A JWST Derivative Design for the Next Large Aperture UV/Optical Telescope

W. B. WhiddonNext Large Aperture Optical/UV Telescope Workshop11 April 2003

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Strategy

JWST is the nation's investment in large space telescopes it is desirable to find ways to capitalize on the technologies

developed

Examine the possibility of developing viable NHST concepts derived from JWST configuration

Philosophy: minimize number of changes to minimize overall risk and cost

A design push rather than a requirements pull

Address required changes and issues with such designs

Identify areas that require technology development

Thanks to the NASA, NGST, Ball, Kodak team for developing the preferred JWST concept and the critical enabling technologies

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Next Large Optical/UV Telescope Workshop

JWST Observatory Architecture

Secondary Mirror (SM) Deployable tripod for stiffness 6 DOF to assure telescope

alignment

7 Meter Primary Mirror (PM) (29.4m2 area) 36 (1 m) hex segments simplify mfg and design Low risk two chord fold deployment Simple semi-rigid WFS&C Tip, tilt, piston, and radius corrections Segment performance demonstrated Stable GFRP/Boron structure over temperature

Tower Isolates telescope from

spacecraft dynamic noise

Integrated Science Instrument Module (ISIM)

3 instruments, fine guidance sensor

23m2 volume Simple three-point

interface

Sunshield Passive cooling of OTE to

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JWST Optical Design Provides Wide FOV With Well-Defined Instrument InterfaceThree mirror anastigmat (TMA) requires few surfaces to provide wide FOV, supporting efficient deep survey scienceSimple on-axis conic prescriptions

Avoids costly fabrication Generous alignment

tolerances between OTE and ISIM

Fine steering mirror provides low cost, straightforward image motion control Eliminates low frequency jitter Provides FOV offsets (dither) Offloads large angles to spacecraft

ACS

Telescope LOS

TowardSpacecraft

ISIM OpticsOTE Optics

SecondaryMirror

PrimaryMirror

TertiaryMirrorFocal SurfaceInterface to ISIM

FineSteeringMirror

7 m flat-to-flat

Simple clean interface keeps costs low: Reduces complexity of the interface Simplifies AI&T and reduces independent verification cost

Simple clean interface keeps costs low: Reduces complexity of the interface Simplifies AI&T and reduces independent verification cost

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Deployment Video

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Concept Comparison

0.2-1 microns0.6-27 micronsWavelength Range

4 pi over a year, ~40% of sky instantaneously4 pi over a year, ~40% of sky instantaneouslyFOR

18 mas72 masResolution

1.5 mas6 mas Pointing stability

Active control of OTE optics to 1mK at 300K, using backplane / PM heaters, lightweightedsunshade, graphite backplane with zero CTE

Passive cooling of OTE to 40K using large deployable 5 layer V-groove radiator

sunshade, cooling in ISIM

Thermal Design

L2L2Orbit

Set and forget alignment and cophasingusing science instruments, tip-tilt-piston and

radius control of individual elements, with deformable mirror added

Set and forget alignment using science instruments, tip-tilt-piston and radius control

of individual elements

WFS&C

10 X 14 arcmin10 X 14 arcminFOV

38nm RMS WFE for Strehl of 0.8152nm RMS WFE for Strehl of 0.8Wavefront Error

Three mirror anastigmat, 36 elements, ULEThree mirror anastigmat, 36 elements, Be or ULE

Optical Configuration

116.7m, f/16.7116.7m, f/16.7Focal Length, f/no.

7m, 29.4m27m, 29.4m2Aperture and area of PM

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Key Challenges Result From New Operating Requirements

Achieve 38nm RMS WFEOver 7m Aperture

Provide active Control at 300Kto Support WFE Requirement

Provide pointing to

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Optical Modifications Require Minimal Technology Development

Low CTE at 300K supports thermal stabilityBackplane and actuator designs already compatible No new technology development needed

Use protected Al or Ag on ULE for 36 one-meter mirror segments

Unchanged optical layout forward of integrated science instruments module provides near-Class II resolution and performance at low cost and risk Maintains same instrument interface (F, f/no., FOV, mechanical interface, same available volume and mass)

Maintain same basic optical configuration

Room temperature DM (25 kg for electronics, processing, and mirror)"Set and forget" operation using image-based wavefront sensingColocation with FSM minimizes number of surfacesSegmented DM option might enable diffraction-limited performance down to

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Thermal/Mechanical Modifications Require No New Technology Development

200 active thermal zones with 1W sheet heaters with 0.1Kdeadband provide ~10 mK control of OTE elementsAdds ~300W, ~25 kgRequires two additional solar array panels, more array regulators, cabling (40 kg*)

Actively heat backplane and mirror segments

Depopulating two layers from JWST design saves up to 30 kg without significant cost/riskAmple observing efficiency without problems of a new designContamination concerns Option for single blanket MLI

Use minimally changed sunshade configuration

Keep same basic ISIM structure and volumeRemove most second surface mirrors and replace with MLI blankets (-10 kg)Add active heating zones to interior (~150W, 3 kg)Mass savings of ~200kg due to removal of active cooling in instruments

Insulate exterior of ISIM

GFRP provides stability for long uninterrupted exposuresUse backplane material with zero CTE at 300K

Pros/Cons/IssuesModification/OptionChallenge: Provide active thermal control at 300K to support the WFE requirement

* Includes increased sizing for ISIM heaters

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Instrument Accommodation Needs No New Technology Development

Deformable mirror for control of mid-spatial frequency wavefront errors (to be proven on Eclipse)Potential for active control (to be traded with active heater control)Potential for higher control authority on the PM segments (increased number and precision of actuators)Apodization masks to compensate for irregular aperture shape, and intersegment gaps

Coronagraphy

Control of particulates and molecular for open, deployable optical configurationPhotopolymerization may occur prior to sunshield deploymentNeed for strict contamination control measures Potential need for protective jettisonable or foldable coccoon(could be >100 kg)

Imaging and spectroscopy in visible and UV

Pros/Cons/IssuesModification/OptionChallenge: Accommodate new Vis/UV instruments

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NHST Concept Derived From JWST

Secondary Mirror (SM) Unchanged

Optical Telescope Element (OTE) ULE optics with thicker facesheets Deformable mirror, apodizing mask

added Same four deployments as JWST

7 Meter Primary Mirror (PM) (29.4m2 area) 36 (1m) hex segments simplify mfg and design Deployable chord fold for thermal uniformity GFRP structure stable over temperature

Tower Unchanged

Integrated Science Instrument Module (ISIM)

Imager, spectrograph, coronagraph, FGS

>1400 kg, ~23 m2available

Sunshield Reduced number of layers from JWST Supports active thermal control

of OTE at 300K Provides ample FOR Momentum balanced

Spacecraft Bus Added two solar array panels, extra regulators

Heritage components for 12 yr life

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Candidate NHST Optical Configuration Minimizes Changes from JWST

Telescope LOS

TowardSpacecraft

SecondaryMirror

PrimaryMirror 36 SegmentProtected Al on ULE

TertiaryMirror

Focal SurfaceInterface to ISIM

FSM/DM

7 m flat-to-flat

Load Spreader

Tip/Tilt/Piston Actuators

Strongback ROC Actuator

TMA provides WFOV with few surfaces for high discovery efficiency Simple on-axis conic prescription avoids costly fabrication, provides generous alignment tolerances

Mid-high frequency optical quality manufactured into segments

Simplified WFS&C (144 actuators) Tip, tilt, piston, and independent ROC control Rigid body corrections do not induce surface

distortions or stressCombined DM/FSMSimple, clean interface for low AI&T and verification costs

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Impacts of First-Look ChangesPower Impacts

+ 300 W (500 W worst case peak) for active heating of OTE + 150 W (200 W worst case peak) for active heating of ISIM - 700 W (at 12 yrs) from adding two solar array panels

Mass Impacts +200 kg for PMA facesheets +25 kg for DM plus electronics and processing +25 kg for PM sheet heaters +40 kg for power system resizing -200 kg for deletion of cooling in ISIM -30 kg for depopulation of sunshield layers -10 kg for reduction in ISIM radiators +3 kg for ISIM heaters Total dry mass change = +53 kg* +3 kg for added propellant Total wet mass change = +56 kg

Launch Vehicle Launch margin on Atlas V541 is 28.9% (including contingencies) For Atlas V551: 39% margin for +$2M For Delta IVH: 64% margin for +$80M

* Does not include potential + 100