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, 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, 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, May 2013, Firenze, Italy

The correction modal base Initial 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, May 2013, Firenze, Italy

Closed loop strategy I Use a broad band sensing channel at 750nm with up to 4632 modes. AO4ELT3, May 2013, Firenze, Italy Close the loop with the modal matrix of 4631 modes.

Segments differential pistons Parameters Value # of modes: 4631 Sampling frequency: 1 kHz Tip-tilt modulation: 5λ/D Integrators gain: 0.6 photons/SA/frame: RON1 e - RMS/frame: AO4ELT3, May 2013, Firenze, Italy

Closed loop strategy II: the second wavelength channel 750nm 850nm 1λ2λ3λ4λ 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, 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, May 2013, Firenze, Italy

2-lambda correction: an example. AO4ELT3, 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 11 AO4ELT3, May 2013, Firenze, Italy

The NGWS optical design AO4ELT3, 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 / 2 nd WL arm: -Glass pyramid -Pupil reimaging lens 92x92 subs TV arm: -beam compressing lens (20 FoV) -Pupil reimaging lens 92x92 subs AO4ELT3, May 2013, Firenze, Italy

Pupil arrangement and AO CCD AO4ELT3, 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, 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: 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, May 2013, Firenze, Italy

System performance: SRs at K band Seeing 0.63 Mag. 8Seeing 1.0 Mag. 8Seeing 0.63 Mag. 12 SR (K band) [%] 95.6± ± ±3.58 Diff. Pist. RMS [nm] 15.3± ± ±25.6 Gain / / 0.4 / 0.2 AO4ELT3, May 2013, Firenze, Italy Conditions: V=8.0, G2V star 120s integration time Seeing =0.63 Wind speed = 12.5m/s GMT TLR for NGSAO SR = 0.89 in K (0.75 instrument SR)

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, 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, May 2013, Firenze, Italy

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