pyramid wavefront sensor performance with laser guide stars

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PYRAMID WAVEFRONT SENSOR PERFORMANCE WITH LASER GUIDE STARS F. Quirós-Pacheco 1 , E. Pinna 1 , A. Puglisi 1 , G. Agapito 1 L Busoni 1 , S. Rabien 2 , S. Esposito 1 1 INAF – Arcetri 2 MPE – Garching

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Pyramid wavefront sensor performance with laser guide stars. F. Quirós-Pacheco 1 , E. Pinna 1 , A. Puglisi 1 , G. Agapito 1 L Busoni 1 , S . Rabien 2 , S . Esposito 1 1 INAF – Arcetri 2 MPE – Garching. INTROduction. - PowerPoint PPT Presentation

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Page 1: Pyramid  wavefront sensor performance with laser guide stars

PYRAMID WAVEFRONT SENSOR PERFORMANCE WITH LASER GUIDE

STARSF. Quirós-Pacheco1, E. Pinna1, A. Puglisi1, G. Agapito 1

L Busoni1, S. Rabien2, S. Esposito1

1 INAF – Arcetri2 MPE – Garching

Page 2: Pyramid  wavefront sensor performance with laser guide stars

AO4ELT3, FIRENZE, 30 MAY 2013 2

• Wave-front sensing with extended sources is characterized by lower sensitivities (w.r.t point sources).

• Larger spots in a focal plane WFS (e.g. Shack-Hartmann) increase noise propagation:

• What is the effect of extended sources on the pyramid WFS?

• Pyramid sensitivity with 2D extended sources

• Pyramid sensitivity with 3D extended sources

• LGS treated as a 3D extended source

• Why pyramid with LGS on an ELT?

INTRODUCTION

Page 3: Pyramid  wavefront sensor performance with laser guide stars

AO4ELT3, FIRENZE, 30 MAY 2013 3

PRELIMINARY EXPERIMENTS WITH FLAO• Laboratory tests @ Arcetri with LBT’s FLAO#2 system in 2011 (E. Pinna et al. AO4ELT2).

• Use of fiber cores (from 0.2 to 1.6 arcsec in diameter) to emulate 2D extended sources.

• One Interaction Matrix done with diff. limited source → modal gain optimization required.

SR(H)~60%

Estimated SR@H ~ 60%

E2Esimulations

2013

FLAO lab test 2011

Page 4: Pyramid  wavefront sensor performance with laser guide stars

AO4ELT3, FIRENZE, 30 MAY 2013 4

SENSITIVITY WITH 2D EXTENDED SOURCES

• LBT (8m) telescope

• FLAO system config:

• 30x30 subaps

• 500 KL modes

• 2D extended sources:

• Uniform illumination

• Diam: 0.4” to 1.6”

• IM calibrations done with

extended sources

• Noise Propagation Coeffs: Noi

se p

rop.

coe

ff.

[nm

/ sl

ope

rms]

𝑝𝑖2= (𝐷𝑇 𝐷 )𝑖𝑖

−1

Page 5: Pyramid  wavefront sensor performance with laser guide stars

AO4ELT3, FIRENZE, 30 MAY 2013 5

2D EXTENSION VS. TILT MODULATION

Is the NGS 2D extension somehow equivalent to a tip-tilt modulation?Round source (top-hat) Tip-tilt round modulation

REO

extended object radiusRM

modulation radius

S = C / RM

Slicing the round sourcein rings of different RM

S = ∫0REO (C /r) [2p r / (p R

EO2)] dr = 2C / REO

Same sensitivity when RM = REO / 2

FT = total source flux

FOCAL P

LANE

PYRAMID EDGES

Ring sensitivity Ring relative intensity

Page 6: Pyramid  wavefront sensor performance with laser guide stars

AO4ELT3, FIRENZE, 30 MAY 2013 6

2D EXTENSION VS. TILT MODULATION

Noi

se p

rop.

coe

ff.

[nm

/ sl

ope

rms]RM = REO / 2

2D extension

(2REO)

TT modulation

(±RM)

0.4” ±5.3 /D

0.8” ±10.6 /D

1.6” ±21.3 /D

FLAO@LBT typical modulations:

±3.0 < RM <±6.0 (l/D)

• LBT (8m) telescope

Page 7: Pyramid  wavefront sensor performance with laser guide stars

AO4ELT3, FIRENZE, 30 MAY 2013 7

HIGH ORDER SENSITIVITY DIFFERENCE

Same sensitivity when RM = REO / 2 ?

RM

FOCAL P

LANE

PYRAMID EDGES

dFor modes projecting energy at a distance

d > RM from the PSF center,

the sensitivity is not affected by the modulation

The light projected at a distance d > RM

does not cross the pyramid edge during the tilt modulation

Page 8: Pyramid  wavefront sensor performance with laser guide stars

AO4ELT3, FIRENZE, 30 MAY 2013 8

2D EXTENSION VS. TILT MODULATION

REO

extended object radius

RM

modulationradius

RM = REO / 2

The external rings of the extended source affect higher modes more than

RM does

Page 9: Pyramid  wavefront sensor performance with laser guide stars

AO4ELT3, FIRENZE, 30 MAY 2013 9

3D EXTENDED SOURCES

Page 10: Pyramid  wavefront sensor performance with laser guide stars

AO4ELT3, FIRENZE, 30 MAY 2013 10

SODIUM VERTICAL DISTRIBUTION EFFECTS• Study case:• Telescope: 39m diameter (obs. 28%)• LGS: launched from center of the pupil• Pyramid WFS: 78x78 subapertures

• Sampling of 0.5m/subap.• Pyramid conjugated to 90km layer..

Na Layer100km

90km80km

100km

80km

h0=90km

Vertical extension of the Na layer generates defocused images

Page 11: Pyramid  wavefront sensor performance with laser guide stars

AO4ELT3, FIRENZE, 30 MAY 2013 11

SODIUM VERTICAL DISTRIBUTION EFFECTS

-22 m-11 mZ4 rms 0 m+9.8 m

3.6”

FoV

Foca

l Pla

neCC

D Pl

ane

𝚫𝒉 +1.5 km 0 km -1.5 km -3 km+3 km

+19 m

• Extended object: 2D Gaussian with 0.8arcsec FWHM; Vertical Intensity Gaussian profile with 3.1km FWHM.

• Pyramid WFS: 78x78 subaps; =589nm; FoV=3.6”

CCD imagefrom all layers

Page 12: Pyramid  wavefront sensor performance with laser guide stars

AO4ELT3, FIRENZE, 30 MAY 2013 12

VERTICAL DISTRIB. EFFECTS ON SENSITIVITY

EQUIVALENT

• Vertical distribution introduces a “radial tilt modulation” increasing in amplitude with the distance from the pupil center

• The pyramid sensitivity decreases in the radial direction with the distance form the pupil center as the SH does (spot elongation)

Sx

Sy

Tip Tilt

Pyramid signals

Wavefront

Focus = increasing tilt amplitude

Linear decreasing of sensitivity

SIGNAL CUT

Page 13: Pyramid  wavefront sensor performance with laser guide stars

AO4ELT3, FIRENZE, 30 MAY 2013 13

• AO system: pyramid WFS 78x78 and DM correction ~4000 KL modes

• IM calibration with 3D extended source → noise propagation coefficients

• Vertical extension: a) 90±3km b) 90±0.5km (aka LGS with refocusing)

0.8arcsecFWHM

± 3.

0 km

± 0.

5 km

MODAL SENSITIVITY VS. VERTICAL EXTENSION

Page 14: Pyramid  wavefront sensor performance with laser guide stars

AO4ELT3, FIRENZE, 30 MAY 2013

WHY PYRAMID FOR LGS ON ELTS?

14

• 2D extension: losing the “classical” pyramid advantage in sensitivity (l/D vs. l/r0)

• Vertical extension: same radial reduction of sensitivity as the SH

BUTWFS people are worried about other killers:

• # of pixels required: 100 pix/SA, 800x800 WFS CCD not yet available

• spot truncation: compromise between # of pixels and allowed WFS FoV

• Na layer variable profiles: multi-peak Na profiles may create spurious signals on the WFS

Is the sensitivity the main SH limitation working on ELT LGS?

Na Layer

SH sub-ap.

SH WFS: detection on the focal planerecover X-Y slopes (2 numbers) from ~100 pixels

Page 15: Pyramid  wavefront sensor performance with laser guide stars

AO4ELT3, FIRENZE, 30 MAY 2013

WHY PYRAMID FOR LGS ON A ELT?

15

Na Layer

WFS FoV(adjustable)

Information on WF slopes optically recollected in the pupil plane

PYR WFS: detection in the pupil planerecover X-Y slopes from 4 pixels(same algorythm as NGS case)

and the killers…

• # of pixels required: 4 pixels per SA(80x80SAs existing 240x240 OCAM2)

• spot truncation: no more truncation (FoV independent on pixel size and number)

• Na layer variable profiles: just a variable focus off-set (slopes information averaged optically, TBC by simulation)

Page 16: Pyramid  wavefront sensor performance with laser guide stars

AO4ELT3, FIRENZE, 30 MAY 2013 16

FUTURE WORK

• Concept: pupil edge lunch of the LGS

• Simulations:

• Closed loop performance with 3D (more GPUs!)

• Multi peak distribution impact (focus offset only?)

• Lab experiment: PEACE bench (Arcetri - ESO - GMT) already equipped with pyramid WFS and Na lamp

• Vertical extension simulated by defocus on ALPAO DM

• Phasing segmented mirror with Na source

... then on-sky experiment?