Characterisation of a Pyramid WFS: an experimental study

Transcription

1 Characterisatin f a Pyramid WFS: an experimental study Charltte Bnd, C. M. Crreia, J.-F. Sauvage, K. El Hadi, Y. Abautret, B. Neichel, T. Fusc, J.-P. Véran, G. Herrit, D. Andersen. M. Lamb, M. Van Kten. 30 th June 2017, AO4ELT V, Tenerife. 1

2 Cntents Mtivatin and intrductin t the bench. Calibratin using different mdal bases. Investigatins f nn-linear behaviur and impact f ptical gain. Clsed lp perfrmance and agreement with mdels. 2

3 Cut-ff freq. SNR Preparatin fr future AO systems Many future instruments will include a Pyramid WFS - KPIC (IR Pyramid n Keck) - Sphere upgrade - Subaru - E-ELT, TMT, GMT Challenges - Explit full ptential f PWFS - Calibratin - NCPA cmpensatin - Nn-linear behaviur Aims - Test nn-linear behaviur - Optimisatin (gain tracking) - Characterisatin f perfrmance. Advantages ver Shack-Hartmann Ptential fr increased sensitivity within the crrectin band. Less susceptible t aliasing. Flexibility: mdulatin allws fr adjustment f linear range fr different cnditins. α λ Pyramid Spatial freq. [1] C. Verinaud

4 LAM bench: ptical layut Cmplete AO lp (WFS, DM, turbulence, etc.). Bench perated using OOMAO simulatin cde. Defrmable-mirrr: 9x9 Alpa. WFS: 62x62 pixels (versampled). 4

5 Gals Gain experience running AO system with Pyramid WFS. Investigate nn-linear behaviur and measure ptical gain. Tlerance fr NCPA crrectin. Develpment f errr budget: characterisatin under different perating cnditins. 5

6 Calibratin and initial tests 6

7 First Pyramid signals Initial images f Pyramid pupil demnstrate rf-tp effect at lw mdulatin. Preferential distributin f light alng ne diagnal. Perfect Pyramid tip Pyramid tip with rf-tp N mdulatin Mdulatin 2λ/D Mdulatin 5λ/D Rf-tp measurement 7

8 System calibratin: measuring the interactin matrix Interactin matrix taken with 65 Zernike mdes (mdulatin 3 λ/d shwn). 10 nm rms fr each applied mde Pyramid sensitivty varies by mde Mdes weighted t maximise the SNR fr each mde WFS signal rms WFS signal rms WFS signal rms WFS signal rms mde number mde number 8

9 Calibratin with Furier mdes Develpment f spatial frequency dmain mdels fr Furier recnstructin and cmputatin f errr budgets (HARMONI, Keck IR Pyramid etc.).* Crrbratin with experimental results. Pyramid respnse t Furier mdes: mdulatin f 3λ/D. x signals κ y y signals κ y S y κ x κ x * Mdel detailed in pster P3033. Pyramid signal fr individual Furier mde 9

10 Calibratin: cmparisn with mdels Measurement Mdel Pyramid respnse in the Furier dmain (x signals).* Slpe-like respnse at lw spatial frequency. Similar features in mdel and measurement. * See pster P

11 Linearity and wrking ff-null 11

12 Linear range Hw des the Pyramid behave in the nn-linear regime (ptical gain < 1)? The linear range varies depending n the mde and increases with mdulatin. Pyramid measurement [nm] /D 2 /D 3 /D fcus Pyramid measurement [nm] /D 2 6/D 3 6/D 4 6/D simulatin (2 6/D) Mde 23 (n = 6, m = 2) rms f applied aberratin [nm] rms f applied aberratin [nm] Labratry measurement f linearity fr different mdes ver a range f mdulatin. 12

13 Optical gain and the Pyramid WFS Diffractin limited, wrking arund 0 Offset peratin, wrking ff null On sky peratin, residual turbulence ptical gain = 1 ptical gain < 1 ptical gain < 1 Diffractin limited spt n the Pyramid. Calibratin state. Cmpensatin f NCPA requires ffset signals. Large NCPA nnlinear regime. Residual turbulence reduced ptical gain. * Investigatins int gain tracking are nging. See pster P

14 Pyramid peratin with ffsets wave-frnt rms [nm] clsed lp stability limit CL wave-frnt pen lp Pyramid measurement NCPA in the AO system requires the WFS t wrk with an ffset. Tight linear range restricts the NCPA which can be accurately crrected rms f applied ffset [nm] Labratry measurement with mdulatin = 1λ/D and fcus ffset applied. Applying an ffset far frm the linear range can destabilise the clsed lp. 14

15 Tracking the ptical gain at the HIA Gain tracking carried ut n HIA Pyramid bench in Victria, Canada. Knwn signal injected int system via mdulatin mirrr. Nn-cmmn path aberratins: ~80 nm rms (astigmatism + fcus) Measured dither signal Clsed lp ptical gain f 1 when n NCPA crrectin applied. Strehl rati Clsed lp strehl f ~80% due t NCPA Clsed lp iteratin Clsed lp iteratin Measurements f the ptical gain (left) and Strehl (right) as the lp is clsed. 15

16 Cmpensatin f the ptical gain Lp clsed with n NCPA crrectin and n gain cmpensatin NCPA crrectin applied but n gain crrectin NCPA and gain crrectin 80% Strehl due t NCPA NCPA crrectin ver-applied NCPA crrectly cmpensated Strehl rati and ptical gain as the lp is clsed and NCPA and gain crrectin are applied. * See pster P

17 Dynamic perfrmance and cmparisn with mdel 17

18 NCPA estimatin n the bench Phase diversity implemented t estimate NCPA n LAM bench. Reference Strehl imprvement frm 85% t 97%. 5 0 rms amplitude [nm] mde number Estimated NCPA between imaging camera and Pyramid WFS f ~25 nm rms. 18

19 Clsing the lp with turbulence Clsed lp example: - 4 λ/d mdulatin - Turbulence with d/r 0 = 3.2 Lng expsure image 29% Strehl (λ = 660 nm) 19

20 Analysis f clsed lp perfrmance Mdulatin imprves perfrmance (larger linear range). Lp gain ptimised in experiment fr maximum Strehl. Gd agreement with simulatin. 40 Realistic mdel 35 includes: 30 - Rf-tp 25 - NCPA 20 - Mdes prjected n DM. 15 Strehl rati [%] Strehl at WFS wavelength (λ = 660nm) 10 5 experiment perfect simulatin realistic simulatin mdulatin [6/D] 20

21 Cnclusins and utlk Lessns learnt Calibratin: gd agreement with mdels. Pyramid WFS can be tricky in nn-linear regime. Different ptins t crrect fr NCPA. Final bench perfrmance limited at lw mdulatin (NCPA and rf-tp effects). Errr budget well understd. Future tests Furier recnstructin. Mdal gain tracking/ n sky calibratin. Bench available t test new cncepts. 21

22 ensing in the When 2-4 Oct 2017 Where Padva (italy) Web site: ( r just Ggle the title ) era 22

23 Acknwledgements The research leading t these results received the supprt f: - the A*MIDEX prject (n. ANR-11-IDEX ) funded by the Investissements d'avenir French Gvernment prgram, managed by the French Natinal Research Agency (ANR). - French FUI gvernment prgram, French CSAA prgram, French ROP netwrk, Régin PACA, - C-funded by the Peple Prgramme (Marie Curie Actins) f the Eurpean Unin s Seventh Framewrk Prgramme (FP7/ ) under REA grant agreement n. PCOFUND-GA , thrugh the PRESTIGE prgramme crdinated by Campus France. 23

24 The Pyramid wave-frnt sensr Cnsists f a 4-sided prism, re-imaging ptics and a CCD camera. Light fcused nt the pint f the Pyramid and 4 pupil images prjected nt the CCD. Re-imaging lens Advantages ver Shack-Hartmann Ptential fr increased sensitivity within the crrectin band. Less susceptible t aliasing. Flexibility: mdulatin allws fr adjustment f linear range fr different cnditins. 4-sided prism Light fcused nt the tip f the Pyramid 4 images f the pupil prjected nt the CCD 24

25 Cut-ff freq. SNR Pyramid signals 4 pupil images 4 pupils n the CCD camera. Identificatin f equivalent pixels in each image. 2 signal maps S x = I 1 + I 3 I 2 + I 4 Σ S y = I 1 + I 2 I 3 + I 4 Σ Σ is a nrmalisatin term dependent n the ttal light n the detectr. Mdulatin Mdulatin larger spt size. Mdulatin angle α = m λ D Larger linear range vs. reductin in sensitivity at sme spatial frequencies. Pyramid α λ Spatial freq. S x S y Increasing mdulatin 25

26 Clsed lp Clsed lp Initial state Initial state Clsing the lp: n turbulence Mdulatin: 1 λ/d Pyramid pupils Wave-frnt Mdulatin: 3 λ/d Pyramid pupils Wave-frnt Imaging camera Imaging camera Pyramid signals Pyramid signals Pyramid pupils Wave-frnt Pyramid pupils Wave-frnt Pyramid signals Imaging camera Pyramid signals Imaging camera 26

27 Clsing the lp: static distrtins clsed lp rms [nm] mdulatin 0 mdulatin 1 6/D mdulatin 2 6/D A small mdulatin increases the linear range. Mdulating achieves a much quicker crrectin clsed lp iteratin 27