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A JWST Derivative Design for the Next Large Aperture UV/Optical Telescope
W. B. Whiddon Next Large Aperture Optical/UV Telescope Workshop 11 April 2003


Next Large Optical/UV Telescope Workshop

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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JWST Observatory Architecture
Optical Telescope Element (OTE) · Beryllium (Be) or ULE optics · Stable, symmetric PSF over wide FOR · Four deployments · Simple and low risk 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 Integrated Science Instrument Module (ISIM) · 3 instruments, fine guidance sensor · 23m2 volume · Simple three-point interface Sunshield · Passive cooling of OTE to <40K · Provides large FOR · Torque balancing limits momentum buildup · Reliable PAMS-type deployment

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

Spacecraft Bus · Passive isolation of wheel noise · Heritage components (12 yr life) · ACS supports fine pointing

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Tower · Isolates telescope from spacecraft dynamic noise


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JWST Optical Design Provides Wide FOV With Well-Defined Instrument Interface
Three mirror anastigmat (TMA) requires few surfaces to provide wide FOV, supporting efficient deep survey science Primary Simple on-axis conic prescriptions
­ Avoids costly fabrication ­ Generous alignment tolerances between OTE and ISIM
Mirror 7 m flat-to-flat Tertiary Mirror Focal Surface Interface to ISIM Fine Steering Mirror Telescope LOS

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

Toward Spacecraft OTE Optics ISIM Optics

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

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Concept Comparison
Requirement
Diffraction limited wavelength Wavelength Range Aperture and area of PM Resolution Focal Length, f/no. Optical Configuration Wavefront Error FOV FOR WFS&C

JWST
2 microns 0.6-27 microns 7m, 29.4m2 72 mas 116.7m, f/16.7 Three mirror anastigmat, 36 elements, Be or ULE 152nm RMS WFE for Strehl of 0.8 10 X 14 arcmin 4 pi over a year, ~40% of sky instantaneously Set and forget alignment using science instruments, tip-tilt-piston and radius control of individual elements L2 Passive cooling of OTE to 40K using large deployable 5 layer V-groove radiator sunshade, cooling in ISIM 6 mas

NHST
<0.5 microns (might extend to <0.2) 0.2-1 microns 7m, 29.4m2 18 mas 116.7m, f/16.7 Three mirror anastigmat, 36 elements, ULE 38nm RMS WFE for Strehl of 0.8 10 X 14 arcmin 4 pi over a year, ~40% of sky instantaneously Set and forget alignment and cophasing using science instruments, tip-tilt-piston and radius control of individual elements, with deformable mirror added L2 Active control of OTE optics to 1mK at 300K, using backplane / PM heaters, lightweighted sunshade, graphite backplane with zero CTE 1.5 mas

Orbit Thermal Design

Pointing stability
= Unchanged

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Key Challenges Result From New Operating Requirements
Provide Diffraction-Limited Operation at 500nm (18mas res'n)

Achieve 38nm RMS WFE Over 7m Aperture

Provide active Control at 300K to Support WFE Requirement

Provide pointing to <1.5 mas and Jitter to <1 mas

Accommodate Vis/UV Instruments

(4X better than JWST)

(Control of OTE to approx 10 mK)

(4X better than JWST)

(Requires stable, WFOV, few elements)

·Requires minor modifications to optical configuration ·Potential for segmented DM technology development

·Requires up to 500W ·No new technologies

·Minor extension of technology beyond JWST, SIM, Eclipse

·Improvements in apodization, contamination control, mirror surface quality

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Optical Modifications Require Minimal Technology Development
Challenge: Achieve 38nm RMS WFE over 7m aperture Modification/Option Maintain same basic optical configuration Pros/Cons/Issues 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) Low CTE at 300K supports thermal stability Backplane and actuator designs already compatible No new technology development needed Increases specific 0ass, adds ~200kg Decreases sensitivity to print-through errors Might obviate need for segmented DM Room temperature DM (25 kg for electronics, processing, and mirror) "Set and forget" operation using image-based wavefront sensing Colocation with FSM minimizes number of surfaces Segmented DM option might enable diffraction-limited performance down to <200 nm Non-segmented design will be flown by Eclipse

Use protected Al or Ag on ULE for 36 one-meter mirror segments Thicken primary mirror facesheets Add deformable mirror coincident with fast steering mirror at pupil

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Thermal/Mechanical Modifications Require No New Technology Development
Challenge: Provide active thermal control at 300K to support the WFE requirement Modification/Option Use minimally changed sunshade configuration Pros/Cons/Issues Depopulating two layers from JWST design saves up to 30 kg without significant cost/risk Ample observing efficiency without problems of a new design Contamination concerns Option for single blanket MLI 200 active thermal zones with 1W sheet heaters with 0.1K deadband provide ~10 mK control of OTE elements Adds ~300W, ~25 kg Requires two additional solar array panels, more array regulators, cabling (40 kg*) GFRP provides stability for long uninterrupted exposures Keep same basic ISIM structure and volume Remove 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

Actively heat backplane and mirror segments

Use backplane material with zero CTE at 300K Insulate exterior of ISIM

* Includes increased sizing for ISIM heaters
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Instrument Accommodation Needs No New Technology Development
Challenge: Accommodate new Vis/UV instruments Modification/Option Imaging and spectroscopy in visible and UV Pros/Cons/Issues Control of particulates and molecular for open, deployable optical configuration Photopolymerization may occur prior to sunshield deployment Need for strict contamination control measures Potential need for protective jettisonable or foldable coccoon (could be >100 kg) 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

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NHST Concept Derived From JWST
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

Secondary Mirror (SM) · Unchanged

Integrated Science Instrument Module (ISIM) · Imager, spectrograph, coronagraph, FGS · >1400 kg, ~23 m2 available 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

Tower · Unchanged

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Candidate NHST Optical Configuration Minimizes Changes from JWST
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

Primary Mirror 36 Segment Protected Al on ULE

7 m flat-to-flat Tertiary Mirror Focal Surface Interface to ISIM FSM/DM Telescope LOS

Secondary Mirror ROC Actuator Strongback

Toward Spacecraft

Load Spreader Tip/Tilt/Piston Actuators

·Simplified WFS&C (144 actuators) · Tip, tilt, piston, and independent ROC control · Rigid body corrections do not induce surface distortions or stress Combined DM/FSM Simple, clean interface for low AI&T and verification costs

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Impacts of First-Look Changes
Power 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 kg for contamination coccoon
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Technologies Available From JWST Can Reduce NHST Risk and Cost
Mirror Actuators (CDA) ULE Mirrors (Corning/Kodak) Mirror System (Brush-Wellman/ Tinsley/Ball) Wavefront Sensing and Control, Mirror Phasing (Ball)

Secondary Mirror Structure Hinges (NGST)

Deployable Optical Telescope Assembly (Alliant)

Half-Scale Sunshield Model (NGST)

Reaction Wheel 1 Hz OTE Isolators Isolators (NGST) (NGST) 14

Primary Mirror Structure Hinges and Latches (NGST)


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Advantages and Challenges
Technical Advantages of JWST Reuse
Large aperture provides near Class II performance at low cost and risk Highest risk elements will have been proven Large FOV with small number of optical elements for high throughput Stable PSF over large FOR >1400kg and >23m2available for instruments

Programmatic Advantages of JWST Reuse
Reuse of proven concepts for critical deployments, OTE design, sunshield reduces risk and cost Reuse of designs, drawings, analytical models Potential for reuse of test facilities and special test equipment Potential for refurbishment and flight of protoflight hardware Potential for multiple buys

Contamination control for large deployable unsealed optics operating in UV Cocoon design, quantification of contamination effects, controls for UV Providing contrast required for coronagraphy Eclipse and other surface quality, DM developments Apodizing Tighter actuator control Cleanup of errors from segmented optics Segmented DMs Pointing and jitter to sub mas levels Detectors, FPAs, coatings, instruments
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Challenges