Ground layer wavefront reconstruction using dynamically refocused Rayleigh laser beacons

Center for AstronomicalAdaptive 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 OpticsSteward Observatory, The University of Arizona

OSA 2005 Adaptive Optics: Analysis and Methods


GLAO - IntroductionGLAO - 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.


GLAO at the MMTGLAO 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.


RLGS Beam Projector at the MMTRLGS 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)


Wavefront Sensor InstrumentWavefront 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.


LGS WFS DataLGS 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.


Wavefront ReconstructionWavefront 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.


Performance with Field AnglePerformance 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.


Reconstructed DataReconstructed Data

Phase reconstruction of ground layer estimate and NGS: Zernike orders 2-6.

Upper row: Shack-Hartmann 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


Zernike Mode TrackingZernike 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.


Residual RMS after correctionResidual 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



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


4 223 285 261 276 269


5 183 220 207 220 220


6 159 184 175 194 170


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



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 ofro = 15cm at 500nm.

Bars on left show the uncorrected measured NGS RMS wavefront error rescaled toro = 15cm.




Ground/Upper layer turbulenceGround/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.


Ground Layer Isoplanatic AngleGround 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)


Mean Height of Ground LayerMean 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


ConclusionConclusion

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.


Future WorkFuture 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”


Dynamic Refocus in ActionDynamic 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.


Details of Beam ProjectorDetails of Beam Projector

FoldMirror

Laser Box

Tip/Tilt Pupil Mirror

Pupil Box

L3

L1

L2

AdaptiveSecondary

6.5m Primary Mirror

Hologram

Optical Axis

Laser Power Supply and Chiller in Yoke Room

Star Imager


WFS Instrument Optical LayoutWFS 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 FWHMEstimated FWHM

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

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.


Substandard DataSubstandard 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.

Documents

Ground layer wavefront reconstruction using dynamically refocused Rayleigh laser beacons