Transcript (Clare.ppt)

Low order wavefront sensor trade study
Richard Clare
NGAO meeting #4
January 22 2007
Outline
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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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Modeling overview
NGS at infinity
generated with guide star statistics
(Bahcall-Soneira, Spagna models)
Sodium LGS at 90km
a(3)
a(2)
Discrete layers
of turbulence, described
by Zernikes, a.
a(1)
•Calculate transformation matrices from
LGS, NGS, science points to aperture
bL  TL a
bn  Tn a  n
bs  Ts a
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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
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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
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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
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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
Field Galaxies case: Latitude=30 deg
J=16.4
J=17.1
J=17.4
J=16.6
J=19.0
J=18.7
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Example Cumulative Density Function
Field Galaxies science case
J band
30th percentile=107nm
Errors are in nm. 1 mas =12.1 nm for a 10m telescope
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Choice of Spectral Band
• Trade-off between:
1. Partial correction
As λ increases, tip/tilt estimate
2. Sky background
3. Zeropoint (number of photons)
4. Spot size
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Spectral Band
Errors 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
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For field galaxies science case, J band, and 1 TTFA + 2TT sensors
LOWFS number & order
Errors are in nm. 1 mas =12.1 nm for a 10m telescope
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
TT=tip/tilt (ie 1x1), TTFA=tip/tilt/focus/astigmatism (2x2)
For field galaxies science case, and J band
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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
Narrow Field (86nm)
Field galaxies (173nm)
Goods N (218nm)
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Median results for science cases/asterisms
Errors are in nm. 1 mas =12.1 nm for a 10m telescope
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
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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
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