Astronomy with LGS AO, Ringberg 1

NAOS with the LGSCommissioning results

Markus KasperNancy Ageorges – Paranal operationsChristian Soenke – Real-time computerGerard Zins – Instrument software

With support from the ESO-Paranal, LGSF and PARSEC (MPE) teams

Astronomy with LGS AO, Ringberg 2

OutlineOverview of NACO in LGS modeLGS limitations and performance predictionThe commissioning processLIDAR (cf. poster of Stefan Hippler, MPIA)Focus control / sodium layer distance trackingNGS tip-tiltJitter Loop / Stabilization of LGS on WFSLoop PerformancePotential upgrade

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NACO LGS Upgrade Requirements

Ensure NAOS NGS functions are left working (Dichroic + flat motorized)Additional tip-tilt sensor for NGSLGS position stabilization on WFSLIDAR for initial LGS height, sodium layer statisticsLGS focus correction (Trombone)LGS WFS calibration (extra calibration source)

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LGS Upgrade, 4 Control Loops

LGS stabilization loop, corrected by LGSF Launch Telescope

NGS tip-tilt loop

LGS focus loop, corrected by trombone

LGS AO loop

LGS calibrationfiber

FieldSelector

ADC

TromboneSTRAP

IC-LCU

RTC

TTM DM

VIS WFS

long term defocus

LGS jitter to LGSF

589 nm

Vis

Dichroic

2face mirror(M4)

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Optical path

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LGS AO performance predictionLGS equivalentmagnitude V~11.5

Specified return flux of 106 phot/s/m2 -> V~10.5WFS Noise increase wrt NGS due to LGS size larger than seeing (σ2 ~ FWHM2 / nph)

Cone effect (Strehl loss around 20% in K-band)Performance limited by tip-tilt residuals

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The commissioning process

Problem: Modify and test an instrument in operation Three “preparation runs” between 2003 and 2005

Installation of new optomechanics (LGS calibration source, LIDAR pick-up optics, STRAP, LGS) and real-time computer upgrade (tip-tilt and defocus handling)

Two commissioning runs in Dec 06 and Jan 07High level software / interfaces with LGSF, operational tests close to zenithoperational tests far from zenith, performance evaluation, new LGS related technical templates, system operation using observation templates

Astronomy with LGS AO, Ringberg 8

Some LIDAR results (cf Hippler poster)

1. Sodium layer width of up to 15 km FWHM2. Mean height between ~86 km and 90 km3. Strong sporadic layers exist

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LGS Focus correction

Average DM defocus is a measure of WFS to sodium layer offsetAdjust WFS focus by optical element in WFS path in closed loopLoop must be too slow to track atmospheric variations, but fast enough to track sodium layer height variations

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STRAP residuals

DIMM Seeing: 0.7”

Jitter from CONICA: S27 camera, 2.15 micron, 50 ms exposures

1. Best perf: ~20 mas rms2. Small open loop jitter

(but fast telescope guiding) of 40-80mas

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With and without NGS tip-tilt (cf. talk of Ric Davies)

LGS + NGS tip-tilt correction22% K-band Strehl ratio, ~85mas FWHM

LGS only correction (full-sky AO)10% K-band Strehl ratio, ~130mas FWHM

Full AO LGS only No AO

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WFS Jitter correction

WFS LGS tip tilt corrected by LGS launch telescope piezo -> large delayLoop implemented by LGSF1-axis jitter variance reduced by ~50% to ~80 mas rms

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LGS and NGSLGS size somewhat larger than expected (typically between 1.5” and 2” FWHM)14x14 lenslet array subapertures have 2.3” FoV only Truncation error and additional flux lossMust use 7x7 array (4.6” FoV)

LGS

NGS

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LGS equiv. magnitude

NGS: R = 13.25NGS WFS noise variance about 4 times higherLGS: Requiv = 11.7(σ2 ~ FWHM2 / nph)

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NGS vs LGS performance

LGS: ~32% K-band Strehl ratio, ~75 mas FWHMNGS (R=13.25):~28% K-band Strehl ratio, ~80 mas FWHM

=> Use LGS AO for guide stars fainter than R ~ 13.5

LGS

NGS

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Error budget and upgrade plan (~2009)

Error Term Value [nm rms]

AO residual WFE (NAOS NGS performance for V=12 guide star in 0.7″ seeing: K-band Strehl 45%)

310

Cone effect (20% Strehl loss in K-band) 165

STRAP residual jitter (18mas 2-axis, ~15% Strehl loss compared to NGS assuming 10mas 2-axis jitter with NGS, tbc)

140

Static correction performance (~10% Strehl loss compared to NGS) 110

Total LGS AO WFE 395

LGS AO K-band Strehl ratio 0.28

1. Replace the RTC -> Increase robustness and flexibility

250

30045%

2. Replace the WFS by L3 CCD with large FoV 14x14 LA3. Replace STRAP by a better performing TTS

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Some pictures (cf. Ric Davies talk)