Real-Time Laser Guide Star Elongation and Uplink

Turbulence in the Lab

Andrew P. Reeves1a, Richard M. Myers1, Timothy J. Morris1, Alastair G. Basden1, and Nazim

A. Bharmal1

Centre for Advanced Instrumentation,The University of Durham, South Road, Durham, DH1

3LE, UK

Abstract. The effects of Laser Guide Star spot elongation and uplink turbulence on Adaptive Opticsperformance must be considered when designing an AO system for use on an Extremely Large Telescope.

The former is the effect of atmospheric turbulence on a LGS as it travels up to excite the mesospheric

sodium layer, resulting in unknown tip/tilt modes and laser plume shape and the latter the effect of

the sodium layer’s finite thickness, degrading Shack Hartmann wave front sensor performance through

elongated spots.

DRAGON is an AO test bench under construction in Durham, which can explore these effects in real

time through the use of a novel LGS emulator, where a laser is projected through a realistic turbulence

simulator into a cell filled with a water solution of fluorescent dye. The resulting plume provides a 3-D

light source analogous to a sodium LGS. The turbulence simulator consists of 4 rotating phase screens,

which can be independently translated in height. DRAGON is now in a state capable of investigating the

effects of uplink turbulence and WFS spot elongation on wavefront sensing accuracy.

1 Introduction

Adaptive Optics (AO) with Laser Guide Stars (LGSs) is essential to achieve a good sky coverage

with both existing telescopes and the coming generation of Extremely Large Telescopes (ELTs).

Though LGSs have been in successful operation on a number of facility instruments around the

world [1–3], aspects of their use is still non-optimal, specifically the effects of the so-called

“spot elongation” and “uplink turbulence”.

The most common form of LGS is to project a sodium laser tuned to the sodium D2 line to

the upper mesosphere, where it excites atoms in the Sodium Layer, causing them to emit light

and hence form an artificial guide star (GS). The sodium layer is a layer of sodium atoms which

exist at around 90 km above the earth and has a mean thickness of 10 km. Because of its finite

thickness, the guide star is an elongated source, and when viewed from off-axis subapertures

on a shack-hartmann WFS, results in elongated WFS spots [4]. For a centre launched laser, this

will produce elongation originating from the centre of the WFS, shown in Figure 1a, and for a

side launched laser, elongation from that side of the WFS, Figure 1b.

Shack-Hartmann WFS spot elongation poses problems for wavefront sensing for AO. An

elongated spot requires a larger subaperture field of view than the non-elongated case, which

reduces the sensitivity of the WFS. To counter this, subapertures can contain more pixels, but

this results in a lower signal to noise ratio across the WFS.

The elongation effectively increases noise in its corresponding direction and the WFS will

consequently have reduced sensitivity to spot motion in that direction if it is not taken into

a a.p.reeves@durham.ac.uk

Third AO4ELT Conference - Adaptive Optics for Extremely Large TelescopesFlorence, Italy. May 2013ISBN: 978-88-908876-0-4DOI: 10.12839/AO4ELT3.13255

(a) (b)

Fig. 1: The DRAGON Shack-Hartmann WFS observing a centre launched (a) and side launched (b) LGS.

For the centre-launched case, the spots are elongated outwards from the centre of the WFS, reducing

sensitivity to radial order aberrations. The larger illuminated spot in the centre of the WFS is caused

by the on-axis laser used to illuminate the LGS emulation cell. In the side launched case the spots are

elongated from the side of the aperture, where the LGS is effectively on-axis.

account when centroiding. A correlation centroiding algorithm can be used to mitigate this

effect if the sodium concentration profile is well characterised, but this not always the case[5],

and the profile cannot be easily approximated as a common function, such as a Gaussian profile.

As the laser passes up through the atmosphere to excite the sodium layer, it also encounters

turbulence. The laser launch aperture diameter is usually of the order of r0 so it is mainly affected

by tip and tilt and causes the laser beacon to move about in the sky, resulting in spurious tip-tilt

modes being observed by the LGS WFS, which cannot be disentangled from those encountered

on the way down. For this reason, LGS AO systems requires a tip-tilt sensor which observes a

Natural Guide Star (NGS), reducing sky coverage and increasing system complexity.

It is possible that the LGS plume may also be affected by higher order aberations when pass-

ing up through the turbulence, which could also have an effect in degrading WFS performance

due to a changing laser spot shape. This is a difficult effect to investigate on-sky as, similarly to

tip-tilt modes, it is impossible to disentangle these aberrations from those encountered on the

return path.

2 DRAGON - The Durham Real-time Tomographic Adaptive-optics Test

Bench

Durham has developed a new AO test-bench, DRAGON, with the aim of optimising the use of

LGSs, tomographic reconstructors and real-time control for existing telescopes and their future

ELT class counterparts. The optical bench emulates a 4.2 m diameter telescope, which is scaled

to an 18 mm pupil on the bench. DRAGON has been designed to be modular and compatible

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Third AO4ELT Conference - Adaptive Optics for Extremely Large Telescopes

(a) (b)

Fig. 2: An image of the DRAGON LGS emulation cell (a), and the emulated sodium layer profile (b). The

cell, (a), is illuminated by an external 532 nm diode laser, which can either be projected from behind the

cell, or up through the turbulence generator to emulate uplink turbulence. The sodium profile illustrated

in (b) is created by placing two cells back to back.

with the CANARY MOAO (Multi-Object AO) demonstrator on the 4.2 m William Herchsel

Telescope[6,7], allowing modules developed on DRAGON to be tested on sky.

2.1 Guide Star Emulation

DRAGON features complex “Star” and “Turbulence” Simulators. In the Star Simulator, we

generate 3 NGSs using diffraction limited optical fibres, which can be placed anywhere in a 3

arcminute field of view. LGSs are emulated using a fluorescent cell, created by sealing two weak

lenses in a lens tube and injecting Rhodamine dye into the space in between. When illuminated

by a 532 nm diode laser, the cell fluoresces in the wavelength range of 550 − 610 nm, co-

incidently close to the wavelength of a sodium LGS. As the cell has a finite depth, this produces

a 3-dimensional source, which, when viewed off-axis on a Shack-Hartmann WFS, results in

spot elongation – analogous to that observed when exciting the mesospheric sodium layer to

generate LGS on-sky. The cell is placed at a plane optically conjugate to 90 km on-sky.

The depth of the cell determines the extent of the observed spot elongation, and it is possible

to combine a number of cells to produce a multi-peak sodium layer profile. Significant elon-

gation would not be visible on a 4.2 m telescope with a LGS at a height of 90 km, so in order

to study spot elongation, the cell depth can be scaled to achieve comparable elongation to that

observed by a 40 m telescope. A potential profile is illustrated in Figure 2b.

In future we plan to implement arbitrary evolving sodium profiles. By setting thin layers of

dye in refractive index matching glue and layering the resulting thin films, we can obtain a more

complex and realistic sodium profile and by rotating the layered films, can emulate an evolving

sodium layer. The current LGS emulation cell is illustrated in Figure 2a.

This method of LGS emulation allows us to introduce up-link turbulence into DRAGON. By

launching the 532 nm diode laser downstream of the turbulence generator, the excitation beam

propagates up through the turbulence before exciting the cell, again analogous to the sodium

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(a) (b)

Fig. 3: An image, (a), and a block diagram (b), of the DRAGON turbulence simulator. The 532 nm

diode laser propagates up through the phase screens, where it experiences uplink turbulence, and into

the flourescent cell. The light from the cell then passes back through the turbulence and to the WFS. The

straight-through arm in (a) leads to the NGS Sources.

laser passing up through turbulence on-sky. When the laser passes up through the turbulence,

the beam diameter is 2 mm, which scales to 46.7 cm on a 4.2 mm telescope.

2.2 Turbulence Emulation

The Turbulence simulator is shown in Figure 3a. It consists of four rotating phase plates. The

ground layer screen was made using a technique pioneered by Rampy et al.[8] to approximate

Kolmogarov atmospheric statistics, with a Fried Parameter, r0 value of 0.75 cm, which equates

to an r0 value of 17.5 cm on a 4.2 m telescope. We are currently investigated other solutions for

upper layer turbulence phase screens with a higher value of r0. Each screen can rotate indepen-

dently and can also be mechanically translated in height while the system is running.

2.3 Wavefront Sensing

Both NGS and LGS are observed by a 31x31 subaperture Shack-Hartmann WFS, where each

sub-aperture has a field of view of 2.7 arcseconds. Each subaperture is 14x14 pixels across and

the cameras run at at 135 frames per second. The WFS has a SNR of 10, and a brightest pixel

selection algorithm is used to dynamically determine background signal level[9]. The centroid

is calculated using a simple centre of gravity algorithm.

3 Investigating the Effects of Laser Guide Stars

DRAGON is currently being used to investigate the effects of LGSs on wavefront sensing ac-

curacy. By back illuminating the cell, such that the excitation laser does not pass through the

turbulence simulator, uplink turbulence can be eliminated. If a very thin cell is used, spot elon-

gation is also eliminated. By introducing uplink turbulence and elongation individually, we can

examine the effects of both in detail.

Currently, only one phase screen is in place on DRAGON which does not provide enough

different phase distortions for the measured power spectrum to converge to the expected Noll

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Fig. 4: The modal power spectra for an LGS with and with uplink turbulence. An increase in the power of

tip-tilt modes is observed. The apparent noise in the spectrum is most likely due to the lack of different

frames of turbulence, as we currently have only one phase screen. This effect will be investigated in

further detail in the coming months.

power spectrum [10]. Additional phase screens are required before significant results can be ob-

tained, these will be received soon and tested soon. From very preliminary results, we do notice

a significant increase in power in tip-tilt Zernike Modes when the laser passes up through turbu-

lence compared to the case where there is no uplink turbulence. Figure 4 shows this preliminary

power spectrum.

4 Conclusions

At present DRAGON consists of laser and natural guide star sources, a turbulence generator and

an on-axis WFS. We are in the process of using this configuration to investigate the impact of

using LGSs on WFS accuracy. We have presented very initially results which show an increased

power tip-tilt Zernike Modes measured when the LGS encounters uplink turbulence. These

results were gathered with only one rotating phase screen, and will be repeated when we have

obtained a further three screens.

In the next year, DRAGON will be significantly expanded. On completion, DRAGON will

feature two correction arms, 3 NGS WFSs, four LGS WFSs, a high resolution “science” camera,

and an on-axis “truth” WFS.

The first correction arm will be operated in closed loop of all WFSs. Correction is performed

by a 97 actuator Xinetics Deformable Mirror (DM), with a large stroke. The second correction

arm is operated in open loop of all WFSs with the exception of a truth sensor. Correction is

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performed by a 1024 actuator Boston Micromachines MEMS Kilo DM. Both the NGS and

LGS WFS have a high spatial resolution of 31x31 subapertures, and are capable of running at

frame rates of 200 Hz.

DRAGON is ideally suited to investigating a number of pressing questions in AO science.

We aim to complement the CANARY MOAO demonstrator, by testing and optimising open

loop AO control in the laboratory, including the required calibration procedures for MOAO.

Individual phase screens in the turbulence simulator can translate up and down in height while

the system is running. We plan to use this feature to investigate the effects of evolving turbulence

on tomographic reconstruction algorithms, such as those used in CANARY.

The combination of a low resolution, high stroke DM, with a high resolution, low stroke

DM, is ideal for further investigation into so called woofer-tweeter correction, where the low

resolution DM corrects the more powerful, but lower order spatial frequencies, whilst the high

order DM corrects for the remaining high spatial frequency aberrations. The configuration of

DRAGON is analogous to that proposed for EAGLE on the E-ELT, where a high order DM will

be correcting in open loop behind a closed loop adaptive M4.

DRAGON can run in real-time and is controlled by DARC, the Durham Adaptive optics

Real-time Controller[11]. It will be used to further develop and test DARC, and also exam-

ine other potential real-time control solutions, such as using GPUs and FPGAs to allow faster

reconstruction.

References

1. P. Wizinowich et al., Publications of the Astronomical Society of the Pacific 118, (2006)

297-309

2. M. Boccas et al., Proc. of SPIE 6272, (2006) 62723L1-9

3. Hayano et al., Proc of SPIE 6272, (2006) 6272471-7

4. L. A. Thompson C. S. Gardner, Nature, 328, (1987) 229-231

5. B. Neichel et al., MNRAS 429, (2013) 3522-3532

6. R. Myers, Z. Hubert, T. Morris et al., Proc. SPIE 7015,(2008) 70150E,

7. E. Gendron, F. Vidal, M. Brangier et al, A&A 529, (2011) L2

8. R. Rampy et al., Proc. of SPIE 7736, (2010) 77362Y1-10

9. A. Basden et al., MNRAS 419, (2012) 1628-1636

10. R. Noll, J. Opt. Soc. Am. 66, (1976) 207-211

11. A. Basden et al., Applied Optics 49, (2010) 6354-6363

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