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,

UA technologies converge to NMSD Large, fast primary mirrors NMSD technology for NGST 6.5-m f/1.25 14 nm rms Adaptive secondary mirrors (thin glass, active control)

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

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!

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.

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 nm Excellent 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

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

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

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

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

NMSD Glass Deblocking Hot Oil-Bath Technique Floats attached to glass Pitch softens and glass 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 Glass resting on support tool, convex side up Vacuum support tool

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.

Actuator coupling to glass

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

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 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