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Pyramid wavefront sensor performance with laser guide stars

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.

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Pyramid wavefront sensor performance with laser guide stars

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  1. Pyramid wavefrontsensor performance with laser guide stars F. Quirós-Pacheco1, E. Pinna1, A. Puglisi1, G. Agapito 1 L Busoni1, S. Rabien2, S. Esposito1 1 INAF – Arcetri 2 MPE – Garching

  2. INTROduction • 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? 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% FLAO lab test 2011 E2E simulations 2013 Estimated SR@H ~ 60% 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: Noise prop. coeff. [nm / slope rms] AO4ELT3, Firenze, 30 May 2013

  5. 2d extension vs. tilt modulation Round source (top-hat) Tip-tilt round modulation Is the NGS 2D extension somehow equivalent to a tip-tilt modulation? REO extended object radius RM modulation radius FOCAL PLANE S = C / RM PYRAMID EDGES Slicing the round source in rings of different RM FT = total source flux S =∫0REO(C /r) [2p r / (pREO2)] dr= 2C / REO Ring sensitivity Ring relative intensity Same sensitivity when RM= REO / 2 AO4ELT3, Firenze, 30 May 2013

  6. 2d extension vs. tilt modulation • LBT (8m) telescope Noise prop. coeff. [nm / slope rms] FLAO@LBT typical modulations: ±3.0 < RM <±6.0 (l/D) AO4ELT3, Firenze, 30 May 2013

  7. High order sensitivity difference Same sensitivity when RM= REO / 2 ? For modes projecting energy at a distance d > RMfrom 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 FOCAL PLANE d RM PYRAMID EDGES AO4ELT3, Firenze, 30 May 2013

  8. 2d extension vs. tilt modulation RM= REO / 2 The external rings of the extended source affect higher modes more than RM does REO extended object radius RM modulation radius AO4ELT3, Firenze, 30 May 2013

  9. 3D EXTENDED SOURCES AO4ELT3, Firenze, 30 May 2013

  10. Sodium Vertical distribution effects 100km 90km 80km • 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 Layer Vertical extension of the Na layer generates defocused images 100km h0=90km 80km AO4ELT3, Firenze, 30 May 2013

  11. Sodium Vertical distribution effects 𝚫𝒉 +3 km +1.5 km 0 km -1.5 km -3 km 3.6” FoVFocal Plane CCD Plane Z4 rms +19 m +9.8 m 0 m -11 m -22 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 AO4ELT3, Firenze, 30 May 2013

  12. Vertical distrib. Effects on sensitivity • 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) Tip Tilt Focus = increasing tilt amplitude Wavefront Sx EQUIVALENT Pyramid signals Sy Linear decreasing of sensitivity SIGNAL CUT AO4ELT3, Firenze, 30 May 2013

  13. Modal sensitivity vs. vertical extension • 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) ± 3.0 km ± 0.5 km 0.8arcsec FWHM AO4ELT3, Firenze, 30 May 2013

  14. Why pyramid for lgs on elts? • 2D extension:losing the “classical” pyramid advantage in sensitivity (l/D vs. l/r0) • Vertical extension: same radial reduction of sensitivity as the SH BUT Is the sensitivity the main SH limitation working on ELT LGS? WFS 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 Na Layer SH sub-ap. SH WFS: detection on the focal plane recover X-Y slopes (2 numbers) from ~100 pixels AO4ELT3, Firenze, 30 May 2013

  15. Why pyramid for lgs on a elt? 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) Na Layer WFS FoV (adjustable) PYR WFS: detection in the pupil plane recover X-Y slopes from 4 pixels (same algorythm as NGS case) Information on WF slopes optically recollected in the pupil plane 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? AO4ELT3, Firenze, 30 May 2013

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