1

Innovative, Non-Classical Optical performance

Verification Methodology:

Applied to the 6.6m James Webb Space Telescope

Allison Barto

November 8, 2012

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

EvolvePlanets, 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

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

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”

5

JWST is BIG!

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

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 component2. Alignment between the optics3. Adjustability of the PMSAs and SMA4. 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

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/

WFEAlignment

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

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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 TestExtrapolate 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 TestExtrapolate Test

Results to Flight

On-Orbit

Deployment

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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

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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 & PrescripXRCF 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

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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

alignmentFixed

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

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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 approximately1.0 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

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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

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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

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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