Adaptive Optics Nicholas Devaney GTC project, Instituto de
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Transcript Adaptive Optics Nicholas Devaney GTC project, Instituto de
Adaptive Optics
Nicholas Devaney
GTC 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 telescopes
System
KECK AO
Telescope
Focus
Keck
Installation
Input
beam
Output
beam
Nº of
actuators
in the
DM
WFS
February
1999
f/15
f/15
349
Nasmyth
ShackHartmann
Cassegrain
1999
f/16
f/26
36
Curvature
SYSTEM
Hokupa´a
Gemini N.
Subaru
AO
system
Subaru
Cassegrain
2000
NAOS
VLT-UT3
Nasmyth
Altair
Gemini N.
MACAO
LBT AO
Guide
star
NGS
LGS in
2001
I/F with the
instruments
Instruments
IR
transmissive
dichroic
Ambient
temperature.
Beamspliter.
KCAM,
NIRSPEC,
NIRC2
QUIRC
NGS
f/12.4
f/12.4
36
Curvature
NGS
LGS is
planned
Beamspliter
(inside the
instrument).
CIAO, IRCS
mid 2001
f/15
f/15
250
2001
f/16
f/16
177
NGS in
2001
LGS in
2003
NGS and
LGS
VLT-UT3
Cassegrain
2002
f/13.4
f/17
64
IR reflective
dichroic
Ambient
temperature.
IR reflective
dichroic
Ambient
temperature.
Dichroic.
Ambient
temperature.
CONICA
Cassegrain
ShackHartmann
14x14
subaps
ShackHartmann
12x12
subaps
Curvature
Large
Binocular
Telescope
Any focus
having
WFS
Late 2002
650
ShackHartmann
NGS and
LGS
NGS and
LGS
NIRI,
GNIRS,
GMOS, NIFS
SPIFFI
System
MMT AO
Telescope
Converted
Multi Mirror
Telescope
(6.5m)
ChAOS
Focus
Installation
Nº of
actuators
in the
DM
Any focus
equipped
with a WFS
2002
336
(Adaptive
Secondary
)
Folded
1995
Apache
Cassegrain
Point (3.5m)
NAOMI
William
Herschel
Telescope
(4.2m)
Nasmyth
2000
LICK AO
Shane
Telescope
(3m)
Cassegrain
1996
PALAO
Hale
Telescope
(5m)
Cassegrain
1998
ADONIS
ESO 3.6m
Telescope
Cassegrain
1993
Nasmyth
1999
Cassegrain
1996
AdOpt@T Telescopio
Nazionale
NG
Galileo
(3.6m)
ALFA
Calar Alto
3.5m
Telescope
WFS
ShackHartmann
12 x 12
subapertur
es
201
Shack(Continuo Hartmann
us
16 x 16
facesheet subapertur
PZT
es
actuators)
228
Shack(Segment Hartmann
ed DM 8 x 8 / 4 x
with 76 4
segments) subapertur
es
127
Shack(Continuo Hartmann
us
37
facesheet subapertur
PMN
es on
actuators) pupil
349
Shack(Xinetics Hartmann
continuou 16 x 16
s
subapertur
facesheet es
DM,
PMN
actuators)
64
ShackHartmann
97
Guide
star
Instruments
NGC
(Sodium
LGS
planned)
NGC and
Sodium
LGC
ChAOS
CAM
NGS
(LGS
planned)
INGRID,
OASIS
NGC and
Sodium
LGS
NICMOS III
NGS
(LGS
planned)
PHARO
NGS
SHARPII+,
COMIC
ShackNGS
Hartmann
8x8/4x
4
subapertur
es &
pyramidic
WFS
96
ShackNGC and
(Xinetics Hartmann Sodium
continuou
LGS
s
facesheet
DM)
NICS, OIG
MAGIC,
CHARM,
OMEGACASS
AO Systems on
3-8m 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:
S comp
5
5
3
D
exp 0.3 N 6
r0
S uncomp
G
r
0
D
2
S comp
Suncomp
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 Gmax 1.6Neff
compensation efficiency,
q
N eff
N
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
Publications
40
30
20
10
2002
2001
2000
1999
1998
1997
1996
1995
1994
1993
1992
1991
0
year
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
d
r0
2
5
3
In order to keep fitting error constant
ndof D
2
The number of pixels in the wavefront sensor
will also scale as D2
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-planets
XAO
• 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