Low order wavefront sensor trade study Richard Clare NGAO meeting #4 January 22 2007.
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Transcript of Low order wavefront sensor trade study Richard Clare NGAO meeting #4 January 22 2007.
Low order wavefront sensor trade studyLow order wavefront sensor trade study
Richard ClareRichard Clare
NGAO meeting #4NGAO meeting #4
January 22 2007January 22 2007
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Outline
• Background on sky coverage simulator• Assumptions and parameter set chosen for NGAO• What spectral band should we use for the LOWFS?• How many LOWFS do we need? • What modes should the LOWFS measure?• What is the sky coverage for different science cases?• What is the effect of the LGS asterism radius on partial
correction and sky coverage?
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Sodium LGS at 90km
Discrete layers of turbulence, describedby Zernikes, a.
NGS at infinitygenerated with guide star statistics(Bahcall-Soneira, Spagna models)
a(1)
a(3)
a(2)
•Calculate transformation matrices from
LGS, NGS, science points to aperture
naTb nn
aTb LL
aTb ss
Modeling overview
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Simulator methodology
• Calculate atmospheric tip/tilt error with minimum variance estimator from transformation matrices and covariance matrices of atmosphere & noise
• Optimize sampling frequency to balance servo lag and noise
• Choose combination of NGS that gives lowest total error• Monte Carlo over many NGS constellations• Generate cumulative density functions of performance
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Checking against an AO Simulation
• Compare to LAOS for 4 asterisms for an 8m telescope with no windshake, no sodium tracking error, integrator control, and 10 phase screens for each asterism
– Generally good agreement, but LAOS results somewhat poorer with noise
Median tip/tilt error (nm)
Asterism LAOS without noise
Sky cov. without noise
LAOS with noise
Sky cov. with noise
Good (equilateral)
71±9 58 115±11 84±2
25th percentile
76±13 86 88±9 119±0
Median 106±16 114 200±15 178±5
75th percentile
192±33 221 480±52 334±4
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Simulation Parameters/Assumptions• Finite outer scale (75m)
• Mauna Kea (7 layer) turbulence and velocity profile
• First 6 Zernike orders considered, only tip/tilt errors are evaluated
• Detector pixels are seeing-limited in V band (0.5 arc sec)
and diffraction-limited in J/H/K bands (λ /D rads)
• NGS are partially corrected in J/H/K bands. Not in V band.
• Integral control with g=0.5
• 7 LGS asterism (1 on-axis, 6 in a ring) = Ralf’s asterism 7a
• LGS measurements are noise-free
• Limiting magnitude is chosen to be 19 for all spectral bands
• At zenith
• Read noise = 10 e
• Run over 500 NGS constellations
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Median Field of View
J=16.4
Field Galaxies case: Latitude=30 deg
J=17.1
J=19.0
J=17.4
J=18.7
J=16.6
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Example Cumulative Density Function
Errors are in nm. 1 mas =12.1 nm for a 10m telescope
Field Galaxies science caseJ band
30th percentile=107nm
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Choice of Spectral Band
• Trade-off between:
1. Partial correction
2. Sky background
3. Zeropoint (number of photons)
4. Spot size
As λ increases, tip/tilt estimate
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Spectral BandErrors are in nm. 1 mas =12.1 nm for a 10m telescope
Tip/tilt error (nm)
Spectral band
10th percentile Median 90th percentile
V 198 326 1140*
J 80 131 221
H 78 127 212
K 99 161 261
For field galaxies science case and 1 TTFA + 2TT sensors
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NGS Patrol Field Diameter
For field galaxies science case, J band, and 1 TTFA + 2TT sensors
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LOWFS number & order
Tip/tilt error (nm)
LOWFS 10th percentile Median 90th percentile
1 TT 152 209 359
1 TTFA 125 215 312
3 TT 90 146 263
1 TTFA + 2TT
80 131 221
Errors are in nm. 1 mas =12.1 nm for a 10m telescope
For field galaxies science case, and J band
TT=tip/tilt (ie 1x1), TTFA=tip/tilt/focus/astigmatism (2x2)
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Different science cases
• Three science cases chosen from NGAO proposal• Science cases have different higher order error, galactic
latitude and science field size
1. Goods N (218 nm, 45 deg, 1.09 arc min)
2. Narrow Field (86 nm, 10 deg, 0.178 arc min)
3. Field Galaxies (173nm, 30 deg, 0.7 arc min)
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Degree of partial correction
• Partial correction depends on LGS asterism radius and higher order error from science case
Goods N (218nm)Field galaxies (173nm)Narrow Field (86nm)
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Median results for science cases/asterisms
Median tip/tilt error (nm)
Science Case
LGS radius
=7”.2
LGS radius
=21”.6
LGS radius
=35”.9
Goods N 317 284 277
Narrow Field 116 96 94
Field Galaxies
156 131 127
Errors are in nm. 1 mas =12.1 nm for a 10m telescope
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Conclusions
• IR WFS (either J or H) is preferable to visible• Multiple NGS WFS significantly improve tip/tilt
estimate• Measuring focus with 1 of the tilt sensors also
helps• A wider LGS asterism improves partial
correction over the field and hence sky coverage