LGS-AO Performance Characterization Plan

AOWG meetingDec. 5, 2003

A. Bouchez, D. Le Mignant, M. van Dam

for the Keck AO team

LGS-AO performance characterization effort

• Purpose (from CfAO proposal):1. Demonstrate and characterize the Keck II LGS AO

system for science via collaboration with the LGS and science AO community.

2. Using the output from goal 1, optimize the Keck LGS AO system and feed back the lessons learned to future LGS AO systems.

• Products:1. Full LGS-AO error budget.2. Accurate planning information for users. 3. New data on sodium layer structure and density

variations.

LGS-AO Characterization Plan

• LGS-AO characterization effort is closely related to • LGS-AO development priorities• demonstration of LGS-AO observing modes.

• Goals• On-axis performance (Strehl & FWHM vs.

magnitude) fully characterized by Feb. 2004.• Off-axis performance (anisoplanatism, dither

accuracy, etc.) documented by Aug. 2004.

Error budgets

• Not one single error budget!1. Performance

• Subsystems• LGS performance• Integrated system performance

2. Throughput3. Field of view

• off-axis FOV• dithering precision

4. Operational aspects• Overheads• Stability

Performance error budget (on-axis)

# Error Term RMS Error (nm)LGS Relevant measurements

1 Camera 113 Investigate in more detail for 40" field.2 Fitting 128 Fixed by DM.3 Telescope 60 Fixed by telescope + AO.4 Bandwidth - TT 1285 Measurement - TT 40

6 Bandwidth - DM 807 Measurement - DM 1008 Measurement - laser jitter 50 Dependence on seeing, residual after UTT implementation9 Measurement -elongation 112 Fixed by projector location10 Focal Anisoplanatism 155 Best estimate from 2002 scidar observations.11 LGS Calibrations 150

12 Telescope focus 3013 Other NGS 105 MvD has been investigating this

RSS = 375

Strehl at 2.1 um = 0.28

Dependence on star magnitude, background (sky, moon, laser), resolved source, moving source, seeing, elevation.

Dependence on laser power, spot quality, sodium density, seeing, elevation

LBWFS calibration, measurement & bandwidth error vs. magnitude & integration time.

Sodium layer characterization

• Continuously monitor mean altitude through WFS focus.

• Continuously monitor sodium density through return power / projected power.

• Nightly map of density structure with unstacked images

NGS Strehl vs. magnitude

6 7 8 9 10 11 12 13 14 150

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

Magnitude (V)

Str

ehl a

t 1

.59

m

Simulated 2.4" Actual 2.4" Actual 1.0"

Subsystem performance• Tip-tilt sensor stage - STRAP Residual tip-tilt as

function of:1. Star magnitude.2. TT reference motion (asteroid, etc.)3. Sky background (moonlight, raleigh).4. Resolved sources (AGN, close binaries).

• Low-bandwidth wavefront sensor - Precision of wavefront measurement as function of:

1. Star magnitude & integration time.2. Limit due to aberrations in LBWFS.3. Off-axis location of tip-tilt stage.4. Pupil rotation rate

• Tip-tilt sensor positioning accuracy1. Repeatability2. Accuracy

• Effect of LGS spot elongation on WFS measurement

LGS performance

• Laser spot size / elongation / power• Temporal stability• How it is affected by seeing• Correlation against laser table diagnostic

measurements

• LGS tip-tilt and jitter• Flexure compensation performance• Magnitude of atmosphere-induced jitter• Residual tip-tilt after up-tip-tilt implementation

• Sodium layer • Track mean altitude• Power returned vs. power out (sodium density)• Layer structure from unstacked images.

Integrated system performance

• Strehl on target versus• Tip-tilt star magnitude.• Tip-tilt star separation.• Spot elongation / sodium altitude / elevation• Laser power / sodium density• Integration time

• Anisoplanatism• Different from NGS?

• Focal anisoplanatism• Predict from Cn2 profiles• Measure from total error budget

• Operational aspects• Acquisition overhead• Dithering overhead• Mean time between failures