JWST mirror and actuator performance at cryo-vacuum

Wolf, Erin M.; Gallagher, Benjamin; Knight, J. Scott; Chonis, Taylor S.; Sullivan, Joseph; Smith, Koby; Rudeen, Andrew; Babcock, Kevin; Hardy, Bruce; Barto, Allison; Coppock, Eric; Davis, Clinton

doi:10.1117/12.2312872

Cited by 10 publications

(7 citation statements)

References 2 publications

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“…Finally, we obtained the output of the flexible legs as d 1 = R 1 (a 1 )a 1 , d 2 = R 2 (a 2 )a 2 , d 3 = R 3 (a 3 )a 3 by substituting the identified amplification ratio function. Then, the relationship between the actuators and the sensors can be rewritten as Equation (9). Additionally, the matrix relationship between the input of the actuator and the output of the sensors is shown in Figure 8.…”

Section: System Control Matrixmentioning

confidence: 99%

“…There are seven low-temperature actuators, customized by Ball Aerospace, behind each mirror. Each actuator has a coarse and fine two-stage adjustment system, and the actuator is connected to the mirror and support mechanism through a kind of flexible structure [9,10]. The co-phase adjustment of most ground-or space-based telescopes depends on the positioning mechanism behind the mirror.…”

Section: Introductionmentioning

confidence: 99%

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Design and Parameter Identification for a Positioning Platform with a Large Stroke and High Precision for Segmented Mirrors

Yin

Qin

et al. 2023

Micromachines

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An active optical system with three segmented mirrors was proposed to verify the co-focus and co-phase progress. In this system, a kind of large-stroke and high-precision parallel positioning platform was specially developed to help support the mirrors and reduce the error between them, which can move in three degrees of freedom out of plane. The positioning platform was composed of three flexible legs and three capacitive displacement sensors. For the flexible leg, a kind of forward-type amplification mechanism was specially designed to amplify the displacement of the piezoelectric actuator. The output stroke of the flexible leg was no less than 220 μm and the step resolution was up to 10 nm. Further, a linear model was established to identify the amplification ratio between the actuator and the flexible leg, which can increase the precision of the positioning platform. Moreover, three capacitive displacement sensors with a resolution of 2.5 nm were symmetrically installed in the platform to accurately measure the position and attitude of the platform. To improve the stability and precision of the platform, particle swarm optimization algorithm was applied to identify the control matrix, which can help the platform achieve ultra-high precision positioning. The results showed that the theoretical matrix parameters had a maximum deviation of 5.67% from the experimental ones. Finally, abundant experiments verified the excellent and stable performance of the platform. The results proved that while bearing the heavy mirror, which is no more than 5 kg, the platform can realize a 220 μm translation stroke and 2.0 mrad deflection stroke, with a high step resolution of 20 nm and 0.19 μrad. These indicators can perfectly cater to the requirements of the proposed segmented mirror system’s co-focus and co-phase adjustment progress.

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Section: System Control Matrixmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Design and Parameter Identification for a Positioning Platform with a Large Stroke and High Precision for Segmented Mirrors

Yin

Qin

et al. 2023

Micromachines

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“…The bearings in the motor are a limited lifetime item, sized for the expected mission lifetime. 57 Sunshield. Passive cooling of the JWST's OTE will be achieved mainly through the use of a large, deployable sunshield, as shown in Fig.…”

Section: James Webb Space Telescopementioning

confidence: 99%

“…3 (Top) Exploded view of the main elements of JWST, including the integrated science module and the thermal management unit. 57 (Bottom) Schematic representation of JWST's sunshield. 58 reminiscent of that of JWST but substantially larger.…”

Section: Large Ultraviolet Optical Infrared Surveyormentioning

confidence: 99%

Review on thermal and mechanical challenges in the development of deployable space optics

Villalba

Kuiper

Gill

2020

J. Astron. Telesc. Instrum. Syst.

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Deployable optics promise a revolution in the capability of observing the universe by delivering drastically reduced mass and volume needs for a desired level of performance compared to their conventional counterparts. However, this places new demands on the mechanical and thermal designs of new telescopes, essentially trading mass and volume for structural and control complexity. We compile the thermomechanical challenges that should be taken into consideration when designing optical space systems, as well as summarize 14 projects proposed to address them. Stringent deployment repeatability requirements demand low hysteresis, whereas stability requirements require high stiffness, proper thermal management, and active optics.

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“…While there are flight contingency procedures for recalibrating the FSC of any of Webb's 132 mirror actuators on-orbit, the processes are time consuming (i.e., on the order of days) and would reduce the time Webb could spend accomplishing its science mission. As a risk reduction exercise, measurements of the FSC errors in each actuator were made during OTE functional testing 6 during the various ground test campaigns in the fully integrated OTIS configuration over the last two years. The following sections describe these tests and their results.…”

Section: Fine Step Count Knowledge Requirements For Webb Commissioninmentioning

confidence: 99%

Characterization and calibration of the James Webb space telescope mirror actuators fine stage motion

Chonis

Gallagher

Knight

et al. 2018

Space Telescopes and Instrumentation 2018: Optical, Infrared, and Millimeter Wave

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The James Webb Space Telescope's (Webb's) deployable primary and secondary mirrors are actively controlled to achieve and maintain precise optical alignment on-orbit. Each of the 18 primary mirror segment assemblies (PMSAs) and the secondary mirror assembly (SMA) are controlled in six degrees of freedom by using six linear actuators in a hexapod arrangement. In addition, each PMSA contains a seventh actuator that adjusts radius of curvature (RoC). The actuators are of a novel stepper motor-based cryogenic two-stage design that is capable of sub-10 nm motion accuracy over a 20 mm range. The nm-level motion of the 132 actuators were carefully tested and characterized before integration into the mirror assemblies. Using these test results as an initial condition, knowledge of each actuator's length (and therefore mirror position) has relied on software bookkeeping and configuration control to keep an accurate motor step count from which actuator position can be calculated. These operations have been carefully performed through years of Webb test operations using both ground support actuator control software as well as the flight Mirror Control Software (MCS). While the actuator's coarse stage length is cross-checked using a linear variable differential transformer (LVDT), no on-board cross-check exists for the nm-level length changes of the actuators' fine stage. To ensure that the software bookkeeping of motor step count is still accurate after years of testing and to test that the actuator position knowledge was properly handed off from the ground software to the flight MCS, a series of optical tests were devised and performed through the Center of Curvature (CoC) ambient optical test campaigns at the Goddard Space Flight Center (GSFC) and during the thermal-vacuum tests of the entire optical payload that were conducted in Chamber A at Johnson Space Center (JSC). In each test, the actuator Fine Step Count (FSC) value is compared to an external measurement provided by an optical metrology tool with the goal of either confirming the MCS database value, or providing a recommendation for an updated calibration if the measured FSC differs significantly from the MCS-based expectation. During ambient testing of the PMSA hexapods, the nm-level actuator length changes were measured with a custom laser deflectometer by measuring tilts of the PMSA. The PMSA RoC fine stage characterization was performed at JSC using multi-wave interferometric measurements with the CoC Optical Assembly (COCOA). Finally, the SMA hexapod fine stage characterization test was performed at JSC using the NIRCam instrument in the "pass-and-a-half" test configuration using a test source from the Aft-Optics System Source Plate Assembly (ASPA). In this paper, each of these three tests, subsequent data analyses, and uncertainty estimations will be presented. Additionally, a summary of the ensemble state of Webb's actuator fine stages is provided, along with a comparison to a Wavefront Sensing & Control (WFSC)-based requirement for FSC errors as they rela...

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JWST mirror and actuator performance at cryo-vacuum

Cited by 10 publications

References 2 publications

Design and Parameter Identification for a Positioning Platform with a Large Stroke and High Precision for Segmented Mirrors

Design and Parameter Identification for a Positioning Platform with a Large Stroke and High Precision for Segmented Mirrors

Review on thermal and mechanical challenges in the development of deployable space optics

Characterization and calibration of the James Webb space telescope mirror actuators fine stage motion

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