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

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Presentation on theme: "GMT The Giant Magellan Telescope Phasing System Brian McLeod Harvard-Smithsonian Center for Astrophysics."— Presentation transcript:

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

2 GMT AO4ELT May 2013 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 2

3 GMT Telescope Observing Modes Natural Seeing – No phasing necessary Ground Layer AO – No phasing necessary 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 3AO4ELT May 2013

4 GMT 3-stage approach to phase the telescope to <65 nm RMS in the LTAO observing mode: 1.Initial phasing using off-axis Phasing Camera 2.Maintain alignment over short timescales using M1 & M2 edge sensors 3.Update edge sensor setpoints using the phasing channel of the On- Instrument Wavefront Sensor. Phasing Strategy: LTAO Mode 4AO4ELT May 2013

5 GMT Outline 5 1.The phasing challenge 2.The toolbox 1.Metrology 2.Optical Sensors 3.Compensators 3.Putting it together into a system AO4ELT May 2013

6 GMT GMT Phasing - Challenges Ohara E6 has non-zero CTE (2.8 × /°C) M1 segment separations are large (30-36 cm) 6AO4ELT May 2013

7 GMT 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’ 7 Challenges: Field dependent piston AO4ELT May 2013

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

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

10 GMT 10 Optical Sensors: Phasing Camera AO4ELT May 2013 Patrol radius = 6’-10’

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

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

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

14 GMT 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 14 Optical Sensors: Phasing camera: performance AO4ELT May 2013 K R nm RMS WFE

15 GMT 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 GMT 16 IOPS waveguide schematic Optical Sensors: Integrated Optics Piston Sensor AO4ELT May 2013 Opto-mechanical design

17 GMT 17 IOPS Performance Error source NGS 30” off-axis (nm RMS) NGS 60” off-axis (nm RMS) Low order Zernike modes 1639 Residual wavefront526 Residual tip-tilt55 Detector noise22 Wavelength bandwidth 66 Total IOPS Error (nm RMS) AO4ELT May 2013

18 GMT 3-stage approach to phase the telescope to <65 nm RMS in the LTAO observing mode: 1.Initial phasing using off-axis Phasing Camera 2.Maintain alignment over short timescales using M1 & M2 edge sensors 3.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 LTAO Phasing Strategy Summarized 18AO4ELT May 2013

19 GMT 1.M1 and M2 edge sensor metrology – startup and for high-speed relative measurement 2.Phasing camera 6-10’ off axis – fringe capture and initial setup 3.NGSAO – Dual wavelength pyramid sensor measures telescope+atmosphere 4.LTAO – Integrated Optics Piston 1’ or Phasing (no measurement of atmospheric piston) AO4ELT May Phasing System Summary


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