David Chaney Space Symposium Radius of Curvature Actuation for the James Webb Space Telescope

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1 2018 Space Symposium Radius of Curvature Actuation for the James Webb Space Telescope David Chaney Optical Engineering Staff Consultant Ball Aerospace 4/2/18 1

JWST Overview James Webb Space Telescope (JWST) is a large infrared (0.6 to 29 microns) observing space telescope which will launch in 2019.

2 JWST Overview James Webb Space Telescope (JWST) is a large infrared (0.6 to 29 microns) observing space telescope which will launch in The observatory is comprised of 4 major subsystems Optical Telescope Element (OTE) Integrated Science Instrument Module (ISIM) Sunshield Spacecraft Bus 2

Telescope Overview The telescope consists of 4 mirrors Segmented Primary Mirror (6 DoF & RoC actuation) Secondary Mirror (6 DoF actuation) Tertiary (Fixed) Secondary Mirror Tertiary Mirror Primary

3 Telescope Overview The telescope consists of 4 mirrors Segmented Primary Mirror (6 DoF & RoC actuation) Secondary Mirror (6 DoF actuation) Tertiary (Fixed) Secondary Mirror Tertiary Mirror Primary Mirror Fine Steering Mirror (flat mirror used for pointing stabilization and very small offset maneuvers) FSM The mirrors are made of beryllium and will operate at ~45 Kelvin The primary mirror segments will be phased (adjusted in 6 DoF & Radius of Curvature) to create what is what is optically a single monolithic mirror 3

4 What is Radius of Curvature For a basic spherical mirror (JWST is an asphere) the shape is a section of a sphere (i.e. a piece of a ball) The Radius of Curvature (RoC) is defined as the radius of the ball size For a segmented primary, each mirror must have the same RoC to form a continuous spherical surface The center of curvature point is defined as the center of the ball. All light rays emanating from the center of curvature will be normal to the mirror surface Center of Curvature Radius of Curvature MIRROR 4

5 Radius of Curvature (RoC) Matching Methods To optimize the phasing of the primary mirror the Radius of Curvature for each segment must be matched There are a number of methods that could be employed to match the RoC JWST has chosen to use the RoC actuation method (PM semi-rigid architecture) due to a number of advantages including: - Relaxation of RoC requirements - Reduction in Polishing Time - Ability to perform real-time RoC adjustments in changing environment Polished Matching RoC Actuation Fixed Compensation Optic Deformable Compensation Optic Simplicity Reduced Alignment Requirements On-Orbit RoC Correction Capability Reduced Polishing Times 5

Actuation System Design The RoC actuation system is

system is designed to change the length of one actuator

6 Actuation System Design The RoC actuation system is completely separate from the 6DoF motion system The system is designed to change the length of one actuator and in turn apply a reaction force at the outer edge of the mirror. A flexure system self-balances the system to assure uniform loads at the six outer points of the mirror 6

RoC Actuation Residual Total Mirror Bending for 1mm RoC Actuation RoC Actuation Residual (Total minus Power) The design of the PMSA RoC actuation system has been optimized to deform the mirror in a

Due to the multitude of constraints, the RoC system produces some additional surface figure deformation above the pure RoC change. This figure change is known as RoC actuation residual.

7 RoC Actuation Residual Total Mirror Bending for 1mm RoC Actuation RoC Actuation Residual (Total minus Power) The design of the PMSA RoC actuation system has been optimized to deform the mirror in a power shape, while simultaneously being robust enough to survive launch effects and minimize both gravity and cryogenic mirror deformations. Due to the multitude of constraints, the RoC system produces some additional surface figure deformation above the pure RoC change. This figure change is known as RoC actuation residual. There is approximately 24nm rms of figure change per millimeter of radius of curvature actuation. The RoC actuation residual is predominately segment level spherical aberration and hexafoil. The amount of measured RoC residual is used to determine how much the mirror s RoC is actuated by. 7

8 RoC Fabrication Outline Manufacturing the mirrors with the correct RoC is part of the overall fabrication process The RoC of the mirror is polished to a very loose requirement During the first cryogenic test the RoC actuation system is used to set the RoC to a tight tolerance. The resulting RoC actuation residual error is simply part of the figure measurement The mirror is returned to the polishing phase where the RoC is not changed while polishing the mirror to match the Target Map. 8

9 RoC Metrology A RoC test method was developed that relied on setting the spacing between a CGH (Computer Generated Hologram) and the mirror segment to a known distance. A CGH acts as a diffractive null lens. Converts spherical test wavefront to an aspheric one. The CGH-to-Mirror spacing was measured using an Leica Absolute Distance Meter The RoC takes into account: spacing measurements, CGH accuracy, Window effects, Interferometer errors, Temperature effects, Data Processing, Gravity effects, and other error sources ADM Interferometer Removable Fold Mirror CGH Window (When testing under vacuum) PMSA 9

RoC Verification The RoC metrology discussed was established at three locations Ball Aerospace, Tinsley, XRCF To assure that the absolute RoC values measured at each location were consistent and

10 RoC Verification The RoC metrology discussed was established at three locations Ball Aerospace, Tinsley, XRCF To assure that the absolute RoC values measured at each location were consistent and accurate over the roughly 3 year test program a calibration optic was manufactured ROCO (Radius of Curvature Optic) is a spherical mirror with the same R# as the primary mirror segment. ROCO is ~ 20 diameter with 200 RoC ROCO s RoC was independently measured by the University of Arizona. ROCO was measured using the same measurement technique as the primary segments at each of the three test locations Testing showed that the RoC metrology met the accuracy and reproducibility requirements 10

Two opportunities were used to verify the RoC at the telescope

11 RoC Verification at Telescope Level After integration of the mirrors into the telescope additional testing was conducted. Two opportunities were used to verify the RoC at the telescope level Goddard Space Flight Center Test (GSFC) Johnson Space Center Test (JSC) 11

The PM was not phased but rather each segment was measured one at a time Since this test did not directly measure the RoC

12 RoC Verification at GSFC A Center of Curvature test at GSFC measured the surface figure of the mirror segments before and after telescope vibro-acoustic testing. The PM was not phased but rather each segment was measured one at a time Since this test did not directly measure the RoC of the segments, data gathered during this test was used to infer the RoC Results showed that the RoC values were within the expected uncertainty which gave confidence moving forward with the I&T plan 12

analyzed by two independent teams. Results showed that the cryogenic RoC matched the predicted value to less than 0.

13 RoC Verification at JSC As part of JSC testing the PM was phased using a multi-wave interferometer & null lens placed at the CoC of the PM The spacing between the null lens and the PM was measured using an ADM The RoC data was analyzed by two independent teams. Results showed that the cryogenic RoC matched the predicted value to less than 0.2mm. These results confirmed that the primary mirror manufacturing process resulted in the proper Radius of Curvature 13

14 Conclusion Summary Ball Aerospace successfully developed a methodology to manufacture and test the JWST primary mirror segments, which included a radius of curvature actuation system. This system was robust in nature and provided numerous advantages including the reduction in mirror polishing time and the ability to change the RoC after launch. Measurements of both the radius of curvature and the RoC actuation residual confirmed that both of these parameters had met the requirements put forth. Telescope level testing confirmed the ability to phase the mirrors in both 6-DOF motion and RoC matching. Acknowledgements The James Webb Space Telescope is an international mission led by NASA in partnership with the European Space Agency (ESA) and the Canadian Space Agency (CSA). Ball Aerospace would like to acknowledge NASA and our partner and observatory contractor for JWST, Northrop Grumman Aerospace Systems, for their support in the development of the PMSA. The authors would also like to acknowledge the exceptional support activities for PMSA testing by the University of Alabama in Huntsville and Marshall Space Flight Center s XRCF facility. Finally, we would like to thank the individuals, companies, and government institutions who supported the telescope level testing at both Goddard Space Flight Center and Johnson Space Center. 14

RADIUS OF CURVATURE MATCHING SYSTEM FOR A SPACE BASED SEGMENTED TELESCOPE

RADIUS OF CURVATURE MATCHING SYSTEM FOR A SPACE BASED SEGMENTED TELESCOPE David Chaney Ball Aerospace, 1600 Commerce St., Boulder, CO 80301 dchaney@ball.com ABSTRACT As space based telescopes continue