1. Lightweight mirror technology using a thin facesheet with active rigid support J. H. Burge, J. R. P. Angel, B. Cuerden, H. Martin, S. Miller University of Arizona D. Sandler ThermoTrex Corp. Advanced lightweight mirror technology being developed at University of Arizona Motivated by Next Generation Space Telescope Builds on UA developments for adaptive optics

2. NGST primary mirror requires a break from conventional technologies Why? Bigger for seeing weaker sources, also diffraction limit in IR Faster focal ratio to limit size into launch vehicle Passively cooled to 40 degrees to allow far IR operation Lighter so it can be sent to a more distant orbit, far from earth background

3. 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.

4. HST Egg crate MMT Lightweight technologies

5. Ideal shape Actuators are driven to compensate Structure deforms, taking membrane with it Membrane with Active Rigid Support

6. 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

7. 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

8. 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?

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

10. Demonstration of a 53-cm prototype 2 mm thick Zerodur membrane, f/1.4 sphere Carbon fiber support made by Composite Optics, Inc 36 screw-type Picomotor actuators from New Focus Total mass of 4.7 kg (21 kg/m 2 ) Figure 33 nm rms after backing out static gravity effects Substrate and some funding provided by NASA Marshall

11. Actuator and glass attachment for 53-cm prototype

12. Figure of shell while it was blocked down 48 nm rms -150 nm 150 nm

13. Optical measurements of 53-cm prototype 53 nm rms Raw measurement Calculated figure after subtracting self-weight deflection 33 nm rms After manually adjusting actuators to optimize the figure 150 nm -150 nm

14. Demonstration of survival of 1-m glass membrane 2.2 mm shell, sagged to 4-m radius supported on 75 dummy actuators, roughly 100/m 2, 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

15. Cryogenic CTE for glasses

16. 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

17. 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

18. 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

19. 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

20. Actuator coupling to glass

22. University of Arizona 2-m NGST demonstration mirror to be measured interferometrically at 35K mid-1999

23. Weight summary for 2-m NMSD

24. 2-m NGST Demonstration Mirror goals and design values Mirror development program from NASA Marshall Space Flight Center

25. Next Generation Space Telescope Error budget for primary mirror Mirror Surface Error 12 nm rms Thermal effects 7.5 nm rms Control system errors 6.7nm rms Mirror fabrication 6.6 nm rms Membrane fabrication 5.2nm rms Null corrector Calibration 4 nm rms Membrane cryo distortion 7.3nm rms 10° variation across segment 1.5nm rms Actuator resolution 6 nm rms Wavefront sensor 3 nm rms (after correction using actuators)

26. 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.