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On-sky validation of LIFT on GeMS

This study validates the Linearized Focal-plane Technique (LIFT) on Gemini South telescope, showcasing its efficiency in sensing low-order modes and increasing sky coverage. The results demonstrate the potential of LIFT for implementation in laser-assisted adaptive optics systems.

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On-sky validation of LIFT on GeMS

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  1. On-sky validation of LIFT on GeMS Strong collaboration with B. Neichelfrom Gemini South C. Plantet1, S. Meimon1, J.-M. Conan1, B. Neichel2, T. Fusco1 1: ONERA, the French Aerospace Lab, Chatillon, France 2: Gemini SouthernObservatory, La Serena, Chile

  2. Outline • Context and description of LIFT • Validation at Gemini South • Extension to the ELT context

  3. Context • Laser assisted AO: need to sense low order modes (tip/tilt/focus) on faint natural stars • Classical solution: Shack-Hartmann 2x2  noise and aliasing issues Sodium layer Laser guide star Turbulent layer Laser • Find a NGS-WFS solution to increase sky coverage • Goal: 1 to 2 magnitude gain Telescope

  4. LIFT: noise effective low order WFS LIFT : LInearized Focal-plane Technique Simple! Focus < 0 Focus = 0 • Linearization  Direct LO linear estimation (≡ WCoG)  Fast  Optimized SNR • Full aperture diffraction-limited focal-plane sensor Focus > 0 • Maximum Likelihood estimation • Astigmatism offset: removes the focus sign ambiguity [Meimon10, Opt. Lett.] • Simple, fast, full aperture gain

  5. Noise propagation comparison with the SH 2x2 and the pyramid • Variance of estimation error in a WFS: • Comparison of noise sensitivity for the estimation of tip/tilt and focus: [Plantet13, Opt. Exp. (accepted)] • Pyramid model: no modulation, pupil sampled on 4 pixels, ML estimation • Much more sensitive than a SH 2x2 • Performance comparable to the pyramid • SH and pyramid proven efficient, let’s do it with LIFT!

  6. Laboratory validation • Linearity of estimation for tip/tilt and focus • Estimation in large spectral bandwidth (R=3.4) • Noise propagation confirmed Collimation and refocusing Source Tilted glass plate for astigmatism Camera [Plantet12, SPIE], [Plantet13, Opt. Exp. (accepted)]

  7. LIFT features • Fast and efficient algorithm • Much less sensitive to noise than a SH 2x2, comparable to a pyramid • Easy to setup, move, debug • Validated in lab  Let’s see on the sky!

  8. Validation at Gemini South

  9. What do we do? • Goal: Prove linearity of estimation on a star  retrieve same graph as in simulations:

  10. Optical system: Canopus Calibration Sources Telescope light DMs Toward IR science camera TTM GSAOI NGSWFS LGSWFS gemini.edu [Bec08, SPIE]

  11. Data acquisition Output Input  Fast On Detector Guide Window to select star 1 Close AO loop 3 Z4 offsets on ref slopes  GSAOI: 2x2 Rockwell HAWAII-2RG 2048 x 2048 + 0.5 rad Z5 offset [McGregor04, SPIE] LIFT 4

  12. History • 04/09/12: Linearity test on calibration sources  OK [Plantet12, SPIE]

  13. Weather problems • 3 nights window… 04/26 04/27 04/28

  14. History • 04/09/12: Linearity test on calibration sources  OK • 04/26/12, 04/27/12 and 05/28/12: bad weather • 04/27/12: Focus estimation on calibration sources with simulated residual turbulence  reproductible estimation • End of 2012  new runs beginning • 2 data sets: • On sky data! But low Strehl ratio (10-15% @K) • Data with SR ~ 25% @K, validation is possible! [Plantet12, SPIE]

  15. Validation of linearity on sky Data Model Linearity and robustness confirmed!

  16. Perspectives • More complete tests to come • Close NGS loop with LIFT? • Measurement of more modes @ low frequency: LIFT as a truth sensor (cut Mickey Mouse ears?) We performed LIFT on an operational system. How would it be on an ELT?

  17. Extension to the ELT context

  18. LIFT for LTAO: efficient low order WFS • Simple solution for a natural guide star WFS • Dedicated DM + camera • Benefits from full aperture gain • Easy to setup, move, use, debug • Very reliable • Feature of ATLAS baseline [Fusco10, SPIE]

  19. LIFT for MCAO: use of scientific detector • MCAO systems  wide field of view  chances of having a natural star • A region of interest can easily be extracted from the image • Add an astigmatism 10% (typically) of the time, and use LIFT on an image taken from the detector • Slow correction of focus and low order modes from all possible origins (sodium layer, internal errors…) • Astigmatism respects the symmetry of the PSF • < 1% loss in Strehl Ratio, unnoticeable on long exposure image

  20. Conclusion • First validation on sky! • Seductive perspectives for ELTs • To come : further comparison with other sensors, study of aliasing, tests for implementation in laser assisted systems Poster: SH + LIFT by Serge Meimon

  21. Thank you!

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