Transcript OSA - Ground Layer AO

Center for Astronomical
Adaptive Optics
Ground layer wavefront reconstruction
using dynamically refocused Rayleigh
laser beacons
C. Baranec, M. Lloyd-Hart, M. Milton, T. Stalcup, M. Snyder, N. Putnam
and R. Angel
Center for Astronomical Adaptive Optics
Steward Observatory, The University of Arizona
OSA 2005 Adaptive Optics: Analysis and Methods
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Adaptive Optics
GLAO - Introduction
Ground layer adaptive optics (GLAO) correction is a method for correcting
the wavefront errors caused by turbulence close to the telescope.
•By using a constellation of guide sources, one can average the measured
wavefronts, giving an estimate of the ground layer turbulence.
•Applying this correction to a DM conjugated near the ground, removes
the wavefront aberration common to a wide field.
•With varying measurements of the ground layer turbulence being up to
2/3 of the total turbulence, this can greatly improve seeing over this same
field.
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GLAO at the MMT
•GLAO will be beneficial for current and future extremely large
telescopes (ELTs). It promises partial wavefront correction and uniform
PSFs over a wide field of view.
•GLAO is a powerful new technique that needs experimental validation.
•We are investigating GLAO as we move forward to testing new AO
techniques for ELT’s at the MMT.
•We have deployed a five beacon Rayleigh laser guide star (RLGS)
source at the MMT to test ground layer and tomographic reconstruction
of atmospheric turbulence. Here, I present our system and first results
in relation to ground layer adaptive optics.
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RLGS Beam Projector at the MMT
• Two 15 W doubled YAG lasers at 532
nm pulsed at 5 kHz.
• The laser beams are combined with a
polarizing beam splitter.
• A computer generated hologram splits
the combined beam into 5 beams that
are projected onto a circle of 2 arc
minutes diameter.
• Projection optics mounted on the
telescope axis behind the secondary
mirror
• Photometry:
•Measured: 760,000 ph/m2/J
•Typical Sodium LGS: 840,000 ph/m2/J
(J. Ge 1998)
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Wavefront Sensor Instrument
Wavefront Sensor (WFS) Instrument mounts to MMT Cassegrain
focus. Run both RLGS and NGS simultaneously.
RLGS WFS:
•Multiple laser guide star Shack-Hartmann wavefront sensor.
•Hexapolar geometry, breaks pupil into 36 subapertures.
•Uses a range gated Lincoln Labs CCID18 chip run at ~55 Hz.
•Dynamic refocus system removes the focus term from each pulse
of the RLGS over its range gate from 20 – 30 km
NGS WFS:
•Optical clone of the MMT-AO NGS WFS camera with an E2V
CCD39 run at ~110 Hz.
•Pupil broken into 12x12 subapertures of which 108 are illuminated.
•Sensor on translation slide, to allow exploration of field in one axis.
RLGS/NGS WFS Synchronization:
•externally controlled LED flashers used to synchronize data
capture for both RLGS and NGS WFS. Flashed once per second.
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LGS WFS Data
•Shack-Harmann patterns
of the five beacons on the
RLGS WFS after
background subtraction.
•Windshake of the
secondary mirror hub bends
the telescope, causes
patterns to move around.
•Flashes due to LED
synchronization.
•Used physically
constrained iterative blind
deconvolution methods to
measure spot positions
•Data Quality.
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Wavefront Reconstruction
Wavefront reconstruction of the ground layer turbulence and the ground
truth natural star:
•RLGS wavefront reconstruction by inversion of synthetic influence
matrix of Zernike modes on our geometry of Shack-Hartmann pattern.
•Estimate of ground layer turbulence by averaging the Zernike
coefficients of each beacon.
•NGS wavefront reconstruction by using the same reconstructor matrix
as used in the closed-loop MMT AO system. The NGS WFS is optically
the same, so we can use the same reconstructor.
•Estimate of GLAO performance by subtracting ground layer estimate
from NGS ground truth.
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Performance with Field Angle
Exploration of GLAO
performance with field angle.
Figure shows the position of
the NGS for each data set in
relation to the RLGS. Data
taken over a period of 2 hours.
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Reconstructed Data
Phase reconstruction of
ground layer estimate
and NGS: Zernike
orders 2-6.
Upper row: ShackHartmann patterns from
RLGS and NGS.
Bottom row:
Reconstructed phase
from ground layer
estimate and NGS. In
good agreement but
show differences due to
non-common turbulence
and measurement error.
RLGS
NGS
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Zernike Mode Tracking
An example comparison of three
Zernike modes between GLAO estimate
and NGS ground truth.
NGS in dashed blue.
GLAO average of the five RLGS in solid
black.
Each
sequence is
approximately
3 seconds.
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Residual RMS after correction
Example RMS wavefront
aberration over 3 seconds
for Zernike orders 2
through 6:
•NGS in blue.
•Average RLGS in black.
•Residual wavefront
aberration of NGS after
GLAO correction in red.
NGS RMS wavefront aberration: 650 nm
Residual NGS RMS wavefront aberration after correction: 380 nm
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Performance with Field Angle
RMS stellar wavefront aberration in nm, averaged over the modes of each Zernike
order. Before correction, top, and after GLAO correction, bottom. Median seeing at
the MMT at 500nm is ro = 15cm, so we were working under poor seeing conditions.
Zernike order
Set 1
Set 2
Set 3
Set 4
Set 5
2
462
572
513
571
559
2 (after correction)
255
316
308
349
343
3
308
404
365
383
379
3 (after correction)
198
283
226
246
258
4
223
285
261
276
269
4 (after correction)
142
181
168
184
190
5
183
220
207
220
220
5 (after correction)
140
166
152
168
168
6
159
184
175
194
170
6 (after correction)
116
143
130
154
143
2-6
645
809
732
797
778
2-6 (after correction)
397
487
463
518
518
ro (cm) @ 500 nm
12.1
9.0
10.3
9.2
9.8
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Performance with Field Angle
GLAO performance as a function of field angle
Over the course of
taking data, ro varied
from 9.0 to 12.1 cm at
500 nm.
To allow direct
comparison, all data
points have been
rescaled to the MMT’s
median seeing of
ro = 15cm at 500nm.
Bars on left show the
uncorrected measured
NGS RMS wavefront
error rescaled to
ro = 15cm.
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Performance with Field Angle
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Ground/Upper layer turbulence
From Hardy (1998), the power in Zernike orders 2-6 is given by:
The overall ground layer corrected residual wavefront error inside the
beacon constellation is 356nm.
This yields values of r0 for the ground and upper layers:
Uncorrected upper layers: r0 = 30 cm
Ground layer: r0 = 19 cm
An approximate division of 2/3 power in the ground layer, and 1/3
power in the free atmosphere. In agreement with other studies done at
Cerro Pachon.
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Ground Layer Isoplanatic Angle
From our data we were able to calculate other atmospheric parameters.
•For each of the five data sets, we were able to find the residual RMS
stellar wavefront aberration using each individual beacon as a correction.
•This gave us 25
measurements of RMS
residual error as a function of
angle.
•Plotting these points and
fitting a curve of the form
y = a + b θ0-ground 5/3 gave us
a measurement of θ0.
•We found θ0-ground = 29
arcsec at 500nm.
Beacon – NGS Separation (arc sec)
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Mean Height of Ground Layer
Given our measurements of θ0-ground and r0-ground, we can
calculate the mean height of the ground layer
turbulence, h. From Hardy (1998):
With a mean sec(ζ ) = 1.05 for these observations, we
calculate:
h = 445 m
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Conclusion
What we have learned about GLAO correction:
•Using five Rayleigh laser guide beacons, we can get a measurement
of the ground layer turbulence.
•The residual RMS stellar wavefront aberration after correction is more
constant in time.
•Ground layer correction is relatively flat within the diameter of the
RLGS constellation with a gradual decay of correction outside.
•Gives modest seeing improvement even into I band.
•Most importantly… We have seen an average 40% improvement in
wavefront error over a 2 arcminute field.
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Future Work
Another run at the MMT next week with much improved instrument
• New CCD for the RLGS that actually works properly!!!
• Made a number of optical improvements to system, easing alignment and
increasing throughput.
• Upgraded the RLGS WFS from 36 to 60 subapertures, allowing wavefront
reconstruction up to Zernike order 9.
• Will allow us better understanding of GLAO
• Will allow us to take the next step and attempt tomographic reconstruction
of the atmospheric turbulence
Future work
• With data collected next week, we will be preparing to run the system in
closed loop with the MMT’s adaptive secondary later this year
• See Michael Lloyd-Hart’s talk on “Development of Multi-Laser Guide Star
Adaptive Optics Techniques for Extremely Large Telescopes”
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Dynamic Refocus in Action
RLGS Shack-Hartmann
patterns with and without
dynamic refocus (DR)
running.
Without DR, off-axis spot
elongation. Can be seen
here as radial streaking of
spots.
Data taken 29th Sept ’04,
11:28 pm.
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Details of Beam Projector
L1
Hologram
L2
Fold
Mirror
L3
Adaptive
Secondary
Pupil
Box
Tip/Tilt Pupil
Mirror
Optical Axis
Star
Imager
6.5m Primary Mirror
Laser Power Supply and
Chiller in Yoke Room
Laser
Box
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WFS Instrument Optical Layout
(1) Wide field imaging optics and camera, (2) Dichroic mirror, (3) Natural
guide star wavefront sensor optics, (4) Closeup of NGS WFS camera, (5)
Dynamic refocus ‘resonator’ and optics, (6) Rayleigh Laser guide star
wavefront sensor arm, (7) Closeup of RLGS WFS Camera.
Estimated FWHM
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Given our GLAO correction of Zernike orders 2 through 6, and
assuming perfect tip/tilt correction, we can calculate the FWHM of a
long exposure image using our current system.
For an ro = 15cm at 500nm, we can see the comparison of the seeing FWHM
and the FWHM after correction for bands in the near IR.
H and K bands are nearly diffraction limited, and there are significant gains in
FWHM into I band.
Band
Wavelength
r0
Seeing
Diffraction
FWHM after correction
μm
m
arcsec
arcsec
arcsec
K
2.2
0.824
0.551
0.0698
0.0733
H
1.6
0.562
0.587
0.0508
0.0592
J
1.25
0.418
0.617
0.0397
0.116
R
0.9
0.282
0.658
0.0286
0.229
I
0.7
0.209
0.691
0.0222
0.268
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Substandard Data
Data quality from previous runs was substandard. Due to a number of
factors:
•Our RLGS WFS CCD was horrible
• Bad MTF caused images on RLGS WFS to look terrible.
• Typical FWHM of Shack-Hartmann spots found to be 3.7 arcsec. When
measured on separate camera was 1.5 arcsec.
• Lots of Noise / Fixed pattern Noise
• Video Dropouts
• Vastly different amplifier biases
•Found our alignment tolerances were very tight and made it difficult to
align in short amount of time we had on the mountain.
•Typical problems getting a prototype system up and running
•Working in 40+ mph winds, which made us stop observing early.