Lightweight mirror technology using a thin facesheet with active rigid support J. H. Burge, J. R. P....
-
Upload
sharyl-owens -
Category
Documents
-
view
217 -
download
0
description
Transcript of Lightweight mirror technology using a thin facesheet with active rigid support J. H. Burge, J. R. P....
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.