The GMT NGS
WFS design
Presented by S. Esposito
The team
L. Fini
G. Agapito
L. Carbonaro
A. Puglisi
L. Busoni
V. Biliotti
A. Riccardi
S. Esposito
E. Pinna
R. Briguglio
F. Quiros
M. Xompero
C. Arcidiacono
A. Bouchez
G. Trancho
F. Santoro
AO4ELT3, 26-31 May 2013, Firenze, Italy
Talk overview
The GMT NGWS, the system
sensing concept & correction
strategy
S_750nm
KL continuous aberrations
S_850nm
Segments differential piston
The WFS system
optomechanical design
The NGS AO system performance
simulation
AO4ELT3, 26-31 May 2013, Firenze, Italy
Key elements of NGSAO of GMT
(1) Adaptive
secondary mirror
of 7 units with
672 acts. Each.
(2) A pyramid WFS
measuring over 4600
continuous modes and
segments differential
piston.
signals for λ/10
AO4ELT3, 26-31 May 2013, Firenze, Italy
The correction modal base
Initial test with a re-ortogonalized KL base, used up to 2500
modes (SPIE2012).
Actuator Influence functions based on FEA of Adsec thin shells
SVD modes of a large sample of statistical phase perturbations
fitted using used 4632 actuators. This base contains the
atmospheric modes and the pistons.
AO4ELT3, 26-31 May 2013, Firenze, Italy
Closed loop strategy I
Use a broad band sensing channel at 750nm with up to 4632
modes.
Close the loop with the
modal matrix of 4631
modes.
AO4ELT3, 26-31 May 2013, Firenze, Italy
Segments differential pistons
Parameters Value
# of modes: 4631
Sampling frequency: 1 kHz
Tip-tilt modulation: 5λ/D
Integrator’s gain: 0.6
photons/SA/frame: 482122
RON1 e- RMS/frame:
AO4ELT3, 26-31 May 2013, Firenze, Italy
Closed loop strategy II: the second
wavelength channel
1λ
850nm
750nm
2λ
3λ
4λ
-APE/VLT multi-lambda on sky test
-Test of GMT segment geometry/mask at
ESO/APE bench
The second wavelength is used to establish the sign of the differential piston.
A correction of +/- 1 is applied accordingly. Correction is iterated.
AO4ELT3, 26-31 May 2013, Firenze, Italy
Closed loop strategy III: the overall
control strategy
S_750nm
KL continuous aberrations
S_850nm
Segments differential piston
Note. The two loops has different integration time:
-1ms for broadband AO channel
-150ms for segments differential pistons
AO4ELT3, 26-31 May 2013, Firenze, Italy
2-lambda correction: an example.
Develop a WFS design accomodating for two channels:
(a) broadband one around 750nm for fast WFS
(b) Narrow band channel around 850nm for segments
differential piston
AO4ELT3, 26-31 May 2013, Firenze, Italy
GMT AO overview
11
AO4ELT3, 26-31 May 2013, Firenze, Italy
The NGWS optical design
GMT f/8
focal plane
LTAO dichroic
NGW board
Instrument window
Tele-corrector
AO4ELT3, 26-31 May 2013, Firenze, Italy
NGWS optical design: details
The optical design consists
of 4 main sections:
Main arm:
-ADC corrector
-modulation mirror
-pupil derotation k-mirror
AO arm / 2nd WL arm:
-Glass pyramid
-Pupil reimaging lens 92x92 subs
TV arm:
-beam compressing lens (20” FoV)
-Pupil reimaging lens 92x92 subs
AO4ELT3, 26-31 May 2013, Firenze, Italy
Pupil arrangement and AO CCD
AO4ELT3, 26-31 May 2013, Firenze, Italy
The mechanical layout
WFS board mounted on translation
stages to patrol a FoV of 1.5 arcmin
radius
Instrument Flange &
entrance window
Light after M3
reflection
AO4ELT3, 26-31 May 2013, Firenze, Italy
Calibration strategy with RR
Adsec and NGAO system can be calibrated in daytime using an approach
similar to the LBT one.
Ellipsoidal mirror
Mechanical diameter: 100mm
Vertex thickness: 17mm
Material: Zerodur
Radius of curvature: 121.6mm
Conic constant: -0.467106
Short focus distance from vertex: 72.2mm; long
focus: 384.2mm
Clear aperture: 96mm
On-axis beam footprint diameter: 83.8mm
F/# (considering clear aperture): F/0.71
F/# (considering on-axis beam footprint): F/0.79
AO4ELT3, 26-31 May 2013, Firenze, Italy
System performance: SRs at K band
GMT TLR for NGSAO
SR = 0.89 in K
(0.75 instrument SR)
Conditions:
V=8.0 , G2V star
120s integration time
Seeing =0.63”
Wind speed = 12.5m/s
Seeing 0.63’’ Mag. 8 Seeing 1.0’’ Mag. 8 Seeing 0.63’’ Mag. 12
SR (K band) [%]
95.6±0.0675
88.5±2.5
90.3±3.58
Diff. Pist. RMS [nm]
15.3±0.835
30.0±18.5
49.7±25.6
Gain
0.6
0.8 / 0.6
0.6 / 0.4 / 0.2
AO4ELT3, 26-31 May 2013, Firenze, Italy
System Performance:
PSFs for 0.63 arcsec
Seeing: 0.63
Mag: 8.0,
Frame rate: 1kHz
Mod: 3lambda/D
SR K band: 95.6
AO4ELT3, 26-31 May 2013, Firenze, Italy
Conclusions
We identified a WFS strategy able to control atmospheric
aberrations including segments differential pistons using an
additional sensing channel aimed to detect ambiguities in
segment differential pistons.
We completed an opto-mechanical design implementing this
two channel WFS that fit in the allocated volume.
End to End simulation of the NGAO system using 4631 modes has
been performed showing 95% SR in K band for a 0.63 seeing and
an 8.0 mag star, so fulfilling the GMT performance requirement of
SR>0.89.
AO4ELT3, 26-31 May 2013, Firenze, Italy
Thanks for your attention !!