Post on 17-Jan-2016
NGST Mirror System Demonstrator NGST Mirror System Demonstrator
from from
the University of Arizonathe University of ArizonaJim Burge
B. Cuerden, S. DeRigne, B. Olbert,
S. Bell, S. Clapp, P. Gohman, R. Kingston, G. Rivlis, P. Woida,
UA technologies converge to NMSD
Large, fast primary mirrors
Adaptive secondary mirrors(thin glass, active control)
NMSD
technology for NGST
6.5-m f/1.25 14 nm rms
• Rely on active control for shape accuracy.
• Use highly optimized lightweight backing structure for rigidity
• Choose facesheet for ease of manufacturing
• Use many position actuators, allows for redundancy
Lightweight mirror using a thin reflective surface with active rigid support
(high authority)Ideal shape
Actuators are drivento compensate
Structure deforms,taking membrane with it
The NGST Mirror System Demonstrator (NMSD)
• 2 meters in diameter• 13 kg/m2
• 2 mm thick facesheet• 166 actuators• 35K operation• Designed for launch
86 pounds total!86 pounds total!
Active mirrorsSince the mirror shape is determined by active control, the emphasis shifts from the optical surface to the control system
Wavefront sensing - This area is fairly mature. NASA, Lockheed Martin, and others have demonstrated accurate wavefront sensing directly from images using phase retrieval methods
Actuators - The actuators are key. These devices can be made to be simple and robust. Also, The system design can accommodate failed actuators.
NMSD Composite Support Structure Designed at UA and Lockheed Martin
Fabricated at Composite Optics, Inc (COI).
Cryogenic actuators
Weighs < 50 g(including cabling)
80-pitch screw
Electromagnetic drive
Tunable step size from 5 - 30 nmExcellent behavior at
ambient and cryogenic
The transition to actuator production
It took many months to develop a procedure and set of specifications that allow efficient actuator production
We have completed and tested 180 units
NMSD support structure - actuator installation
Fabrication of glass membrane
The concept is to work the glass while it is rigidly bonded in place
Glass substrate cast from Ohara E6 glass
Two pristine chunks of E6
Cast in UA 8-m spinning oven
Completed casting from Ohara E6 borosilicate
Substrate 100 mm thick borosilicate casting
Generate, grind, polish using conventional methods
Ground to concave sphere, R = 20 m
Supported with hydraulic actuators
Fabrication of blocking body for NMSD membrane
Start with 40 mm thick blank
Generate, grind, polish using conventional methods
Polished to convex sphere, R = 20 m
Supported with pitch pads on a convex blocking tool
Carefully polished to assure removal of subsurface damage from generating and grinding
Fabrication of convex (back) side of NMSD membrane
Blocking of 2.2-m membrane
Blocking body
Glass dam holding pitch
Oven hearth
Hydraulic support
Membrane support
Finishing 2 mm thick glass shellGenerate,grind and polish to thickness
Completed 2 mm shell
~ 0.5 µm rms, but smooth
NMSD Glass DeblockingHot Oil-Bath Technique
Floats attached to glass
Pitch softens andglass floats to the surface
Preparations for deblocking
Set up “the hot shack” with 10’ insulated stock tank with heaters and circulation pump
We went through a full scale test using float glass.
The learning curve...
Despite our positive test results, the silicone did not hold up at temperature, and the deblocking was aborted.
We switched materials to solve this problem
Successful deblocking
Floating in hot oil
Lifting from the oil using 18-point whiffle tree attached to floats
Cleaning and handling the glass
The finished glass membrane
Prepare for integration
Vacuum support toolGlass resting on support tool, convex side up
A fracture of unknown origin
Site of fracture initiation
Crack tips found by etchingthen stop drilled
Central site fully excised
The calculated Kt is 3.4, allowing ~900 psi stress. This would withstand launch and all handling operations as long.
Actuator coupling to glass
Bonding of attachment hardware
For safety factor of 3 for all loads in handling and operation:
All pucks are proof tested at 1.45 lbs in shear
Subloadspreaders are proof tested at 1.6 lbs in tension
Additional coupons have been tested long term in vacuum at 3.5 lbs
Pucks are attached with 12 µm thick bond of PRC 1564
Thickness maintained using microspheres
Bond area controlled by controlling glue volume
Puck positions maintained to 125 µm with template, tooling
Attachment of primary loadspreaders
Remaining tasks
• Complete loadspreader integration• Complete wiring and system testing electronics• Coat optical surface• Ambient testing at University of Arizona under tower• Cryogenic testing at MSFC XRCF• Operation using phase diversity wavefront sensing
System Performance
FE Analysis
• distortion due to cooling• annealing residual strains• blocking strains• membrane support
27 nm rms27 nm rms
Cryo distortions corrected by actuators, not by iterative polishing based on cryo measurements