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 !!