Adaptive Optics

Nicholas DevaneyGTC project, Instituto de Astrofisica de Canarias

1. Principles

2. Multi-conjugate

3. Performance & challenges

Overview

• Overview of current AO systems and Instruments

• Measures of performance

• Challenges for current systems

• Challenges for the future

AO systems on 8-10m telescopesSystem Telescope Focus Installation Input

beamOutputbeam

Nº ofactuators

in theDM

WFS Guidestar

I/F with theinstruments

Instruments

KECK AOSYSTEM

KeckNasmyth

February1999

f/15 f/15 349 Shack-Hartmann

NGSLGS in2001

IRtransmissivedichroicAmbienttemperature.

KCAM,NIRSPEC,NIRC2

Hokupa´a Gemini N. Cassegrain 1999 f/16 f/26 36 CurvatureNGS

Beamspliter. QUIRC

SubaruAOsystem

Subaru Cassegrain 2000 f/12.4 f/12.4 36 Curvature NGSLGS isplanned

Beamspliter(inside theinstrument).

CIAO, IRCS

NAOS VLT-UT3 Nasmyth mid 2001 f/15 f/15 250 Shack-Hartmann14x14subaps

NGS in2001LGS in2003

IR reflectivedichroicAmbienttemperature.

CONICA

Altair Gemini N. Cassegrain 2001 f/16 f/16 177 Shack-Hartmann12x12subaps

NGS andLGS

IR reflectivedichroicAmbienttemperature.

NIRI,GNIRS,GMOS, NIFS

MACAO VLT-UT3 Cassegrain 2002 f/13.4 f/17 64 Curvature NGS andLGS

Dichroic.Ambienttemperature.

SPIFFI

LBT AO LargeBinocularTelescope

Any focushavingWFS

Late 2002 650 Shack-Hartmann NGS and

LGS

System Telescope Focus Installation Nº ofactuators

in theDM

WFS Guidestar

Instruments

MMT AO ConvertedMulti MirrorTelescope(6.5m)

Any focusequippedwith a WFS

2002 336(AdaptiveSecondary

)

Shack-Hartmann12 x 12subapertures

NGC(SodiumLGSplanned)

ChAOSApachePoint (3.5m)

FoldedCassegrain

1995 201(Continuo

usfacesheet

PZTactuators)

Shack-Hartmann16 x 16subapertures

NGC andSodiumLGC

ChAOSCAM

NAOMI WilliamHerschelTelescope(4.2m)

Nasmyth 2000 228(Segment

ed DMwith 76

segments)

Shack-Hartmann8 x 8 / 4 x4subapertures

NGS(LGSplanned)

INGRID,OASIS

LICK AO ShaneTelescope(3m)

Cassegrain 1996 127(Continuo

usfacesheet

PMNactuators)

Shack-Hartmann37subapertures onpupil

NGC andSodiumLGS

NICMOS III

PALAO HaleTelescope(5m)

Cassegrain 1998 349(Xinetics

continuous

facesheetDM,PMN

actuators)

Shack-Hartmann16 x 16subapertures

NGS(LGSplanned)

PHARO

ADONIS ESO 3.6mTelescope

Cassegrain 1993 64 Shack-Hartmann

NGS SHARPII+,COMIC

AdOpt@TNG

TelescopioNazionaleGalileo(3.6m)

Nasmyth 1999 97 Shack-Hartmann8 x 8 / 4 x4subapertures &pyramidicWFS

NGS NICS, OIG

ALFA Calar Alto3.5mTelescope

Cassegrain 1996 96(Xinetics

continuous

facesheetDM)

Shack-Hartmann

NGC andSodiumLGS

MAGIC,CHARM,OMEGA-CASS

AO Systems on3-8m Telescopes

Measures of performance

• Image quality– Strehl ratio and fwhm

• Astronomy– Results– Publications– Citations

• Efficiency– Correction achieved vs. Possible– Use of observing time

Wavefront correction Quality

Ref: Rigaut et al. In ‘High-resolution imaging by interferometry’, ESO conf. 1991

Image quality

Ref: Roddier & Rigaut in ‘Adaptive Optics in Astronomy’

Image fwhm

Ref: Roddier & Rigaut in ‘Adaptive Optics in Astronomy’

AO Compensation Efficiency

• Roddier (PASP, 110, 1998) defined compensation efficiency based on the following argument:

Gmax=1.6 N at D/r0 = 2.4 N. At Gmax, S0.3

An AO system with N actuators behaves as an ideal system with Neff actuators

compensation efficiency,

uncomp

comp

uncomp

comp

S

SG

D

rS

Nr

DS

2

0

6

53

5

0

3.0exp

effmax NG 6.1

N

Nq eff

Compensation Efficiency of some systems

Roddier (PASP, 110, 1998)

Images !

University of Hawaii AO http://www.ifa.hawaii.edu/ao/

Faint companion detection

University of Hawaii AO http://www.ifa.hawaii.edu/ao/

Keck I AO

http://www2.keck.hawaii.edu:3636/realpublic/inst/ao/ao.html

Galactic center with Keck AO

Astronomical publications based on AO in refereed journals

0

10

20

30

40

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

year

Pu

bli

ca

tio

ns

http://www2.keck.hawaii.edu:3636/realpublic/inst/ao/ao_sci_list.html

Efficiency

• Marco et al. (PASP, 113, 2001) observing efficiency of ADONIS over 3 years– Efficiency = Science ‘shutter time’/ Available dark time

= 10%-30%

Other instruments = 50%-80%

• Detector readout accounts for 5% of observing time; 60% of observations had exposure time < 5s

• Extra overheads for AO include closing the loop and optimization (typ. 5 minutes), centering coronographic masks.

• Loose time if loop opens during integration.

Challenges

• For Current Systems– Characterise and Improve correction efficiency– Improve Observing efficiency– Improve astronomical productivity

• Prototype development– MCAO for 8-10m

• Future– AO for ELTs

AO Scaling laws

• Recall wavefront fitting error

In order to keep fitting error constant

The number of pixels in the wavefront sensor will also scale as D2

3

5

0

2

rd

2Dndof

AO scaling laws

• In order to maintain bandwidth the pixel readout rate also has to increase as D2.

• Using a full matrix-multiply, the required computing power increases as D4

• Keck AO has 349 actuator; scale to 30m– 3000 actuators– on 128x128 if quad cell (just!)– 1kHz sampling => 16.4 MHz pixel rate– Computing power ~10 Gflop

Scale to OWL

• If we scale the same system to OWL...– 35000 actuators– 512x512 CCD– 1kHz sampling => 262 MHz pixel rate– computing power 103 Gflops !!

• Even given Moores’ law, need to develop sparse matrix techniques

• Note that noise propagation error increases as the ln(ndof) so need brighter guide stars

Ref: Donald Gavel in ‘Beyond conventional Adaptive Optics’ 2001

Scaling issues• Deformable mirrors

– current piezomirros cost 1k$ per actuator– 7mm per actuator => 1.3m DM (ok)– MEMS promising but currently too small. – Stroke scales with D but outer scale will keep it to 5-

10 m

• Laser guide stars– Elongation – Optical errors due to finite distance (P.Dierickx)

• Tolerances !

MCAO on ELTs

• For MCAO need 2-3 Deformable mirrors with similar number of actuators and 2-5 wavefront sensors

• Sky coverage with natural guide stars may be sufficient – 42% at b=50 for multi-fov LO on OWL

(Marchetti et al., Venice 2001)

AO on Euro50

Detection of exo-planetsXAO

• Jupiter-Sun intensity ratio ~ 109

• Need very high order and very fast AO to suppress uncorrected halo.

• Also need correction of scintillation. • Smooth optics • Sandler et al. Claim can detect Jupiter at m~4 stars

with 3.5 hour integration• XAO for OWL will require 100k DM

Other concepts

• Ground-conjugate wide field AO

– 1 DM conjugate to ground

– 10-20´ field of view

– improved fwhm rather than diffraction-limited

• FALCON

– Division of field of view into multiple areas

– WFS/DM ‘buttons’ placed on guide stars around several objects in field

– micro-DMs correct each object (low order correction)

– Used in combination with Integral Field Spectroscopy