1. NGST Mirror System Demonstrator from the University of Arizona Jim Burge B. Cuerden, S. DeRigne, B. Olbert, S. Bell, S. Clapp, P. Gohman, R. Kingston, G. Rivlis, P. Woida,

2. UA technologies converge to NMSD
Large, fast primary mirrors
NMSD
technology for NGST
6.5-m f/ nm rms
Adaptive secondary mirrors
(thin glass, active control)

3. Lightweight mirror using a thin reflective surface with active rigid support (high authority)
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

4. 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!

5. Active mirrors
Since 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.

6. NMSD Composite Support Structure Designed at UA and Lockheed Martin Fabricated at Composite Optics, Inc (COI).

7. Cryogenic actuators Weighs < 50 g (including cabling)
80-pitch screw
Electromagnetic drive
Tunable step size from nm
Excellent behavior at ambient and cryogenic

8. 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

9. NMSD support structure - actuator installation

10. Fabrication of glass membrane
The concept is to work the glass while it is rigidly bonded in place

11. Glass substrate cast from Ohara E6 glass
Two pristine chunks of E6
Cast in UA 8-m spinning oven

12. Completed casting from Ohara E6 borosilicate

13. Fabrication of blocking body for NMSD membrane
Substrate 100 mm thick borosilicate casting
Generate, grind, polish using conventional methods
Ground to concave sphere, R = 20 m
Supported with hydraulic actuators

14. Fabrication of convex (back) side of 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

15. Blocking of 2.2-m membrane
Membrane support
Glass dam holding pitch
Blocking body
Hydraulic support
Oven hearth

16. Finishing 2 mm thick glass shell
Generate,grind and polish to thickness
Completed 2 mm shell
~ 0.5 µm rms, but smooth

17. NMSD Glass Deblocking Hot Oil-Bath Technique
Floats attached to glass
Pitch softens and
glass floats to the surface

18. 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.

19. 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

20. Successful deblocking
Floating in hot oil
Lifting from the oil using 18-point whiffle tree attached to floats

21. Cleaning and handling the glass

22. The finished glass membrane

23. Prepare for integration
Glass resting on support tool, convex side up
Vacuum support tool

24. A fracture of unknown origin
Crack tips found by etching then stop drilled
Central site fully excised
Site of fracture initiation
The calculated Kt is 3.4, allowing ~900 psi stress. This would withstand launch and all handling operations as long.

25. Actuator coupling to glass

26. Bonding of attachment hardware
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
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

27. Attachment of primary loadspreaders

28. 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

29. System Performance FE Analysis 27 nm rms
distortion due to cooling
annealing residual strains
blocking strains
membrane support
27 nm rms
Cryo distortions corrected by actuators, not by iterative polishing based on cryo measurements