pyramid wavefront sensor performance with laser guide stars
DESCRIPTION
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 PresentationTRANSCRIPT
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
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
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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
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
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
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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
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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
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
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3D EXTENDED SOURCES
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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
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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
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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
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• 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
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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
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)
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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?