opening new frontiers with the gmt – seoul, october 4, 2010

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 HartPhil Hinz, Antonin Bouchez

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.

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GMT TMT E-ELT


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?

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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.

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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)

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These modes all facilitated by an adaptive secondary mirror

Thermal IR enhanced by an adaptive secondary mirror, including chopping


Present state of adaptive secondaries

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Telescope Diameter (m) Actuators Status

MMT 0.64 336 Operational (22% of scheduled observing time since Jan 2010)

LBT 1 0.91 672 Running on sky

LBT 2 0.91 672 Complete; systems testing underway

Magellan 0.85 585 Under construction

VLT 1.16 1170 Under construction

GMT 7 x 1.06 7 x 672 (TBC) Planned

Telescope Diameter (m) Actuators Status

MMT 0.64 336 Operational (22% of scheduled observing time since Jan 2010)

LBT 1 0.91 672 Running on sky

LBT 2 0.91 672 Complete; systems testing underway

Magellan 0.85 585 Under construction

VLT 1.16 1170 Under construction


The shape of the GMT ASMGMT’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.

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Anatomy of the GMT ASM

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Telescope structure

Hexapod legs

Electronics crates

Dust shroud

Cold plate

Light-weighted aspheric reference body


The LBT’s ASM number 1

ASM #1 installed (with its cover) on the SX side of LBT

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Cold plate

Reference body


• Pyramid wavefront sensor with 30x30 subapertures• 1 kHz update rate• 400 corrected modes

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LBT secondary #1 saw first light on May 25


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.

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Not bad for first light!!


Triple star in H band

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• 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


M92 in H band

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HST WFC320 minute exposure

LBT with AO10 minute exposure

~15”


The textbook AO point-spread function

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Star: HD175658 AO correction speed: 1 kHz

R magnitude: 6.5 # of corrected modes: 400Exposure time: 20 s Seeing: 0.9”Wavelength: 1.6 m Image core width:

0.040”

Diagonal stripe from diffraction offsingle 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


Application of an adaptive secondary mirror to ground-layer AO

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The MMT’s multi-laser AO system

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Laser type 2 x doubled YAG (15 W each)

Wavelength 532 nm

Pulse rep rate 5.2 kHz

Average power 30 W

Launch telescope location Behind secondary mirror

Number of beacons 5, arranged as a regular pentagon

Enclosed field of view 2 arc minutes

Beacon type Rayleigh scattering

Range gate 20-29 km with dynamic refocusing


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.

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K-band GLAO imaging of M3

• Exposure time = 60 s in each case (with and without correction)• Hart et al., Nature, 466, 727, 2010.

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Central and edge sub-fieldsUncorrected full field Uncorrected GLAO corrected


GLAO performance on MMT

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• Seeing = 0.61” in K band• Mean corrected FWHM = 0.22”• Corrected FWHM uniformity:

0.015” rms

K band


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

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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.

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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.

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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.

Opening New Frontiers with the GMT, Seoul, October 4 2010 7 segments on discrete hexapods

shellreference

body

electromagnets

cold plate

Permanent magnets

Central flexure

Zerodur shell, 1.7 mm thick

capacitiveposition 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

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Model of the GMT on-axis reference body

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GLAO performance on MMT

• FWHM averaged over the 2’ field:– J = 0.29”– H = 0.29”– K = 0.22”

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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????

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Andersen et al. (2006) study of GLAO on Gemini

GLAO must not cost more in observing efficiency than it delivers.


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 beamswhen looking at the top beacon.

The affected areas of the pupil are shownas lines with the offending beacon’s color.

Simulated Shack-Hartmann WFS affectedby fratricide

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opening new frontiers with the gmt – seoul, october 4, 2010

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