Lightweight mirror technology using a thin facesheet with active rigid

supportJ. H. Burge, J. R. P. Angel, B. Cuerden, H. Martin, S. Miller

University of Arizona

D. SandlerThermoTrex Corp.

Advanced lightweight mirror technology being developed at University of Arizona

Motivated by Next Generation Space Telescope

Builds on UA developments for adaptive optics

NGST primary mirror requires a break from conventional technologies

HST NGST

Collecting area 4.5 m2 25-40 m2

Mass 800 kg 600 kg

Mass/area 180 kg/m2 15 kg/m2

Operating temperature Warm 40 K

Focal ratio F/2.4 F/1.2

Why? Bigger for seeing weaker sources, also diffraction limit in IRFaster focal ratio to limit size into launch vehiclePassively cooled to 40 degrees to allow far IR operationLighter so it can be sent to a more distant orbit, far from earth background

Conventional Mirror Technology

• Use glass because of its stability. Once the mirror is figured, it will maintain its shape.

• Make the mirror thick enough to have rigidity against dynamic loads and parasitic forces.

• Make the mirror rigid using mass efficiently -- attach facesheet to backsheet with ribs.

• Support the mirror by controlling the applied forces.

• HST Egg crate• MMT• Lightweight technologies

Ideal shape

Actuators are driven to compensate

Structure deforms, taking membrane with it

Membrane with Active Rigid Support

These mirrors depart from conventional thinking

• The mirror surface itself has little tendency to take the correct shape on its own.

• Uses rigid position actuators• Relies on active control with bandwidth defined by time scales

of instability or thermal drift of structure• Actuator length is driven to accommodate errors in the support

structure (different from AO DM which drives surface to have figure errors that compensate the atmosphere)

• All system rigidity comes from support structure and connections to glass

Key advantages of active mirrors• Achieves weight and figure goals of NGST• Robust system, can correct unexpected problems• Optimum use of materials

– Carbon fiber structure for light weight, stiffness– Glass for stable, high-quality optical surface

• Facilities and techniques now exist to make 8-m NGST– make the parent and cut into segments

• Actuators are key elements– Mass produced and tested economically– System is designed to tolerate failed actuators

Obvious questions • How can such a membrane be manufactured?• Will this really work?• Can it survive launch?• What are glass properties at 35K?• Are actuators available that have nm resolution at 35K?• Will such a complicated system be reliable?• How does one choose the number of actuators?• What is the next step in developing this technology • Can this technique provide the NGST primary mirror in

time for 2007 launch?

Fabrication of glass membraneThe concept is to work the glass while it is rigidly bonded in place

Demonstration of a 53-cm prototype

2 mm thick Zerodur membrane, f/1.4 sphereCarbon fiber support made by Composite Optics, Inc36 screw-type Picomotor actuators from New FocusTotal mass of 4.7 kg (21 kg/m2)Figure 33 nm rms after backing out static gravity effectsSubstrate and some funding provided by NASA Marshall

Actuator and glass attachment for 53-cm prototype

Figure of shell while it was blocked down

48 nm rms-150 nm

150 nm

Optical measurements of 53-cm prototype

53 nm rms

Raw measurementCalculated figure after subtracting self-weight deflection

33 nm rms

After manually adjusting actuators to optimize the figure

150 nm

-150 nm

Demonstration of survival of 1-m glass membrane

• 2.2 mm shell, sagged to 4-m radius

• supported on 75 dummy actuators, roughly 100/m2, giving ~400 Hz fundamental frequency

• aluminum backing plate

• Survived 3 dB over Atlas IIAS load in Lockheed Martin’s acoustic test facility

• Membrane survived shipping mishap as well as acoustic test

Cryogenic CTE for glasses

0 20 40 60 80temperature (K)

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

CTE

(ppm

/°K)

Borosilicate

Fused silica

Zerodur

ULE

Li A lum inasilicate

Cryogenic actuators• Early prototype designed and built by ThermoTrex and U of A (uses

proprietary ThermoTrex mechanism)• Concept demonstrated, now being optimized for production• Achieves 25 nm resolution at 77K• Requires zero hold power • 5 mm total travel, F > 100 g• total mass of 72 grams

0 10 20 30 400

100

200

300

400

500

steps

posi

tion

(nm

)

Wavefront control system• Wavefront sensors are under development (phase retrieval from

images and interferometry with star light appear feasible)

• Make correction at primary, rather than inducing opposite distortion into a deformable mirror

• Close the loop using an on-board computer

• Adjust figure every few observations, or every few days, depending on stability.

• These types of systems are mature for ground based systems

(a) initial state (b) after adaptive correction.Segmented mirror built by TTC showing interference fringes for=351 nm.

Prototype for the thin shell adaptive secondary mirror. This optichas a 2-mm membrane supported on 25 actuators with bandwidth >100 Hz.

Effect of failed actuators

• Failed actuators will be retracted, leaving area unsupported

• Coupled with membrane strain in a complicated way• if 5% of the actuators fail (8 actuators out of 150 on the

NMSD), the cryo performance will degrade by ~3 nm rms, depending on details of glass

System design and optimization

• Determine statistics of CTE variations within glass• fixed weight -- optimum actuator density vs membrane

thickness found by differentiation • Careful analysis of all launch loads• Adjust actuator density and location from FEM• Design coupling from actuator to glass

Actuator coupling to glass

University of Arizona 2-m NGST demonstration mirrorto be measured interferometrically at 35K mid-1999

Weight summary for 2-m NMSD

2-m NGST Demonstration Mirror goals and design values

Parameter Requirement Specified goal Predicted value

Diameter >1.5 meters 2 m 2 m

Figure ( = 633 nm)

Mid-spatial errors

Finish 2.0 nm rms 1.0 nm rms 1.0 nm rms

Areal density 15 kg/m2 < 15 kg/m2 12 kg/m2

Lowest structural resonance Not specified Not specified ~70 Hz

Lowest resonance of membrane Not specified Not specified 360 Hz

Mirror development program from NASA Marshall Space Flight Center

Next Generation Space TelescopeError budget for primary mirror

Mirror Surface Error

12 nm rms

Thermal effects

7.5 nm rms

Control systemerrors

6.7nm rms

Mirror fabrication 6.6 nm rms

Membrane fabrication5.2nm rms

Null correctorCalibration4 nm rms

Membrane cryo distortion

7.3nm rms

10° variationacross segment

1.5nm rms

Actuator resolution6 nm rms

Wavefront sensor

3 nm rms

(after correction using actuators)

Scale up for flight mirror

6-m monolith

System mass < 400 kg

• Same basic design, actuator density, membrane thickness

• We can make the mirror in any geometry op to 8.5-m f/1

• The difference from the 2-m is the CF backing structure. We had baselined a 6-m monolith for NGST, but are now designing segments for proposed deployed systems.

• Now looking at real fabrication issues for this.