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