Calibration of residual aberrations
 in coronagraphic instruments with ZELDA:
 validation in VLT/SPHERE

Calibration of residual aberrations


in coronagraphic instruments with ZELDA:


validation in VLT/SPHERE

Arthur Vigan (LAM), Mamadou N'Diaye (Observatoire de la Côte d’Azur)

K. Dohlen, A. Caillat, A. Costille, P. Blanchard, J. Le Merrer,
 D. Le Mignant, F. Madec, G. Moreaux (LAM)


J.-F. Sauvage, T. Fusco (ONERA), J. H. Girard, G. Zins (ESO)
 J.-L. Beuzit, D. Mouillet, A. Carlotti (IPAG)

P. Janin-Potiron, P. Martinez, M. Carbillet (Observatoire de la Côte d’Azur) AO4ELT5

Tenerife - 28/06/2017

2

Context

• Direct imaging and spectroscopy of exoplanets

‣ VLT/SPHERE, Gemini/GPI, Subaru/SCExAO, etc

‣ disks, warm or massive gas giant planets

‣ high contrast (Δmag>10) at small separations (0.1”-0.5")

• Instrument limitations

‣ quasi-static aberrations

‣ temporal stability

• Need of a clean PSF for optimal starlight rejection

‣ Calibration of pre-coronagraph aberrations

b c

d e HR8799 VLT/SPHERE

51 Eri b

T0 T1 T0 - T1

GJ 758b

VLT/SPHERE Macintosh et al. 2015, Zurlo et al. 2016, Vigan et al. 2016

3

Direct imaging of colder or lighter exoplanets

• Residual aberrations:

‣ How to calibrate them?

‣ Their origin?

‣ Their temporal evolution?

• Our solution:

‣ Zernike wavefront sensor

‣ N’Diaye et al. A&A 2013, 2016

4

Zernike wavefront sensor

• Conversion of the phase aberrations into intensity variations

‣ Ic=α sin φ + β

‣ Small aberrations: Ic =αφ + β

A B C

Focal plane mask (FPM) π/2 phase shift

Entrance pupil Camera

N’Diaye et al. 2013

5

Linearity range of the sensor

• Linearisation of the amplitude ➙ expression valid only near zero phase error

• Limited capture range: -0.14 λ

➙ 0.36 λ

• Possible extension of the capture range in closed loop

-0.2 -0.1 0.0 0.1 0.2 0.3 0.4 Wavefront error in λ₀ unit

0 1 2 3

Normalized exit pupil intensity

Dynamic range for b=0.5 Negative part

-0.140λ₀

Positive part 0.360λ₀

-400 -200 WFE in nm at λ0 200₀=1.642μm400 600

ZELDA intensity signal Linear reconstruction 2nd-order reconstruction

Closed-loop phasing

-3λ/2 -λ/2 -λ/4 0 λ/4 λ/2 3λ/2

Edge piston (λ) -1.0

-0.5 0.0 0.5 1.0

Normalized signal

0 1 2

3

3 2 1

Corrections

After correction 3 After correction 2 After correction 1 Initial position

Vigan et al. 2011, N’Diaye et al. 2013

6

Implementation in VLT/SPHERE

ZELDA

Zernike sensor for Extremely accurate measurements of Low-level Differential Aberrations

Deformable mirror

Stellar

coronagraph Image post processing

Image formation of an exoplanet Wavefront

sensor Real time

computer

Extreme adaptive optics (ExAO)

Measurement

Science

Non common path aberrations Telescope

Speckles ZELDA Optics

Optics

• Our proposal:

‣ ZELDA a concept based on phase-contrast technique

•

‣ VLT/SPHERE: off-line phase diversity

‣ GPI: Mach-Zehnder interferometer behind coronagraph

7

Current implementation in SPHERE

ZELDA

Zernike sensor for Extremely accurate measurements of Low-level Differential Aberrations

CPI

NIR corono / ZELDA

IRDIS

IFS

ZIMPOL

DTTS

ZELDA

J.-L. Beuzit’s talk
 Thursday morning

8

ZELDA prototype in SPHERE

• Fused silica substrate

• Mask by photolithographic


reactive ion etching (SILIOS, France)

• Within 1% of the specifications

Diameter D=70.7 µm Depth z=0.815 µm

Installation during SPHERE reintegration at Paranal in April 2014 λ=1.62 µm (H-band)

F/40 beam

ZELDA in SPHERE
 coronagraph wheel

Measured mask profile

−15 0 15

0 20 40 60 80 100 120

Lateral position (μm)

−830

−815

−800

Depth (nm)

9

Validating ZELDA in SPHERE

Z1 Z2 Z3 Z4 Z5

Z6 Z7 Z8 Z9 Z10

Zernike modes introduced with 400 nm PV on the DM

• Internal point source

• IRDIS pupil-imaging mode, λ = 1642 nm (Fe II filter)

• PSF centered manually + closed loop on near-IR DTTS

• Zernike and Fourier modes, amplitude ramps: -250 ➙ 600 nm PtV

N’Diaye et al. 2016

10

Quantitative performance assessment

• theory vs. measurements:

‣ excellent agreement!

• low sensitivity to wavelength of measure

-100 -50 0 50 100 150

Measurement in nm rms

5 cycles/pupil, FeII filter λ=1642nm, Δλ=24nm

theory experiment

-100 -50 0 50 100 150

Wavefront error in nm rms -5

0 5

Difference

-100 -50 0 50 100 150

Measurement in nm rms

5 cycles/pupil, H2 filter λ=2124nm, Δλ=31nm

theory experiment

-100 -50 0 50 100 150

Wavefront error in nm rms -5

0 5

Difference

-100 -50 0 50 100 150

Measurement in nm rms

Defocus

theory experiment

-100 -50 0 50 100 150

Wavefront error in nm rms -5

0 5

Difference

nanometer accuracy

Reference wavelength (H-band)

Measurement in K-band

N’Diaye et al. 2016

11

NCPA measurement and compensation

Before correction After correction

Before correction [filtered]

After correction [filtered]

-150 -100 -50 0 50 100 150

Phase errors [nm]

45 nm RMS

30 nm RMS

N’Diaye et al. 2016

12

NCPA measurement and compensation

Before correction After correction

Before correction [filtered]

After correction [filtered]

-150 -100 -50 0 50 100 150

Phase errors [nm]

45 nm RMS

30 nm RMS

35 nm RMS

16 nm RMS

Residual

tip-tilt

!

Tip-tilt:

~12 nm RMS

N’Diaye et al. 2016

13

Impact on coronagraphic images

Before calibration

0.85"

0.20"

Apodised pupil Lyot coronagraph, H-band

N’Diaye et al. 2016

After calibration

14

Impact on coronagraphic images

Apodised pupil Lyot coronagraph, H-band

N’Diaye et al. 2016

0.85"

0.20"

10^-7 10^-6 10^-5 10^-4

Normalized standard deviation

0 5 10Angular separation [λ/D]15 20 25 30

16 14 12 10

Contrast [mag]

Before correction After correction SPHERE APLC (theory)

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Angular separation [as]

1 10

Gain

15

Contrast gain after ZELDA calibration

Before calibration

After calibration

x10 gain

@ 0.2"

perf. limit of SPHERE coronagraph

➙ ZELDA will be used for NCPA calibration in SPHERE this year

N’Diaye et al. 2016

Towards ZELDA on sky

• internal NCPA calibration

• calibrated reference slopes applied on-sky

• on-par with 2015

• 5-10 contrast gain

• no gain in contrast!

• reason unknown:

‣ chromatic beam-shift?

‣ near-IR ADCs?

‣ amplitude aberrations?

New tests in March 2017

Procedure

Internal performance

On-sky performance

16 Vigan et al. in prep

Towards ZELDA on sky

• internal NCPA calibration

• calibrated reference slopes applied on-sky

• on-par with 2015

• 5-10 contrast gain

• no contrast gain yet!

• reason unknown:

‣ chromatic beam-shift?

‣ near-IR ADCs?

‣ amplitude aberrations?

New tests in March 2017

Procedure

Internal performance

On-sky performance

17 Vigan et al. in prep

18

ZELDA in E-ELT/HARMONI high-contrast mode

• Goal:

‣ spectro-imaging of young giants

‣ R=3000-20000 ; 10^-6 contrast at 0.2’’

and closer, in H & K bands

• No ADC in the instrument:

‣ Dispersed beam & PSF

‣ SCAO sensing at 0.8 um & 


science at 1.45-2.45 um:

‣ significant NCPA

• ZELDA @ 1.25 um, prospects:

‣ NCPA calibration: less constraints on surface quality of upstream optics

‣ Pupil centering follow-up (0.5%

accuracy): good for pupil masking

‣ Fine E-ELT cophasing

Courtesy from A. Carlotti (IPAG)

Opto-mechanical design of 
 high-contrast module

• Fine phasing sensor 


in diffraction-limited regime

‣ For each segment,

measurements of piston, tip, tilt

• ZELDA-Phasing sensor

‣ Mode estimation 


with nanometric accuracy

‣ Closed-loop wavefront control 
 for fine segment alignment

‣ promising option 


for fine cophasing of ELTs

19

ZELDA-Phasing Sensor

Janin-Potiron et al. 2017

Closed-loop control of combined piston, tip-tilt ZELDA-PS: principle

91 segment configuration

20

Conclusions

• ZELDA for the calibration of residual aberrations

‣ easy to manufacture

‣ simple alignment

‣ no calibration required

‣ fast and straightforward data analysis

• Validation in VLT/SPHERE

‣ excellent agreement between measurements and theory

‣ NCPA compensation: gain x10 in contrast at 0.2"

‣ implementation in the calibration plan of SPHERE in 2017

• Powerful diagnostic tool for current and future AO facilities

‣ internal and on-sky measurements

‣ several SPHERE examples: low-wind effects, internal turb., derotator behavior

Quasi-static speckles