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The GMT NGS WFS designPresented by S. Esposito
The teamL. FiniG. AgapitoL. Carbonaro A. PuglisiL. BusoniV. Biliotti A. Riccardi S. EspositoE. PinnaR. Briguglio F. Quiros M. Xompero C. ArcidiaconoA. BouchezG. TranchoF. Santoro
AO4ELT3, 26-31 May 2013, Firenze, Italy
Talk overview
The GMT NGWS, the system sensing concept & correction strategy
The WFS system optomechanical design
The NGS AO system performance simulation
AO4ELT3, 26-31 May 2013, Firenze, Italy
S_750nm
S_850nm
KL continuous aberrations
Segments differential piston
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 baseInitial test with a re-ortogonalized KL base, used up to 2500 modes (SPIE2012).
SVD modes of a large sample of statistical phase perturbations fitted using used 4632 actuators. This base contains the atmospheric modes and the pistons.
Actuator Influence functions based on FEA of Adsec thin shells
AO4ELT3, 26-31 May 2013, Firenze, Italy
Closed loop strategy IUse a broad band sensing channel at 750nm with up to 4632 modes.
AO4ELT3, 26-31 May 2013, Firenze, Italy
Close the loop with the modal matrix of 4631 modes.
Segments differential pistonsParameters Value# of modes: 4631Sampling frequency: 1 kHzTip-tilt modulation: 5λ/DIntegrator’s gain: 0.6 photons/SA/frame: 482122RON1 e- RMS/frame:
AO4ELT3, 26-31 May 2013, Firenze, Italy
Closed loop strategy II: the second wavelength channel
750nm
850nm
1λ 2λ 3λ 4λ
The second wavelength is used to establish the sign of the differential piston. A correction of +/- 1 is applied accordingly. Correction is iterated.
-APE/VLT multi-lambda on sky test-Test of GMT segment geometry/mask at ESO/APE bench
AO4ELT3, 26-31 May 2013, Firenze, Italy
Closed loop strategy III: the overall control strategy
Note. The two loops has different integration time: -1ms for broadband AO channel -150ms for segments differential pistons
S_750nm
S_850nm
KL continuous aberrations
Segments differential piston
AO4ELT3, 26-31 May 2013, Firenze, Italy
2-lambda correction: an example.
AO4ELT3, 26-31 May 2013, Firenze, Italy
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
GMT AO overview
11AO4ELT3, 26-31 May 2013, Firenze, Italy
The NGWS optical design
Instrument window
LTAO dichroic
Tele-corrector
GMT f/8focal plane
NGW board
AO4ELT3, 26-31 May 2013, Firenze, Italy
NGWS optical design: detailsThe 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
Instrument Flange & entrance window
Light after M3 reflection
WFS board mounted on translation stages to patrol a FoV of 1.5 arcmin radius
AO4ELT3, 26-31 May 2013, Firenze, Italy
Calibration strategy with RRAdsec and NGAO system can be calibrated in daytime using an approach similar to the LBT one.
Ellipsoidal mirrorMechanical diameter: 100mmVertex thickness: 17mmMaterial: ZerodurRadius of curvature: 121.6mmConic constant: -0.467106Short focus distance from vertex: 72.2mm; long focus: 384.2mmClear aperture: 96mmOn-axis beam footprint diameter: 83.8mmF/# (considering clear aperture): F/0.71F/# (considering on-axis beam footprint): F/0.79
AO4ELT3, 26-31 May 2013, Firenze, Italy
System performance: SRs at K band
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
Conditions: V=8.0 , G2V star120s integration timeSeeing =0.63”Wind speed = 12.5m/s
GMT TLR for NGSAOSR = 0.89 in K (0.75 instrument SR)
System Performance: PSFs for 0.63 arcsec
Seeing: 0.63Mag: 8.0, Frame rate: 1kHzMod: 3lambda/DSR K band: 95.6
AO4ELT3, 26-31 May 2013, Firenze, Italy
ConclusionsWe 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 !!
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