to verify the feasibility of the method.

The James Webb Space Telescope (JWST) Optical Simulation Testbed (JOST) is a tabletop experiment designed to reproduce the main aspects of wavefront sensing and control (WFSC) for JWST. To replicate the key optical physics of JWST's three-mirror anastigmat (TMA) design at optical wavelengths we have developed a three-lens anastigmat optical system. This design uses custom lenses (plano-convex, plano-concave, and bi-convex) with fourth-order aspheric terms on powered surfaces to deliver the equivalent image quality and sampling of JWST NIRCam at the WFSC wavelength (633~nm, versus JWST's 2.12~micron). For active control, in addition to the segmented primary mirror simulator, JOST reproduces the secondary mirror alignment modes with five degrees of freedom. We present the testbed requirements and its optical and optomechanical design. We study the linearity of the main aberration modes (focus, astigmatism, coma) both as a function of field point and level of misalignments of the secondary mirror. We find that the linearity with the transmissive design is similar to what is observed with a traditional TMA design, and will allow us to develop a linear-control alignment strategy based on the multi-field methods planned for JWST.

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 Center's (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.

Unlike supervised machine learning methods, reinforcement learning allows an entity to learn how to deploy a task from experience rather than labeled data. This approach has been used in this paper to correct piston misalignment between segments in a segmented mirror telescope. It was proven in simulations that the algorithm converges to a point where it learns how to move the piston actuators in order to maximize the Strehl ratio of the wavefront at the intersection.

James Webb Space Telescope (JWST) has a segmented Primary Mirror (PM). PM is made of 18 beryllium hexagonal shaped segments. Flat-to-flat dimension of a segment is 1.315 meters. The PM is an ellipsoid of~ 6.5 meters in diameter with a conic constant of -0.99666 and a radius of curvature ~16 meters. After the PM, telescope are assembled and instruments are installed the observatory will go through environmental testing. The environmental test consist of acoustic and vibration test. The objective is to measure the change in the surface astigmatism of the Primary mirror segments at center of curvature before and after vibration and acoustic test. At the final stage of assembly the inner segments of the PM have no external fiducials. The challenge is to separate the alignment astigmatism from surface astigmatism without any external fiducials. This paper describes an alignment method that uses the print-through in the mirror segments as fiducials to separate the two astigmatisms.

From its orbit around the Earth-Sun second Lagrange point some million miles from Earth, the James Webb Space Telescope (JWST) will be uniquely suited to study early galaxy and star formation with its suite of infrared instruments. To maintain exceptional image quality using its 6.6 meter segmented primary mirror, wavefront sensing and control (WFS&C) is vital to ensure the optical alignment of the telescope throughout the mission. WFS&C design architecture includes using the Near-Infrared Camera (NIRCam) to provide imagery for ground-resident image processing algorithms which determine the optimal alignment of the telescope. There are two distinct mission phases for WFS&C, both of which use algorithms and NIRCam imagery to determine the required segment updates. For the first phase, WFS&C commissioning, the telescope is taken from its initial deployed state with each of the 18 primary mirror segments acting like independent telescopes, to its final phased state with each segment acting in concert as a part of a single mirror. The second phase, Wavefront Monitoring and Maintenance, continues for the rest of the mission. Here the wavefront quality is evaluated, and when needed, the mirror positions are updated to bring it back to an optimal configuration. This paper discusses the concept of operations for the commissioning and on-going maintenance of the telescope alignment using WFS&C.

Segmented mirrors have quickly become an integral part of large telescope design in optical astronomy. They mitigate many of the problems associated with monolithic mirrors, such as rigidity, fabrication and transport, and even allow for foldable primary mirrors such as for the James Webb Space Telescope (JWST). However, one significant disadvantage is the need to cophase the separate segments to ensure they conform to the optimum mirror shape. Many cophasing approaches have been proposed and employed in practice, but all suffer from significant problems. Most notably the introduction of non-common path error, a persistent problem plaguing both the fields of active and adaptive optics. One recent proposed cophasing algorithm eliminates non-common path error by removing the requirements for additional hardware and instead concentrating on measuring piston and tip/tilt aberrations by their effects on images. Fizeau Interferometric Cophasing of Segmented Mirrors (FICSM) yields a large capture range and allows phasing to interferometric precision; for these reasons it was recently selected as the backup phasing strategy for the JWST. Here we present an overview of numerical simulations and results of optical testbeds, including the first lab demonstration of FICSM which successfully phased a segmented mirror with more than 5 wavelengths of piston to an RMS of 25nm, a result consistent with the limit set by the accuracy of segment motion. These results suggest this approach is well suited to the task of segment cophasing for future space missions.

The cophase calibration system is applied for adaptive optical phasing with a large segmented telescope mirror, which commonly uses 3-DOF micro-position device with three micro-displacement actuators to drive segments in parallel, making the entire segmented mirror in phase at one time and obtaining the desired sensor readings accordingly. In order to run the active control system to make the segments cophase, it is necessary to calculate the coordinates of the three driving points correctly for segmented mirrors specially limited in geometric parameters, especially for the stability and sensitivity of the micro-positioning device. The mirrors will be supported on a massively parallel system of electrostatically controlled, interconnected microactuators that can be coordinated to achieve precise actuation Adjusting posture of each segment independently so as to obtain the co-phasing errors and to control them at a nanometer level. Several generations of individual actuators as well as parallel arrays of actuators with segmented mirrors have been designed. A mechanical model of the system has been constructed and simulated numerically to obtain the actual position of three actuators using the RPY angle describing means. A three-channel parallel control scheme has been developed and implemented on a segmented mirror array. A universal evaluating method for optimization is prompted and will be a good guide to the design optimization of micro-positioning device for each segmented mirror when the mirrors are groundbased horizontally.

On-orbit cophasing is essential for high-resolution imaging of space telescopes with a segmented primary mirror. A model-based fine phasing method for a segmented mirror is proposed and demonstrated in this Letter. The tip-tilt error is related to the second moment of spot intensity, and the piston error is related to the Strehl ratio. Chebyshev segmented piston-tip-tilt (CSPTT) modes are used to express cophasing errors to suppress the effect of imaging noises. The CSPTT modal coefficients are estimated by the "2N + 1" algorithm that introduces bidirectional biases for each mode's estimation. The proposed method is proved robust to image noises and figure errors.

The one-meter Testbed Telescope (TBT) has been developed at Ball Aerospace to facilitate the design and implementation of the wavefront sensing and control (WFSC) capabilities of the James Webb Space Telescope (JWST). We have recently conducted an end-to-end demonstration of the flight commissioning process on the TBT. This demonstration started with the Primary Mirror (PM) segments and the Secondary Mirror (SM) in random positions, traceable to the worst-case flight deployment conditions. The commissioning process detected and corrected the deployment errors, resulting in diffraction-limited performance across the entire science FOV. This paper will describe the commissioning demonstration and the WFSC algorithms used at each step in the process.

The next generation of space telescopes for direct imaging and spectroscopy of exoplanets includes telescopes with a monolithic mirror, such as the Wide Field Infrared Survey Telescope (WFIRST) [1] and Large Ultra-Violet Optical Infrared (LUVOIR) telescopes with segmented primary mirror, like ATLAST [2, 3] or HDST [4].

The next generation of large space telescopes, including ATLAST, SAFIR, IXO and Generation-X will require the development of key technologies to enable their development at an affordable cost. This includes technologies for the rapid, low cost fabrication of ultra-light weight primary mirror segments, active figure control of primary mirror segments, high speed wavefront sensing and control, highly-packageable and scalable deployment techniques, and active vibration and thermal control for light weight structural elements to supply good pointing stability. In this paper we discuss the current state-of-the-art for these technologies and roadmaps for future development in these areas.