The GMT Adaptive Optics System

Antonin Bouchez Michael Hart, Phil Hinz, Steve Shectman, M. van Dam

GMT2010, Seoul, 4 Oct. 2010

GMT2010, Seoul, 4 Oct. 2010 2

Outline

1.  Introduction 2.  AO system design 3.  Wavefront sensors 4.  Primary & secondary mirror phasing 5.  Performance models and simulations

3

Adaptive Optics on the GMT

•  Enormous benefits in resolution and sensitivity. -  Resolution ∞ D -  Point source sensitivity ∞ D2 (integration time ∞ D-4)

In practice current AO systems achieve ∞ D1.5

•  Smaller image size can allow significant reduction in narrow-field instrument size.

JWST 6.5m

GMT 25m

Simulation of globular cluster around Cen A (3.8 Mpc)

H-band

1.2ʹ′ʹ′

GMT2010, Seoul, 4 Oct. 2010 4

AO Features Unique to the GMT

•  AO correction built into telescope through an adaptive secondary mirror (ASM).

•  ASM allows low background observations at > 2 µm. –  On 25 m telescope, AO correction is beneficial even at 10 µm. –  Exoplanet imaging and planet formation science drivers are

strengthened by this design choice.

•  ASM and wide-field telescope design enables GLAO. –  Will increase the sensitivity and resolution of the planned multi-

object NIR and visible spectrographs for GMT. –  Galaxy assembly and high-z science drivers are strengthened by

this design choice.

GMT2010, Seoul, 4 Oct. 2010 5

Design Constraints

•  The system is designed to maximize science return with minimal technical development.

–  Adaptive Secondary Mirrors are near-replicas of LBT, VLT design.

–  Expected AO performance is similar to MMT/LBT systems. –  Laser Guide Star system can use existing commercial lasers.

•  Within these programmatic constraints, the system performance and design are derived from the science requirements and the science instrument needs.

6 6

Mode Description

Natural Guide Star AO (NGSAO)

High Strehl over a narrow field of view using bright guide stars. Requirement: 160 nm RMS WFE for R<8 stars.

Laser Tomography AO (LTAO)

Moderate Strehl over narrow field of view with high sky coverage Requirement: 200 nm RMS WFE over 80% of sky at b=60°.

Ground Layer AO (GLAO)

Image improvement over very wide field of view by correcting only low-altitude turbulence.

Goal: 2-4x image size reduction in near-IR over 8’ diameter.

First Generation GMT AO Modes

GMT2010, Seoul, 4 Oct. 2010

AO System Components

7

LGS Projector

Laser beam relay

Laser enclosure

4704 actuator adaptive secondary mirror

Wavefront Sensors •  NGSAO: 1 visible pyramid

sensor •  LTAO: 6 LGS sensors, 3

visible NGS or 1 IR NGS •  GLAO: 6 LGS sensor, 3

visible NGS •  Optical Phasing Sensor

top view

Instruments GMT2010, Seoul, 4 Oct. 2010

8

The GMT Adaptive Secondary

•  Seven 1.1 m diameter segments, each with 672 actuators •  Segments are matched 1-to-1 with primary mirror segments •  Hexapods on each segment provide alignment control •  Primary segment misalignments can be compensated with secondary motions

GMT2010, Seoul, 4 Oct. 2010

GMT2010, Seoul, 4 Oct. 2010 9

Laser Guide Star Facility

Laser enclosure on telescope azimuth platform •  7x 30 W CW 589 nm lasers (1 spare) •  Single projection system •  6 beacons on variable radius, 35” (LTAO) to 4’ (GLAO)

3 mirrors relay beams to launch telescope

50 cm launch telescope is located behind the secondary mirror top view

Project between segments minimizes fratricide effect.

LTAO Wavefront Sensing Geometry

GMT2010, Seoul, 4 Oct. 2010 10

•  3 stars in visible to measure tip/tilt/focus. •  1 star to measure high-order calibration errors.

< 90”

35”

•  1 star in near-IR to measure tip/tilt/focus. •  1 star in visible to measure high-order calibration errors.

< 45”

GLAO Wavefront Sensing Geometry

GMT2010, Seoul, 4 Oct. 2010 11

•  3 visible stars in an annulus around science FOV used to measure tip/tilt in the visible. •  Telescope active optics sensors measure high-order calibration errors on far off-axis star.

4’ 5’-7.5’

~4’

NGSAO/LTAO Wavefront Sensor Layout

GMT2010, Seoul, 4 Oct. 2010 12

Instrument cryostat window (reflect <1 µm)

LGS wavefront sensors (folded upward)

Acquisition camera field

NGS wavefront sensor field (180” diameter)

60x40 cm tertiary mirror located 1 m above instrument platform

Visible WFS Assembly (mounted to instrument)

Science & IR WFS field (180” diameter)

Fixed beamsplitter (reflect <600 nm)

Fixed beamsplitter (reflect 589 nm)

Gregorian focus 20 arcmin. field

13

GLAO Wavefront Sensor Layout

Dichroic GLAO WFS

Science Light Instrument Platform

Summary of Wavefront Sensors

Name Location N λ Patrol Track1 Rotate2 Comments

Active Optics IP 3 Visible 15’ diam N N GLAO calib.

Phasing IP 1 Ks, Vis. 15’ diam N Y

GLAO LGS IP 6 589 nm 8’ diam N Y

GLAO NGS SI 3 Visible 8’ annulus N N

NGSAO VWFS 1 Visible 3’ diam N N LTAO calib.

LTAO LGS VWFS 6 589 nm 1.2’ (fixed) N Y

LTAO NGS VWFS 3 Visible 3’ diam N N

LTAO IR SI 1 IR 1.5’ diam N N

IP = Instrument Platform SI = Science Instrument VWFS = Visible WFS Assy.

1 = Sensor must track sky rotation in image plane. 2 = Sensor or pupil mask must rotate to track pupil.

14 GMT2010, Seoul, 4 Oct. 2010

15

Phasing the GMT mirror segments

•  To achieve diffraction-limited images, the GMT segments must be phased to ~1/40 of a wavelength of light.

•  Due to the fact that laser guidestars are incoherent, they cannot be used to measure the optical path length difference between disjoint optical surfaces.

•  Without a phasing sensor, the GMT AO system would superimpose 7 images with the diffraction limit of an 8.4 m telescope.

GMT2010, Seoul, 4 Oct. 2010

16

Phasing for three AO Modes

•  GLAO – Not diffraction limited •  NGSAO – Use a pyramid NGS sensor to measures both

telescope & atmosphere. •  LTAO – No sufficiently bright star within the isoplanatic

region to measure optical path difference. Proposed solution: Slow (~1 min): Optical Phasing Sensor Fast (100+ Hz): Relative metrology of adjacent segments Atmospheric piston: Ignore and accept 120 nm RMS WFE

GMT2010, Seoul, 4 Oct. 2010

17

NGSAO Pyramid Sensor •  30x30 pyramid WFS recently demonstrated at LTB. •  Can measure piston across segment gaps. •  Cannot be used in LTAO as piston error has high

spatial order and must be measured <20” from science target.

Pupil images with 40 nm piston on upper-right segment.

18

LTAO Optical Phasing Sensor Concept

315

270

225

180

135

90

45

0

GMT2010, Seoul, 4 Oct. 2010

19

Optical Phasing Sensor Design

~2” reflective aperture

Iris AO DM (segment tip/tilt)

Field stop

Pupil mask Lenslet array IR array

dichroic

Optical channel (tip/tilt sensing)

•  Use “Enhanced IR Chanan test” •  Place 1.5 m subaperture across each segment gaps •  Sense & correct tip/tilt across each segment to increase fringe contrast •  Requires K~15 star within the off-axis patrol field •  Prototype currently being designed at CfA

20

AO System Performance Wavefront error source RMS wavefront error (nm)

NGS LTAO Primary mirror figure 20 20 Secondary mirror figure 20 20 Non-common path optics 40 40 Science instrument 60 60 Fitting error 80 121 Atmospheric temporal lag 93 93 WFS measurement noise propagation 30 50 Tomography error 95 Piston error (Tel + Atm.) 30 130

High order total 151 240 Tip/tilt measurement 12 30 Tip/tilt anisoplanatism 148 @ 60” Residual windshake 50 50

Total: On-axis 160 247

Total: Off-axis 288

21

NGSAO simulations

NGSAO simulation •  Shack-Hartmann

WFS with 50x50 subapertures

•  25th percentile seeing (r0=20 cm)

•  Bright star (V=5), 1 kHz frame rate

•  Resulting K Strehl ratio: 72%

Residual Wavefront, first 0.1 s

22

LTAO simulations

LTAO simulation •  6 LGS in 80” diam.

hexagon •  Tomographic

reconstruction of turbulence on 4 layers

•  Use 3 NGS in the visible, up to 90” off-axis (R=14, 16, 17)

•  Resulting K Strehl ratio: ~30%

•  This simulation limited by static aberrations due to LGS-WFS sampling

GMT2010, Seoul, 4 Oct. 2010

AO development Plan

•  Working towards an AO PDR in December 2011 •  U. Arizona performing preliminary design study for NGS & GLAO •  Australia National U. performing preliminary design study for

LTAO •  CfA building a prototype phasing sensor for on-sky tests at Las

Campanas in late 2011. •  ADS/Mircrogate to complete preliminary design study for ASM. •  About to kick off ASM optics prototyping contracts.

•  Critical path to AO at first light: Expect ASM production to take 7 years from Phase C start.

GMT2010, Seoul, 4 Oct. 2010 23