GMTThe Giant Magellan Telescope Phasing System

Brian McLeodHarvard-Smithsonian Center for Astrophysics

GMT

2AO4ELT3 - 30 May 2013

The GMT Phasing Team

Systems EngineeringAntonin Bouchez - GMTO

Integrated Optics Phasing Sensor (IOPS)Francis Bennet - Australian National University

Phasing CameraBrian McLeod - Harvard-Smithsonian CfA

Pyramid WFSSimone Esposito, Enrico Pinna – INAF Osservatorio Astrofisico di Arcetri

M1 Edge sensorsD. Scott Acton - Ball Aerospace

M2 Edge sensorsRoberto Biasi, Mauro Manetti - Microgate

GMTTelescope 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

3AO4ELT3 - 30 May 2013

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

4AO4ELT3 - 30 May 2013

GMT

5

Outline

1. The phasing challenge2. The toolbox

1. Metrology2. Optical Sensors3. Compensators

3. Putting it together into a system

AO4ELT3 - 30 May 2013

GMTGMT Phasing - Challenges

• Ohara E6 has non-zero CTE (2.8 × 10-6 /°C)• M1 segment separations are large (30-36 cm)

6AO4ELT3 - 30 May 2013

GMT

7

• 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’

Challenges: Field dependent piston

AO4ELT3 - 30 May 2013

GMT

8

Metrology: M2 Capacitive Edge Sensors

(Microgate Corp.)

Expected piston sensitivity: 20nm RMS

AO4ELT3 - 30 May 2013

Considering alternative layout with additional single-axis sensors (green) to improve tilt sensitivity

GMT

9

Metrology: M1 distance interferometers

Renishaw distance- measuring interferometers

Expected short-term piston sensitivity: 13nm RMS

AO4ELT3 - 30 May 2013

GMT

10

Optical Sensors: Phasing Camera

AO4ELT3 - 30 May 2013

Patrol radius = 6’-10’

GMT

11

Optical sensors: Phasing camera: Basics

Form pupil image onMEMS array

Reimage pupil onto masked lenslet array.

EMCCDShack-Hartmann loop

Dichroic

Grism array

VisIR

Dispersed fringesAO4ELT3 - 30 May 2013

GMT

12

Deployed at Magellan July 2012

Optical sensors: Phasing camera: Prototype

AO4ELT3 - 30 May 2013

GMT

13

Optical Sensors: Phasing camera: Data analysis

AO4ELT3 - 30 May 2013

GMT

14

• 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

Optical Sensors: Phasing camera: performance

AO4ELT3 - 30 May 2013

K

R

nm RMS WFE

GMT

15

Optical sensors: Integrated Optic Piston Sensor

Poster 13236: Integrated Optic Segment Piston Sensor for the GMT, F. Bennet et al.

AO4ELT3 - 30 May 2013

GMT

16

IOPS waveguide schematic

Optical Sensors: Integrated Optics Piston Sensor

AO4ELT3 - 30 May 2013

Opto-mechanical design

GMT

17

IOPS Performance

Error sourceNGS 30” off-axis (nm RMS)

NGS 60” off-axis (nm RMS)

Low order Zernike modes

16 39

Residual wavefront 5 26

Residual tip-tilt 5 5

Detector noise 2 2

Wavelength bandwidth

6 6

Total IOPS Error (nm RMS)

18.6 47.6

AO4ELT3 - 30 May 2013

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

18AO4ELT3 - 30 May 2013

GMT

19

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 Sensor @ 1’ or Phasing Camera @6-10’ (no measurement of atmospheric piston)

AO4ELT3 - 30 May 2013

Phasing System Summary