Adaptive Optics for the Giant Magellan Telescope. Marcos van Dam Flat Wavefronts, Christchurch, New Zealand

Transcription

1 Adaptive Optics for the Giant Magellan Telescope Marcos van Dam Flat Wavefronts, Christchurch, New Zealand

2 How big is your telescope? 15-cm refractor at Townsend Observatory.

3 Talk outline Introduction to adaptive optics GMT first light AO modes Natural Guide Star Adaptive Optics (NGS AO) Ground Layer Adaptive Optics (GLAO) Laser Tomography Adaptive Optics (LTAO) Future AO modes Multi-conjugate Adaptive Optics (MCAO) Extreme Adaptive Optics (ExAO) Segment phasing

4 Introduction to Adaptive Optics Neptune No AO AO

5 Telescope Laser Projector Adaptive Secondary Mirror top view 7 8-m Primary Mirrors

6 Adaptive Optics Compensation

7 Wavefront sensing All wavefront sensors work on the following geometric optics principle: A slope in the wave-front causes an incoming photon to be displaced by Δx= zw x Light propagates in the direction normal to the wavefront

8 Shack-Hartmann wavefront sensor The aperture is subdivided using a lenslet array. Spots are formed underneath each lenslet. The displacement of the spot is proportional to the wavefront slope All wavefront sensors measure wavefront derivatives

9 Angular Anisoplanatism Objects in different parts of the sky have different wavefront aberrations The ground layer turbulence is constant over a wide angle High altitude turbulence decorrelates quickly Anisoplanatism is due to high altitude turbulence

10 Laser guide stars Natural guide stars can only be used to correct 1-5% the sky We can create our own stars anywhere in the sky with lasers

11 Limitations of Laser Guide Stars LGS is displaced on way up as well as on way down Can't determine overall tip-tilt of wavefront Tip-tilt causes causes image motion or jitter

12 Limitations of Laser Guide Stars Solution: use one (or more) natural guide stars to sense tip-tilt Can use much fainter stars Can use stars further from science object NGS: mr < 15, θ < 20 LGS: mr < 18, θ < 60

13 Cone Effect LGS located 90 km, astronomical objects at infinity Different turbulence is sampled Problem worse for big telescope Cone effect or focal anisoplanatism

14 Multiple Laser Guide Stars GMT will have six LGSs situated to avoid light contamination

15 Laser Tomography Adaptive Optics Use multiple laser guide stars to estimate the wavefront over many layers Place the contribution from all the layers on the DM. DM = 0 km 5 km 10 km

16 Resolving Distant Stellar Systems with AO Globular Cluster around Centaurus A HST Gemini GMT 4mas pixels, 1.65 microns 2 Laser Tomography Adaptive Optics

17 Resolving Distant Stellar Systems with AO Globular Cluster around Centaurus A Gemini 8m GMT 25m 17

18 Ground Layer Adaptive Optics A high proportion of turbulence is near the ground Ground layer aberrations are constant over a wide field of view Correct the ground layer only Modest improvement in image quality over large field of view

19 Performance of GLAO Adapted from simulations by Flicker 0.6 FWHM (arcsec) Uncorrected GLAO Wavelength (microns) Ground layer AO works well at visible wavelengths, too!

20 Ground Layer Adaptive Optics Use multiple laser guide stars to estimate the wavefront over many layers Place the ground layer contribution on the DM. DM = 0 km

21 Ground Layer Adaptive Optics Another way to see this: average the wavefront over many angles and place this on the DM. DM = N N

22 Multi Conjugate Adaptive Optics Have an additional DM optically conjugate to 12 km Estimate the wavefront over many layers, like LTAO Project the contribution from all the layers onto the 2 DMs DM 12 km DM -200 m

23 MCAO Performance No AO Classical AO 1 DM / 1 NGS MCAO 2 DMs / 5 NGS 165 MCAO gives a more uniform PSF over wider field of view at slightly reduced image quality

24 Extreme Adaptive Optics Used for high contract imaging (e.g., planet hunting) Second DM with very high actuator density Use the science camera as a second WFS to eliminate static and slowly varying aberrations See Olivier Guyon's talk this afternoon 10-5 companion

25 Comparison of Adaptive Optics Modes Mode K-band Strehl Field of View Sky Coverage NGS AO 70% 40 5% LTAO 60% 40 90% GLAO <1% % MCAO 50% % ExAO >90% 5 <<1% Fictitious but representative values comparing different AO modes

26 Segment Piston Error There are seven 8.4-m segments separated by 8.7-m There is no wavefront derivative information between segments Recall: wavefront sensors measure wavefront derivatives => Can't measure segment piston with wavefront sensors!

27 Measuring Piston with WFS Maybe you can measure slope across segments? Segments Slope measuring device Slope measurement This works if piston error is smaller than λ/4. For bigger discontinuities, you run into phase wrapping problems. Measurement is inefficient!

28 Sources of Piston Error Mirror alignment Slowly varying Measured interferometrically on off-axis star Windshake Phasing system Quickly varying Measured from position of segments Atmospheric Quickly varying Must be measured with AO guide stars? AO system?

29 Phasing System Overview Secondary edge sensors Provide high bandwidth relative piston & tilt. Primary edge sensors Provide high bandwidth relative piston & tilt Phasing camera Provides optical relative piston every 60s

30 Phasing camera concept 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 V~13 star within 20 off-axis patrol field. Optical channel (tip/tilt sensing) Field stop Iris AO DM (segment tip/tilt) dichroic Pupil mask ~2 reflective aperture Lenslet array IR array

31 Secondary Edge Sensor Concept Laser metrology from truss to reference bodies. 10 nm RMS relative precision possible at 1 khz.

32 Primary Edge Sensor Concepts 1. Optical metrology to rear of primary face sheet from Zerodur bars spanning segments Optical metrology from near the focal plane, off secondaries, to primary edges. 3. Optical metrology from secondary support truss to primary edges, along primary radius of curvature. 2 1

33 Atmospheric Piston Error The wavefront is reconstructed is the smoothest possible wavefront consistent with the measured wavefront slopes Unfortunately, the true wavefront is not as smooth as the smoothest possible wavefront!

34 Atmospheric Piston Error The error due to the segment discontinuities has been 5/ 6 calculated and simulated to be: 35 r 0 nm For r0 = 0.2 m, this is about 130 nm Total wavefront error should be < 200 nm! Need a way to measure segment piston interferometrically With segment piston Segment piston removed

35 Atmospheric Piston Error Atmospheric piston is due to high spatial frequencies in the wavefront and changes quickly with time. Needs to be measured and corrected at 250 Hz! Simulated error as a function of frame rate of ideal piston sensor

36 Atmospheric Piston Error Error is reduced for Laser Tomography AO because wavefront slope can be measured where meta-pupils overlap Meta-pupil overlap above ~1000 m

37 Conclusion First light AO modes NGS AO Several existing systems GLAO MMT 2010 experiments LTAO Keck & VLT 2013, MMT experiments 2011 Future AO modes MCAO Gemini South 2010 ExAO Gemini South 2011 Atmospheric piston No-one! GMT has to pioneer segment phasing techniques!

38 Who did you steal the slides from? Some slides and images were stolen from Patrick McCarthy, Phil Hinz, Michael Hart, Antonin Bouchez and James Lloyd If you steal from one author, that's plagiarism. If you steal from many, that's research.

39 Acknowledgements This material is based in part upon work supported AURA through the National Science Foundation under Scientific Program Order No. 10 as issued for support of the Giant Segmented Mirror Telescope for the United States Astronomical Community, in accordance with Proposal No. AST submitted by AURA Thanks to the conference organisers for bringing me here!