1. Telescope Phasing Slides Antonin Bouchez 1GMT AO Workshop, Canberra 22-24 Nov. 2010

2. 2 Phasing System Baseline Design GLAO: No phasing necessary NGSAO: Use a pyramid NGS sensor to sense both telescope & atmospheric piston. LTAO: Slow (~1 min) Optical Piston Sensor with large patrol area + fast (100 Hz) relative metrology of adjacent segments. Average over atmospheric piston and accept 120 nm RMS WFE. Edge sensors must measure relative piston of M1 segment surface to ~15 nm RMS at 100 Hz. GMT AO Workshop, Canberra 22-24 Nov. 2010

3. 3 Edge sensor location (12 total) 2 sensors per location (for Redundancy) Primary Mirror Edge Sensors GMT AO Workshop, Canberra 22-24 Nov. 2010

4. 4 Capacitive Sensors (ASM) Zerodur reference body Capacitive sensors

5. GMT AO Workshop, Canberra 22-24 Nov. 20105

6. 6 Overhanging metrology bar concept RetractedDeployed

7. 7 M1 Edge Sensor Options 3 Capacitive sensors (Keck, TMT, E-ELT) -Must be cantilevered from mirror side walls. Mirror front-back temperature gradient will cause large drift. Laser truss between mirror sidewalls -Simpler to implement mechanically, can be made insensitive to first- order effect of front-rear temperature gradients. -Still susceptible to asymmetric temperature gradients. -However, it’s a matter of timescales: Truss must keep drift <15nm RMS over ~few min. Modeling of M1 thermal transients required. Laser ranging to M1 front surface -Measures desired quantity. -Big emissivity problem. GMT AO Workshop, Canberra 22-24 Nov. 2010

8. 8 Phasing / Edge Sensing - Conclusion We only have a rough concept for how to put this all together at this point. JPL Laser Truss is potentially the most appealing. Needs modeling to verify timescale of M1 sidewall deformation. Interaction between AO and piston control loops not yet modeled or understood. Plan: Hire a controls engineer at the Project Office to take the lead on this. GMT AO Workshop, Canberra 22-24 Nov. 2010

9. End 9GMT AO Workshop, Canberra 22-24 Nov. 2010