Opening New Frontiers with the GMT – Seoul, October 4, 2010

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

2. The ELT farm E-ELT • 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. TMT GMT

3. 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 D4 point-source sensitivity game. • What roles can the GMT take that distinguish it scientifically from other telescopes?

4. 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.

5. 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) Thermal IR enhanced by an adaptive secondary mirror, including chopping These modes all facilitated by an adaptive secondary mirror These modes all facilitated by an adaptive secondary mirror

6. Present state of adaptive secondaries

7. 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.

8. Anatomy of the GMT ASM Electronics crates Telescope structure Dust shroud Hexapod legs Cold plate Light-weighted aspheric reference body

9. The LBT’s ASM number 1 ASM #1 installed (with its cover) on the SX side of LBT Cold plate Reference body

10. Pyramid wavefront sensor with 30x30 subapertures • 1 kHz update rate • 400 corrected modes LBT secondary #1 saw first light on May 25

11. 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. Not bad for first light!!

12. Triple star in H band • Credit for this work and the next few slides goes to Simone Esposito and his team at Arcetri Observatory l = 1.6 mm Triple star Separation = 0.16” With correction Without correction

13. M92 in H band ~15” HST WFC3 20 minute exposure LBT with AO 10 minute exposure

14. The textbook AO point-spread function 10, count ‘em, 10 Airy rings Strehl ratio = 80% No deconvolution, no shift-and-add, no trickery Diagonal stripe from diffraction off single secondary support arm Alternating rings lighter/darker caused by central obstruction of aperture Outer radius of correction imposed by band-limited wavefront compensation Star: HD175658 AO correction speed: 1 kHz R magnitude: 6.5 # of corrected modes: 400 Exposure time: 20 s Seeing: 0.9” Wavelength: 1.6 mm Image core width: 0.040”

15. Application of an adaptive secondary mirror to ground-layer AO

16. The MMT’s multi-laser AO system

17. 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.

18. Central and edge sub-fields Uncorrected GLAO corrected Uncorrected full field K-band GLAO imaging of M3 • Exposure time = 60 s in each case (with and without correction) • Hart et al., Nature, 466, 727, 2010.

19. GLAO performance on MMT K band • Seeing = 0.61” in K band • Mean corrected FWHM = 0.22” • Corrected FWHM uniformity: 0.015” rms

20. 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

21. 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 AW that can be exploited • In both these areas, GMT can outcompete the larger ELTs.

22. 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. 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. Anatomy of an Adaptive Secondary Mirror Central flexure • 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 Permanent magnets Zerodur shell, 1.7 mm thick cold plate shell electromagnets reference body capacitive position sensors 7 segments on discrete hexapods

24. Model of the GMT on-axis reference body

25. GLAO performance on MMT • FWHM averaged over the 2’ field: • J = 0.29” • H = 0.29” • K = 0.22”

26. 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???? Encircled energy (%) Andersen et al. (2006) study of GLAO on Gemini GLAO must not cost more in observing efficiency than it delivers.

27. 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