Use of a Pathfinder Optical Telescope Element for James Webb Space Telescope Risk Mitigation

Lee D. Feinberga*,Ritva KeskiKuhaa, Charlie Atkinsonb, Scott C. Texter” aNASA Goddard Space Flight Center, Greenbelt, MD, USA 20771; 1’Northrup Grumman, Redondo Beach, CA USA

ABSTRACT

A Pathfmder of the James Webb Space Telescope (JWST) Optical Telescope Element is being developed to check out critical ground support equipment and to rehearse integration and testing procedures. This paper provides a summary of the baseline Pathfmder configuration and architecture, objectives of this effort, limitations of Pathfinder, status of its development, and future plans. Special attention is paid to risks that will be mitigated by Pathfmder. Keywords: Space Telescope, JWST, OTE, James Webb Space Telescope, Pathfmder, Risk

1. INTRODUCTION The testing of JWST Optical Telescope Element 1(OTE) and Integrated Science Instrument System (ISIM) occurs as an integrated entity called the OTE-ISIM (OTIS) at the Johnson Space Center”. In order to assure the test is successful with asfewcryogenic cycles as is possible,anOTE Pathfinder is being funded by the prime contractor Northrop Grumman as an IR+D funded effort. NGAS’s current plan emphasizes optical checkout at cryogenic temperatures though an augmentation to the Pathfmder plan is in the process of being added to the program that would add thermal checkout and dynamics objectives.

2. BIsT0RY OF PATHFINDER The OTE Pathfmder has its roots in the Verification Engineering Test Articles implemented on the Chandra program wherein unfinished flight optics were used to aid in the development of, and to demonstrate the integration operations and X-ray optical testing of, the critical optical components of the telescope using GSE that was uniquely designedfor the purpose. A significant component of the VETA-I and VETA-Il effort was the rehearsal and demonstrations of integration and testing operations. This served to significantly reduce the implementation risk of the GSE and integration and X-Ray optical testing of the flight optical assembly. Similarly, the JWST OTE Pathfinder is intended to reduce the implementation risk of the assembly, integration, and cryogenic optical test of the JWST optical assembly. A key goal for the OTE Pathfinder is to allow early checkout of the optical test procedures and test equipment to be used for cryogenic optical testing prior to their use in testing the flight hardware. This is the original key objective of Pathfinder because it significantly reduces the likelihood that the Flight OTIS will require extra JSC test cycles because of optical GSE not performing as expected. Additional unplanned JSC cryogenic cycles pose a risk to flight hardware, risks performance degradation, and have large cost and schedule implications risks, as a single cool-down and warm-up cycle is expected to take of order two months to complete.

—

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imaging

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work

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using

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uses

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hologram

via

interferometer

identification

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measurement

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phase

r(MWIF)

the

together,

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test

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to,

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to

the

will

means

as

the

surface

but

provided not

the

BIA

well

and

also

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to

configurations.

planned

sources

two

not

to

imaging

OTE

camera

monitor

to

collected

of

phasing,

as

will

as

be

though

enable

assess

on

PMSAs

identical observed

the

for

during

exercised.

Pathfinder

by

take

a

on

for

the

intent

will

the

of

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the

the a

the

via

and the

the

OTIS;

the the

via

BIA,

be

to,

of

For In

by

provides

fields The parameter part

and

illumination Exercising

Focal

in include comparing of Exercising

3000

the

defocus,

the

re-imaged

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of

nm

presence

of

Surface

SMA

the

the

view

a

mis)

to

OTE

check

the

the

the

providing

location

of and

the

by

relative

that

at

half-pass

focus, half-pass

3

of

optical

phase

PM

the

PMSAs

tilting

of

the

will

the

in

OTE

test

and

astigmatism

background

additional

to

1 retrieval

also

location

design.

the

g

test

test

the

configuration

that

in

of

focal

ACF(s).

exist

the

the

AOS will

enables

collimated

flight

surface.

algorithms.

It

AOS

of

diagnostic

also

in

and

levels

interface will

the

the

practicing

The

allow

configuration and

coma

sources

also

uses

OTIS

that

light

Figure

The

limiting

source

produce

capability.

in

for

upward-looking

from

The

will

images

test

is

six

relative

WFE

practicing

4:

reflected

larger

plates,

exist

degrees

due

the

aperture

Half

and pupil

recovery

are

retrieved

to

to

in

range

partial

Past

along

the

steered

the

the

back

intensity

of

determination

in

imperfect

freedom.

sources

TM. Test

JSC

this

of

with

in

through phase

illumination

motion

to

the

case

chamber

Configuration

It

maps

various

an

presence

on

also

map

is initial

imaging

Other

the

the

of

of

the

from

provides

to

the

during

OTE

field

field

AOS

of

ACF

AOS

practice

Plate

that

valuable

of

the

BIA

the

at

tilt

locations

by

the

expected

Source

the

aperture

inward

testing.

2 AOS

a

(tens

the

and

PMSAs

residual

check

run

Cassegrain

Source

information

Autocollimating

image

Plate.

of

for

sources,

representing

which

by

of

mm)

in

imaging uncorrected

the

the

centroiding.

the

The

enables

focus

location

includes

source

an

that

OTE

light

important

and

the

will that

Pathfmder Flats

locations

several

is

WFE

of

image

partial

various

is

be

collimated

In

the

an

(ACF5)

gained

addition,

(up

input

OTE

analysis waves

inherent

SI to where only a single ACF is used. During the Pathfmder test, the BIA will take the place of the Science Instruments as in the Half Pass test. Once again, phase retrieval using through-focus images recovers the WFE.

Other important optical testing includes pupil testing and ciyo photogrammetry which measures alignment of the system. The testing will also check out the rogue path illumination system shown in Figure 5. During the Pathfmder test the Signal to Background ratio and overall illumination levels can be assessed whereas during the flight OTIS testing, these rays should be blocked and no illumination should be detected. This enables the Pathfinder test to demonstrate the rogue path test process and also demonstrate that a sufficient signal should be present if the OTIS rogue path mitigation is in error. It also checks the aligmnent of the rogue path sources to the 0Th under cryogenic conditions.

String(6x)of sources ouId

L— Rogue Path I Sources +v1

H

Figure 5: Rogue Path Configuration

The Pathfinder testing will also check out the cryo position metrology which consists of an Absolute Distance Meter (ADM) and the Photogrammetry system, which includes targets, cameras, rotating mechanisms and data collection and processing.

5. DYNAMICS

The risk reduction aspects of the Patbfmder testing at JSC as it applies to dynamics is entirely within the realm of ensuring that viable optical data can be obtained. To that end, the JSC test configuration will have the suspension system isolators and TMDs and the HOSS damper, plus some representation of the shorts to ground provided by the cables and the HOSS cooling lines. Ballast mass is added to the HOSS to replicate the expected mass and CG of the OTIS so that the suspension system can be tuned. Successfully collecting optical performance data with the interferometer in the COCOA during the first Pathfinder cryo test, and then with the Pass-and-a-half test during the second Pathfinder cryo test, will provide the desired risk reduction associated with dynamics effects on the optical measurements. / •

risoiator Ballast

6 Isolators to chamber

TMD j WingTMD L14,L123L14 (GSE) 71

HOSSSpring a: &MagDamper

1_BallastMass Figure 6: Dynamics Configuration

Before the OTE Pathfinder test, a dynamics model will be run using the input PSDs measured at JSC during its certification, the model of the test configuration representing the design as implemented on the JSC chamber, and a dynamics model of the OTE Pathfmder to provide an expectation for the magnitude of the vibration in the optical measurements. In addition, planning is ongoing to add cryo accelerometers and stingers to help check out the dynamics and also to address the cryo performance of the flight dynamic system.

6. THERMAL

The thermal configuration of the OTE Pathfmder will be quite different than for the OTIS test. Most notably, the ISIM and IEC will not be present, nor will the TMS hardware including the ISIM radiators, +/-V2 radiators, PTR and the MU and SLI closeouts of the OTE. What will be present are the 2 non-flight PMSAs (one uncoated), non-flight SMA (uncoated), and the AOS. The flight FSM de-contamination heater will be in place and used during warm up. The method for warming up the Tertiary Mirror and SMA is undefined for the flight OTIS test, but will be replicated for the Pathfinder test at JSC. The PMSA warm-up method has been defined for the flight OTIS test. Its implementation for the Pathfmder OTE will be similar, using the OTIS decontamination heaters and some Pathfmder-specific local blanketing.

Work is ongoing to define a thermal augmentation effort that will include development of a 3D thermal model using the PMBSS thermal model. The augmentation test will propose to test the same temperature profile planned for the flight

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experience, including The

dynamics. OTIS. enables removed a The be monitoring

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authors OTE

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et efforts

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during

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points

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to

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during

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to practicing

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through

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for

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OTHER

REFERENCES

the

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