Innovative, Non-Classical Optical Performance Verification Methodology
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The document describes the optical performance verification methodology for the James Webb Space Telescope (JWST). The methodology involves (1) testing the optical performance of each component, (2) measuring the alignment between components, (3) verifying the adjustability of movable mirrors, and (4) validating wavefront sensing and control algorithms. This data is used in an integrated telescope model to predict final optical performance on-orbit after wavefront correction. The verification program uses a combination of component tests, subsystem tests, and system-level tests to build confidence while addressing the technical challenges of JWST's large size and cryogenic operation.
Innovative, Non-Classical Optical Performance Verification Methodology
- 1. 1 Innovative, Non-Classical Optical performance Verification Methodology: Applied to the 6.6m James Webb Space Telescope Allison Barto November 8, 2012
- 2. 2 Fluctuations condense into first stars and proto- galaxies A Brief History of TimeA Brief History of Time Big Bang Particle Physics Today Atoms & Radiation First Galaxies Galaxies Evolve Planets, Life & Intelligence 3 minutes 1 Gyr Z ~ 5 14 Gyrs Z ~ 0 400 Myr Z ~ 12 Cosmic Dark Zone Cosmic Dark Zone Fluctuations in microwave background 300,000 yr Z ~ 5000 The JWST mission JWST will be a space based astronomical observatory that explores the gap between the distant microwave background and the nearer universe observed by today’s observatories. JWST will be able to “see through the dust” to probe galaxy and planet formation The four science themes are: – First light and re-ionization – Assembly of galaxies – Birth of stars and protoplanetary systems – Planetary systems and origins of life Adapted from: H. S. Stockman, ed. “the Next Generation Space Telescope, “Visiting a Time When Galaxies were Young”, Space Telescope Science Institute, (June 1997.)
- 3. 3 JWST: Large, Cold and Deployable Science objectives require observations of very faint objects in NIR and MIR – 0.6 mm to 27 mm wavelengths – Diffraction limited performance at 2 mm wavelengths and longer – Collecting aperture > 25 m2 Warrants large aperture telescope operating at cryogenic temperatures 32K to 59K Large size requires telescope be stowed for launch in folded configuration, then deployed on orbit
- 4. 4 OTE Architecture Overview SM Support Structure (SMSS) • Deployable four-bar linkage strut assembly • Secondary Mirror Mount (SMM) Thermal Management Subsystem (TMS) • Panel Roof Radiators • Panel +/- V2 Radiators • ISIM Enclosure (MLI) • Parasitic Tray Radiator Aft Optics Subsystem (AOS) • Fixed tertiary mirror • Fine steering mirror • Baffle and pupil mask SM Assembly (SMA) • Low mass Beryllium mirror • 6 DoF pose control PM Segment Assembly (PMSA) • 18 low mass Beryllium mirrors • 6 DoF pose control • Radius of curvature control PM Backplane Assembly (PMBA) • Fixed Center Section supports 12 segments • Two deployable wings support 3 segments each Deployable Tower Assembly • Deployable telescoping tube • Two deployable harness trays • Cryocooler line accommodation Isolator Assembly • 1-Hz passive isolators • Tower support TMS (continued) • Deployable “bat wings” • Fixed diagonal shield • Deployable stray light “bib”
- 6. 6 Optical Verification Methodology Key Features of the JWST architecture make a traditional “test as you fly” ground test challenging from both a technical and cost perspective: – 6.5 meter aperture – Passively cooled to cryogenic temperatures (~40 K) – Thermal Stability effects on Optical Performance (requirements allow DT ~ 0.15 K) – Final flight configuration cannot be tested on the ground (alignment is not deterministic) Test program has been designed to support a final performance prediction via analysis – High fidelity verification at lower levels of assembly – Focus system-level testing on measuring data that cannot be obtained at a lower level - Alignment between major components – “Crosscheck” tests at the integrated system level to confirm lower-level test data can be relied upon – Extensive model validation program - Supported by crosscheck testing and independent modeling efforts – Alignment range on-orbit - Because system is aligned in flight, margin in actuator range gives added confidence to ability to achieve aligned and phased state on orbit.
- 7. 7 Optical Verification Overview In order to verify Observatory optical performance on-orbit, there are four main aspects of the system that need to be understood: 1. Optical performance of each optical component 2. Alignment between the optics 3. Adjustability of the PMSAs and SMA 4. Performance of the WFS&C Algorithms Each of these pieces of data is then used by the Integrated Telescope Model (ITM) to analytically align the telescope and predict final performance StrehlStrehl WFE stabilityWFE stability EE StabilityEE Stability Initial AlignmentInitial Alignment Image MotionImage Motion Thermal distortionThermal distortion FGS-FSM-ACS loopFGS-FSM-ACS loop Disturbance Response Disturbance Response Component WFE (PM, SM, TM, FM, SI) Component WFE (PM, SM, TM, FM, SI) WFSC algorithms WFSC algorithms Ground to flight effectsGround to flight effects Test Uncertainties (Alignment & WFE) Test Uncertainties (Alignment & WFE) STOP Analysis Using As-Built Models Encircled Energy Encircled Energy (Used for Sensitivity Calculation) Monte Carlo As- Built In-Flight Telescope/SIs WFS&C Commissioning Simulations ITM
- 8. 8 Relationship Between Test Performance and Final Verification Uncertainty Fixed Alignment Errors AOS TM FSM ISIM to AOS Compensation by PMSA actuation Compensation by SMA actuation Compensation by SMA actuation Residual WFE after SMA compensation Adjustable Alignment Errors (incl PM Prescrip & all uncert) PMSA level & PM figure Alignment SMA level & SM to AOS Alignment OTE Optic Figure Optic Figure Science Inst. Non-Common Path Uncert Prescription (SM/TM/FSM) Figure/ WFE Alignment Legend Residual WFE after all compensation Other Error Introduced by WFS&C •Sensing Error •Control Error •Field Dependent Error Compensation by SMA actuation PM to AOS Alignment Errors Compensation by PMSA actuation Mid/High Frequency Errors not compensatable Final uncertainty after PMSA/SMA compensation through WFS&C: Therefore, the verification program must: – Show verification uncertainties of alignment and low frequency figure errors are within the capture range of the adjustable optics – Show the remaining uncertainties are sufficiently bound to ensure Image Quality reqs are met - Mid/High frequency optic figure - Actuator Control - Sensing During WFS&C commissioning, low frequency errors are compensated by the PM & SM – Fixed alignment errors – Low frequency mirror figure errors
- 9. 9 Actuator Range Verification Optical performance budgets are based on the assumption that we have enough actuator range to reach an aligned configuration on orbit Test and analysis uncertainties combine to impact total range needed on orbit and magnitude of error that needs to be corrected using WFS&C Actuator Range is verified through a combination of: – Ground alignment during Observatory testing in JSC Chamber-A – Careful tracking of uncertainties of each constituent of the range budget – Analysis of ground to orbit alignment and figure changes Nominal Deployed Position Actuator Range for Alignment Tolerance (OTE-252) 50% Actuator Range Margin (OTE-258) 50% Actuator Range Margin (OTE-258) Ground Measured Range Extrapolated to FlightPredicted “Ground Nominal” Position Ground measured Range Example Acceptable Actuator Position Deployed Flight Position Algorithm Capture Range (OTE-254) Req & Budget Range Allocations JSC Actuator Range Test Extrapolate Test Results to Flight On-Orbit Deployment Nominal Deployed Position Actuator Range for Alignment Tolerance (OTE-252) 50% Actuator Range Margin (OTE-258) 50% Actuator Range Margin (OTE-258) Ground Measured Range Extrapolated to FlightPredicted “Ground Nominal” Position Ground measured Range Example Acceptable Actuator Position Deployed Flight Position Algorithm Capture Range (OTE-254) Req & Budget Range Allocations JSC Actuator Range Test Extrapolate Test Results to Flight On-Orbit Deployment
- 10. 10 Optical Performance of Each Optical Component Tests highlighted with a green box supply data input directly into the ITM model and are considered verification tests Tests not highlighted with a green box are crosscheck are model validation tests of optical performance PMSA Optical Testing SMA Optical Testing TMA Optical Testing FSM Optical Testing Individual SI Optical Testing COCI Testing of Integrated PM at JSC AOS Optical Test using AOS Source Plate during Pathfinder Testing at JSC Integrated ISIM Optical Testing with OSIM at GSFC AOS/ISIM Optical Test using AOS Source Plate Inward Sources at JSC Pass-and-a-Half Observatory Optical Test using AOS Source Plate Outward Sources at JSC PMSA Optical Testing SMA Optical Testing TMA Optical Testing FSM Optical Testing Individual SI Optical Testing COCI Testing of Integrated PM at JSC AOS Optical Test using AOS Source Plate during Pathfinder Testing at JSC Integrated ISIM Optical Testing with OSIM at GSFC AOS/ISIM Optical Test using AOS Source Plate Inward Sources at JSC Pass-and-a-Half Observatory Optical Test using AOS Source Plate Outward Sources at JSC
- 11. 11 Installation Align (to AOS) Actuator Range & Stow/Deploy Pre/Post Vibe Mechanical Gaps Alignment to AOS Actuator Performance Actuator Range Ambient Tests Cryogenic Tests Simplified PMSA Optical Test Flow Other Test Data Mirror Fab (CGH-T) (in process figure measurements) BOTS (CGH-B1) Prescription Alignment XRCF 1 (CGH-B2) Cryo Hit Map Early Fig & Prescrip Actuators Prescrip Align Tinsley Final (CGH-T) Figure & Prescrip XRCF 2 (CGH-B2) Final Figure BOTS (CGH-B1) Prescription Alignment Prescription Alignment Actuator range/resolution Prescrip Align Ambient Tests Cryogenic Tests Integrated PM Optical Test Flow Other Test Data SSDIF Pre/Post Vibe PMSA figure (optical testing under review) JSC Ambient (Null Lens) PMSA figure including Astig JSC Cryo (Null Lens) Final RoC & Low freq figure Alignment to AOS Gaps (optical) In addition to the system-level optical crosschecks, several independent measurements of mirror prescriptions are made at the subsystem level The figure below shows a simplified test flow for the PMSAs as an example of all the in-process measurements that are made – Individual optics are tested both at ambient and cryogenic temperatures – Testing before and after repeated mounting verifies mount distortion – Testing before and after environmental testing verifies allocation for launch induced changes Repeated Optical Testing Builds Confidence & Crosschecks Performance
- 12. 12 Alignment Verification Between Each Optical Component ISIM Verification Data OTE Verification Data ISIM Crosscheck Data OTE Crosscheck Data Legend OTE / ISIM Verification Data OTE / ISIM Crosscheck Data Element I&T Observatory I&T SMSS alignment PMBSS cryo shift PMBSS 1-g/0-g release PMBSS shape SI internal alignment AOS internal alignment PMSA-PMSA Alignment SMSS Deployment ISIM to AOS alignment PM to AOS alignment & PMSA Ground Test Actuator Range SM to AOS alignment & SMA Ground Test Actuator Range Fixed Alignments Adjustable Alignments PMBA Wing Deployment Subsystem Test AOS internal alignment ISIM to AOS alignment SI to ISIM alignment Fixed Alignments Alignment between major optical components is the primary system-level cryo test output for optical verification – Understanding alignment of the optics is critical to verifying actuator range requirements are met – Uncertainty in alignment testing is one of the primary contributors to the capture range needed by WFS&C System cryo testing is the only opportunity to obtain many alignments Critical fixed alignments that cannot be measured directly are obtained in lower-level tests
- 13. 13 OTE Cassegrain Focal Surface – downward pointing IR sources. Images at AOS focal point (up to 3000 nm rms WFE) AOS TM FSM Backplane I/F (3 places) Alignment Verification During System Cryogenic Test Alignment between major optical components is verified at cryogenic temperatures using photogrammetry and the inward sources on the AOS Source Plate – Photogrammetry measures alignment to 100 micrometer / 150 micron levels – ASPA measures alignment of the AOS to the ISIM to approximately 1.0 mm despace, 0.5 mm decenter, 0.25 mrad tip/tilt, 1.2 mrad clocking The system-level “Pass-and-a-Half” optical test described earlier serves as a crosscheck to the alignments measured through the verification tests. Photogrammetry Measures alignment of: - PM to AOS - SM to AOS COCI measures PMSA to PMSA alignment (Photogrammetry is crosscheck for outer 12 PMSAs) AOS Source Plate inward facing sources are used during the Field Alignment Test to measure ISIM to AOS Alignment WFE retrieved by SI WFS; wavefront power extracted to determine defocus Targets Photogrammetry AOS Source Plate PG Cameras on four windmills
- 14. 14 Adjustability Verification of the PMSAs and SMA PMSA/SMA Adjustability Verification PMSA/SMA Adjustability is dominated by actuator performance. Therefore, the test program focuses on detailed verification at the actuator level – Each actuator is tested to the sub-nanometer level for resolution and accuracy at both ambient and cryogenic temperatures (requirements are 10 nm step size with 3 nm accuracy) Adjustability of the fully-integrated PMSAs and SMA are further tested during component cryogenic testing. During these tests, actuator performance is verified to the 2-4 step level – Also compared to hexapod models developed using actuator-level test data Total actuator range is verified at all the test points above During the system-level cryogenic testing, adjustability is crosschecked when mirrors are exercised during PM alignment and when aligning the SMA for the Pass-and-a-Half test. – PMSA motions can be measured to the 2-4 step level through the COCI. Wavefront Sensing Verification Performance of the algorithms is verified using the Integrated Telescope Model – Both individual algorithm performance and full end-to-end commissioning simulation is performed Performance of WFS&C components and Science Instrument detectors is verified at the component and instrument level
- 15. 15 Communication This all sounds pretty complicated – How do you communicate such an idea? – How do you show nothing has been missed? 1. Graphically explain the big picture – keep it to one page! – Because the key JWST optical verification is building a representative model of the as-built hardware, we developed a “Verification Roadmap” to show the top-level data flow (NOT event flow (i.e. not from a schedule). Address how test uncertainty is handled – The roadmap delineates key concepts to understand how the pieces fit together: - At what level of integration is the data collected? (component, system) - Are there key differences between how the data is collected and how it must be used? (temperature, gravity orientation) - What is the fidelity of the test (verification test, model validation test) - Identify system cross check tests – what pieces of data do they crosscheck? 2. Catalog the piece parts to show all are accounted for – We developed a “Crosscheck Matrix” - each row represents a critical parameter (figure of an optic, alignment between optics) - columns represent all major test events (component, assembly, system) - Data within each cell shows the requirement we are trying to measure and the uncertainty of that test
- 16. 16 Summary JWST Optical Performance Verification program has been developed to balance technical challenges with the desire to “test-as-you-fly” Data for each component of the system is verified at the component level and input directly into the as-built Observatory model – I&T Workmanship concerns, actuator range, and combined uncertainties have been considered Flight alignment process/algorithms applied directly to the as-built model This approach gives an accurate prediction of final performance – Use of analysis minimizes the technical difficulties of verification of system performance to requirement- level through an end-to-end test – Allows a more accurate verification and understanding of the effect of test uncertainties on the final in- flight alignment and optical performance of JWST Non-classical approaches to verification programs can be successfully implemented and may be both more technically accurate and more cost effective than traditional approaches Clear communication, careful tracking, and model validation help ensure success Work was supported in part by Ball Aerospace & Technologies Corp subcontract with Northrop Grumman Aerospace Systems (NGAS) under the JWST contract NAS5-02200 with NASA GSFC. The JWST system is a collaborative effort involving NASA, ESA, CSA, the astronomy community, and numerous principal investigators