1. The Giant Magellan Telescope Phasing System
Brian McLeod
Harvard-Smithsonian Center for Astrophysics

2. The GMT Phasing Team Systems Engineering
Antonin Bouchez - GMTO
Integrated Optics Phasing Sensor (IOPS)
Francis Bennet - Australian National University
Phasing Camera
Brian McLeod - Harvard-Smithsonian CfA
Pyramid WFS
Simone Esposito, Enrico Pinna – INAF Osservatorio Astrofisico di Arcetri
M1 Edge sensors
D. Scott Acton - Ball Aerospace
M2 Edge sensors
Roberto Biasi, Mauro Manetti - Microgate
Reorganized by subsystem/activity rather than by institution.
AO4ELT May 2013

3. Telescope Observing Modes
Natural Seeing
No phasing necessary
Ground Layer AO
Natural Guidestar AO
Correct telescope segment piston error
Correct atmospheric segment piston error (~120 nm RMS)
Laser Tomography AO
Only faint, off-axis natural guidestars available
Therefore, can correct only telescope segment piston error
AO4ELT May 2013

4. Phasing Strategy: LTAO Mode
3-stage approach to phase the telescope to <65 nm RMS in the LTAO observing mode:
Initial phasing using off-axis Phasing Camera
Maintain alignment over short timescales using M1 & M2 edge sensors
Update edge sensor setpoints using the phasing channel of the On-Instrument Wavefront Sensor.
AO4ELT May 2013

5. Putting it together into a system
Outline
The phasing challenge
The toolbox
Metrology
Optical Sensors
Compensators
Putting it together into a system
AO4ELT May 2013

6. GMT Phasing - Challenges
Ohara E6 has non-zero CTE (2.8 × 10-6 /°C)
M1 segment separations are large (30-36 cm)
Replaced 2D drawings of M1 with rendering
AO4ELT May 2013

7. Challenges: Field dependent piston
Dual segmentation leads to potential for field-dependent segment piston:
Sensitivity: 1 μrad M2 segment tilt compensated by M1 segment tilt leads to 10 nm of segment piston 1’ off-axis.
Performance limited by stability of M2 edge sensor system and ability to make piston measurement close to field center
Expected uncertainty in current design: 30nm at 10’
AO4ELT May 2013

8. Metrology: M2 Capacitive Edge Sensors
Expected piston sensitivity: 20nm RMS
Considering alternative layout with additional single-axis sensors (green) to improve tilt sensitivity
Added Mauro diagram.
(Microgate Corp.)
AO4ELT May 2013

9. Metrology: M1 distance interferometers
Expected short-term piston sensitivity: 13nm RMS
Added “short term”
Renishaw distance-
measuring interferometers
AO4ELT May 2013

10. Optical Sensors: Phasing Camera
Patrol radius = 6’-10’
Added context drawing
AO4ELT May 2013

11. Optical sensors: Phasing camera: Basics
Reimage pupil
onto masked
lenslet array.
Form pupil
image on
MEMS array
Dichroic IR
Vis
EMCCD
Shack-Hartmann loop
Grism array
AO4ELT May 2013
Dispersed fringes

12. Deployed at Magellan July 2012
Optical sensors: Phasing camera: Prototype
Deployed at Magellan July 2012
AO4ELT May 2013

13. Optical Sensors: Phasing camera: Data analysis
AO4ELT May 2013

14. Optical Sensors: Phasing camera: performance
For R<15, K<12, median seeing, 60 sec, get RMS<~50 nm with 85% sky coverage at SGP.
Fringe capture range is +/- 50 μm K R
nm RMS WFE
AO4ELT May 2013

15. Optical sensors: Integrated Optic Piston Sensor
Poster 13236: Integrated Optic Segment Piston Sensor for the GMT, F. Bennet et al.
AO4ELT May 2013

16. Optical Sensors: Integrated Optics Piston Sensor
IOPS waveguide schematic
Opto-mechanical design
Added optomechanical diagram
AO4ELT May 2013

17. IOPS Performance Error source NGS 30” off-axis (nm RMS)
Low order Zernike modes 16 39
Residual wavefront 5 26
Residual tip-tilt
Detector noise 2
Wavelength bandwidth 6
Total IOPS Error (nm RMS)
18.6
47.6
AO4ELT May 2013

18. LTAO Phasing Strategy Summarized
3-stage approach to phase the telescope to <65 nm RMS in the LTAO observing mode:
Initial phasing using off-axis Phasing Camera
Maintain alignment over short timescales using M1 & M2 edge sensors
Update edge sensor setpoints using the phasing channel of the On-Instrument Wavefront Sensor. OR
Update edge sensor setpoints using Phasing Camera if no star for IOPS
AO4ELT May 2013

19. Phasing System Summary
M1 and M2 edge sensor metrology – startup and for high-speed relative measurement
Phasing camera 6-10’ off axis – fringe capture and initial setup
NGSAO – Dual wavelength pyramid sensor measures telescope+atmosphere
LTAO – Integrated Optics Piston 1’ or Phasing (no measurement of atmospheric piston)
AO4ELT May 2013