opening new frontiers with the gmt seoul october 4 2010
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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. The ELT farm. E-ELT. GMT is the smallest of the three planned ELTs.

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opening new frontiers with the gmt seoul october 4 2010
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

the elt farm
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.



importance of ao
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?
unique features of gmt to be exploited
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.
ao capabilities
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

the shape of the gmt asm
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.

anatomy of the gmt asm
Anatomy of the GMT ASM

Electronics crates

Telescope structure

Dust shroud

Hexapod legs

Cold plate

Light-weighted aspheric reference body

the lbt s asm number 1
The LBT’s ASM number 1

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

Cold plate

Reference body

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

LBT secondary #1 saw first light on May 25

lbt ao first light
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!!

triple star in h band
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

m92 in h band
M92 in H band



20 minute exposure

LBT with AO

10 minute exposure

the textbook ao point spread function
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”

closed loop glao operation at the mmt
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.
k band glao imaging of m3
Central and edge sub-fields


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.
glao performance on mmt
GLAO performance on MMT

K band

  • Seeing = 0.61” in K band
  • Mean corrected FWHM = 0.22”
  • Corrected FWHM uniformity: 0.015” rms
decadal survey
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
asm support of gmt adaptive optics
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.
  • 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.

anatomy of an adaptive secondary mirror
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



reference body




7 segments on discrete hexapods

glao performance on mmt1
GLAO performance on MMT
  • FWHM averaged over the 2’ field:
    • J = 0.29”
    • H = 0.29”
    • K = 0.22”
extended glao performance
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.

laser guide star facility
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