Transcript MCAO for Dummies - Gemini Observatory

MCAO
Principles, Performance and Limitations of
Multi-conjugate Adaptive Optics
F.Rigaut 1, B.Ellerbroek 1 and R.Flicker 2
1Gemini
Observatory
2Lund Observatory
Talk available at www.gemini.edu (adaptive optics documents page)
March 31, 2000
SPIE CONFERENCE 4007, MUNICH
1
A Reality Check
Acquisition Cam:V Band, 30 sec
Seeing limited (0.85 arcsec)
22’’x22’’ subimage
Hokupa’a+Quirc: K’ filter
15mn. FWHM ~ 92 mas
K’lim=24.1, 5 in 1hr
March 31, 2000
M13 with Gemini
SPIE CONFERENCE 4007, MUNICH
2
Tomography
Altitude Layer
(phase aberration =
+)
Ground Layer = Pupil
(phase aberration = O)
Tomography = Stereoscopy
WFS#1
March 31, 2000
SPIE CONFERENCE 4007, MUNICH
WFS#2
3
Multiconjugate AO Set-up
Turb. Layers
#2
#1
Telescope
WFS
DM1
DM2
Atmosphere
UP
March 31, 2000
SPIE CONFERENCE 4007, MUNICH
4
Effectiveness of MCAO
Numerical simulations:
• 5 Natural guide stars
• 5 Wavefront sensors
• 2 mirrors
• 8 turbulence layers
• MK turbulence profile
• Field of view ~ 1.2’
• H band
March 31, 2000
SPIE CONFERENCE 4007, MUNICH
5
MCAO Performance Summary
Early NGS results, MK Profile
No AO
320 stars / K band / 0.7’’ seeing
March 31, 2000
Classical AO
MCAO
1 DM / 1 NGS
2 DMs / 5 NGS
165’’
SPIE CONFERENCE 4007, MUNICH
Stars magnified for clarity
6
Example of MCAO Performance
•
•
•
•
•
•
•
13x13 actuators system
K Band
5 LGSs in X of 1 arcmin on
a side
Cerro Pachon turbulence
profile
200 PDE/sub/ms for
H.Order WFS
Four R=18 TT GS 30” off
axis (MCAO)
One R=18 TT GS on
axis(AO)
March 31, 2000
SPIE CONFERENCE 4007, MUNICH
7
• Sensitivity to noise
is fairly better than with AO
1
Prop noise AO / Prop noise MCAO  sqrt( NGS )
• Predictive algorithms possible ?
March 31, 2000
4
.5
+
+
+
+
+
Profile number
SPIE CONFERENCE 4007, MUNICH
2
+
0
Strehl St. dev across FoV % (+)
• Robustness
Average Strehl (triangles)
Other nice features of MCAO
8
Generalized Fitting
(Finite number of DMs)
Strehl in Fov
AverageFitting/AO
Fitting
Generalized
Geometry of the problem
Geometry of the problem
Highest spatial frequencies
d
projected out of the command
5/3
c(h) = (h)-(h).h
dact
Simulations

0
4
Model
1.75
13
13
.h
/ dact
Altitude [km]
March 31, 2000
< c(h) 2 >
vs .h
0.23(dact /r0)5/3
Error [rd2]  (.h)5/3
SPIE CONFERENCE 4007, MUNICH
9
Generalized Fitting
(Finite number of DMs)
Error [rd2]  (.h)5/3
Design Criteria e.g. Error balanced  hmax(,dact)
DM Spacing = 2 x hmax
dact
FoV [arcmin]
hmax [m]
NDM/GS
0.5
1
3000
3
0.2
1
1200
5-6
0.2
10
120
50
March 31, 2000
SPIE CONFERENCE 4007, MUNICH
10
Generalized Anisoplanatism
(Finite number of Guide Star)
March 31, 2000
SPIE CONFERENCE 4007, MUNICH
11
Generalized Anisoplanatism
(Finite number of Guide Star)
•
•
•
•
Turbulence altitude estimation error
OK toward GS, but error in between GS: Strehl “dips”
Maximum FoV depends upon DM pitch.
Example for 7x7 system
100”
FoV = 70”
March 31, 2000
SPIE CONFERENCE 4007, MUNICH
12
Generalized Anisoplanatism goes
down with increasing apertures
2D info only
3D info
3D info
2D info only
Aperture
March 31, 2000
SPIE CONFERENCE 4007, MUNICH
13
LGS Related Problems: Null modes
• Tilt Anisoplanatism : Low order modes > Tip-Tilt at altitude
 Dynamic Plate Scale changes
• Within these modes, 5 “Null Modes” not seen by LGS (Tilt
indetermination problem)
 Need 3 well spread NGSs to control these modes
• Detailed Sky Coverage calculations (null modes modal control,
stellar statistics) lead to approximately 15% at GP and 80% at
b=30o
March 31, 2000
SPIE CONFERENCE 4007, MUNICH
14
On MCAO for ELTs
• Generalized Fitting  FoV
irregardless of D
5/3
for a fixed DM configuration,
NGSs or LGSs ?
• NGSs -> FoV of 15-20 arcmin to get S.C > 50% with 4 stars
• Gen.Fitting error blows up the error budget, unless many DMs
are used
• Many DMs mean many GS -> 20 arcmin not enough
-> NGS do not work for ELTs. Need LGS.
March 31, 2000
SPIE CONFERENCE 4007, MUNICH
16
Available on Earth
sooner than you
think ?
March 31, 2000
SPIE CONFERENCE 4007, MUNICH
17