1. Opening New Frontiers with the GMT, Seoul, October 4 2010 Giant Magellan Telescope How does an adaptive secondary mirror support the unique qualities of the GMT? Michael Hart Phil Hinz, Antonin Bouchez Opening New Frontiers with the GMT – Seoul, October 4, 2010

2. Opening New Frontiers with the GMT, Seoul, October 4 2010 The ELT farm GMT is the smallest of the three planned ELTs. BUT – besides being the first to be built, it offers design features that make it uniquely suited to many science applications – The primary mirror is made from large segments. – The optical configuration is Gregorian. – Instruments are at the direct focus. – It will implement high-order adaptive correction at the secondary mirror. 2 GMT TMT E-ELT

3. Opening New Frontiers with the GMT, Seoul, October 4 2010 Importance of AO The ELTs in general, including GMT, need AO to exploit their full potential. – Seeing-limited light buckets can be built better and more cheaply in other ways. Enhancements in resolution and sensitivity are crucial. All the ELTs will have AO correcting to the diffraction limit. – GMT is at risk in the D 4 point-source sensitivity game. What roles can the GMT take that distinguish it scientifically from other telescopes? 3

4. Opening New Frontiers with the GMT, Seoul, October 4 2010 Unique features of GMT to be exploited An adaptive secondary mirror – Critical for wide field ground-layer AO with FIVE TIMES the étendue of TMT in this mode. Gregorian secondary is optically conjugated to ground layer => bigger corrected field. – Low, low thermal background. No need to build a truck-sized freezer for thermal cleanliness (NFIRAOS) before you get to the instrument. Large rigid optically smooth primary mirror segments – Supports ground-layer AO. – Easier to control PSF side lobes for very high contrast. 4

5. Opening New Frontiers with the GMT, Seoul, October 4 2010 AO capabilities The GMT facility adaptive optics systems is an integral component of the telescope. The design supports multiple modes of operation: – Natural Guide Star AO (NGSAO) – Laser Tomography AO (LTAO) – Ground Layer AO (GLAO) – Extreme AO (ExAO) – Multi-conjugate AO (MCAO) 5 These modes all facilitated by an adaptive secondary mirror Thermal IR enhanced by an adaptive secondary mirror, including chopping These modes all facilitated by an adaptive secondary mirror

6. Opening New Frontiers with the GMT, Seoul, October 4 2010 Present state of adaptive secondaries 6 TelescopeDiameter (m)ActuatorsStatus MMT0.64336Operational (22% of scheduled observing time since Jan 2010) LBT 10.91672Running on sky LBT 20.91672Complete; systems testing underway Magellan0.85585Under construction VLT1.161170Under construction GMT7 x 1.067 x 672 (TBC)Planned TelescopeDiameter (m)ActuatorsStatus MMT0.64336Operational (22% of scheduled observing time since Jan 2010) LBT 10.91672Running on sky LBT 20.91672Complete; systems testing underway Magellan0.85585Under construction VLT1.161170Under construction

7. Opening New Frontiers with the GMT, Seoul, October 4 2010 The shape of the GMT ASM GMT’s adaptive secondary is segmented in the same way as the primary. ASM segments are sized to match the 8.4 m M1 segments. Minimal superstructure between the segments helps to reduce thermal background. 7

8. Opening New Frontiers with the GMT, Seoul, October 4 2010 Anatomy of the GMT ASM 8 Telescope structure Hexapod legs Electronics crates Dust shroud Cold plate Light-weighted aspheric reference body

9. Opening New Frontiers with the GMT, Seoul, October 4 2010 The LBT’s ASM number 1 ASM #1 installed (with its cover) on the SX side of LBT 9 Cold plate Reference body

10. Opening New Frontiers with the GMT, Seoul, October 4 2010 Pyramid wavefront sensor with 30x30 subapertures 1 kHz update rate 400 corrected modes 10 LBT secondary #1 saw first light on May 25

11. Opening New Frontiers with the GMT, Seoul, October 4 2010 LBT AO first light Seeing of 0.6”-1.5” in H band Achieved Strehl ratios > 80% in H band (120 nm rms wavefront error). These Strehl ratios are among the best ever seen at a telescope of 8-10 m. 11 Not bad for first light!!

12. Opening New Frontiers with the GMT, Seoul, October 4 2010 Triple star in H band 12 Credit for this work and the next few slides goes to Simone Esposito and his team at Arcetri Observatory Without correction With correction Separation = 0.16” Triple star = 1.6  m

13. Opening New Frontiers with the GMT, Seoul, October 4 2010 M92 in H band 13 HST WFC3 20 minute exposure LBT with AO 10 minute exposure ~15”

14. Opening New Frontiers with the GMT, Seoul, October 4 2010 The textbook AO point-spread function 14 Star:HD175658AO correction speed:1 kHz R magnitude:6.5# of corrected modes:400 Exposure time:20 sSeeing:0.9” Wavelength:1.6  mImage core width:0.040” Diagonal stripe from diffraction off single secondary support arm 10, count ‘em, 10 Airy rings Outer radius of correction imposed by band-limited wavefront compensation Alternating rings lighter/darker caused by central obstruction of aperture Strehl ratio = 80% No deconvolution, no shift-and-add, no trickery

15. Opening New Frontiers with the GMT, Seoul, October 4 2010 Application of an adaptive secondary mirror to ground-layer AO 15

16. Opening New Frontiers with the GMT, Seoul, October 4 2010 The MMT’s multi-laser AO system 16 Laser type2 x doubled YAG (15 W each) Wavelength532 nm Pulse rep rate5.2 kHz Average power30 W Launch telescope locationBehind secondary mirror Number of beacons5, arranged as a regular pentagon Enclosed field of view2 arc minutes Beacon typeRayleigh scattering Range gate20-29 km with dynamic refocusing

17. Opening New Frontiers with the GMT, Seoul, October 4 2010 Closed-loop GLAO operation at the MMT Closed-loop ground-layer mode is now operational – Correction signal is computed from the average of the five beacons – Applied to the MMT’s adaptive secondary, the result is partial seeing compensation over the 2’ field spanned by the beacons Technical details: – Corrections are applied at a rate of 400 Hz – A basis set of 45 disc harmonic modes is used to build the wavefront correction – A single natural star is needed for tip-tilt correction, but can be as faint as V ~ 18 and > 1’ off axis. 17

18. Opening New Frontiers with the GMT, Seoul, October 4 2010 K-band GLAO imaging of M3 Exposure time = 60 s in each case (with and without correction) Hart et al., Nature, 466, 727, 2010. 18 Central and edge sub-fields Uncorrected full fieldUncorrectedGLAO corrected

19. Opening New Frontiers with the GMT, Seoul, October 4 2010 GLAO performance on MMT 19 Seeing = 0.61” in K band Mean corrected FWHM = 0.22” Corrected FWHM uniformity: 0.015” rms K band

20. Opening New Frontiers with the GMT, Seoul, October 4 2010 Decadal Survey The astro2010 report lists a Giant Segmented Mirror Telescope as the third priority for ground-based astronomy – Risks seen in timescale for completion, and technology development The report also highlights AO as a key technology for further investment – Seen as fundamental to the success of GSMT The ongoing work to demonstrate the performance of AO modes enabled by adaptive secondaries is important to encouraging the US National Science Foundation to make an investment in GMT 20

21. Opening New Frontiers with the GMT, Seoul, October 4 2010 ASM support of GMT adaptive optics Natural Guide Star and Laser Tomography AO – Thermal IR science requires high-order correction because of GMT’s large size – ASM delivers it with the cleanest possible thermal background, and also supports chopping Ground-layer AO – Would be very challenging without an adaptive secondary because of the large A  that can be exploited In both these areas, GMT can outcompete the larger ELTs. 21

22. Opening New Frontiers with the GMT, Seoul, October 4 2010 Summary Adaptive secondary technology has reached a level of maturity sufficient to be deployed in routine daily operation at the world’s largest telescope. Ground-layer AO with an adaptive secondary is now a demonstrated image sharpening technique with enormously broad application. A second generation ASM at the LBT is producing higher quality imaging than any other astronomical AO system. 22 GMT, through ground-layer and thermal IR AO enabled by its adaptive secondary, will offer multiplexing and sensitivity superior to any other telescope, present or planned.

23. Opening New Frontiers with the GMT, Seoul, October 4 2010 7 segments on discrete hexapods shell reference body electromagnets cold plate Permanent magnets Central flexure Zerodur shell, 1.7 mm thick capacitive position sensors Anatomy of an Adaptive Secondary Mirror Projected actuator spacing is 23 cm Settling time is 0.5 ms Edge sensors and hexapods provide inter-segment control Leverages development of ASM's for MMT/LBT/VLT 23

24. Opening New Frontiers with the GMT, Seoul, October 4 2010 Model of the GMT on-axis reference body 24

25. Opening New Frontiers with the GMT, Seoul, October 4 2010 GLAO performance on MMT FWHM averaged over the 2’ field: – J = 0.29” – H = 0.29” – K = 0.22” 25

26. Opening New Frontiers with the GMT, Seoul, October 4 2010 Extended GLAO performance The value of GLAO extends to shorter wavelengths. Encircled energy improvement still to be had in the visible. If you can have better seeing, why wouldn’t you???? 26 Andersen et al. (2006) study of GLAO on Gemini Encircled energy (%) GLAO must not cost more in observing efficiency than it delivers.

27. Opening New Frontiers with the GMT, Seoul, October 4 2010 Laser Guide Star Facility Provides a general purpose artificial beacon for LTAO and GLAO Six beacon geometry uses GMT pupil to minimize fratricide Variable radius from 35” (LTAO) to 4' (GLAO) Fratricide from other beams when looking at the top beacon. The affected areas of the pupil are shown as lines with the offending beacon’s color. Simulated Shack-Hartmann WFS affected by fratricide 27