1

On-sky validation of LIFT on GeMS

C. Plantet1, S. Meimon1, J.-M. Conan1, B. Neichel2, T. Fusco1

1: ONERA, the French Aerospace Lab, Chatillon, France2: Gemini Southern Observatory, La Serena, Chile

Strong collaboration with B. Neichel from Gemini South

2

Outline

I. Context and description of LIFT

II. Validation at Gemini South

III.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

Find a NGS-WFS solution to increase sky coverage

Goal: 1 to 2 magnitude gain

Sodium layer

Laser guide star

Turbulent layer

Laser

Telescope

LIFT : LInearized Focal-plane Technique

• Maximum Likelihood estimation

Focus = 0

Focus < 0

Focus > 0

LIFT: noise effective low order WFS

Optimized SNR• Full aperture diffraction-limited

focal-plane sensor

• Astigmatism offset: removes the focus sign ambiguity

• Linearization Direct LO linear estimation (≡ WCoG) Fast

Simple, fast, full aperture gain

Simple!

[Meimon10, Opt. Lett.]

5

Noise propagation comparison with the SH 2x2 and the pyramid

• Comparison of noise sensitivity for the estimation of tip/tilt and focus:

CoefficientSH 2x2 (WCoG)

LIFT Pyramid

Photon noise (α)

8.19 1.71 2.3

Read-out noise (β)

334 87 62[Plantet13, Opt. Exp.

(accepted)]

• Variance of estimation error in a WFS:

• 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

[Plantet12, SPIE], [Plantet13, Opt. Exp. (accepted)]

Source

Tilted glass plate for

astigmatism

Camera

Collimation and

refocusing

LIFT features

7

• 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!

Validation at Gemini South

What do we do?

9

• Goal: Prove linearity of estimation on a star retrieve same graph as in simulations:

10

Optical system: Canopus

DMs

TTM

Toward IR science camera

Calibration Sources

LGSWFS

NGSWFS

Telescope light

GSAOI

[Bec08, SPIE]gemini.edu

11

Data acquisition

Z4 offsetson ref slopes

3

Input Output

+ 0.5 rad Z5 offset

4

1

Close AO loop

 GSAOI: 2x2 Rockwell HAWAII-2RG 2048 x 2048

 Fast On Detector Guide Window to select

star

[McGregor04, SPIE] LIFT

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

Linearity and robustness confirmed!

Data Model

Perspectives

16

• 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?

Extension to the ELT context

LIFT for LTAO: efficient low order WFS

18

• 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!