adaptive optics nicholas devaney gtc project, instituto de astrofisica de canarias 1. principles 2....
TRANSCRIPT
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