Laser Guide Star for 3.6- and 8-m telescopes: Performance and astrophysical implications

Louarn, Miska Le; Foy, R.; Hubin, N.; Tallon, Michel

doi:10.1046/j.1365-8711.1998.01223.x

Cited by 37 publications

(32 citation statements)

References 17 publications

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“…For science targets that are themselves too faint to permit wavefront sensing, or too diffuse to sustain efficient wavefront correction (see Rousset 1994), a bright (V ≤ 13), nearby (∆θ ≤ 30 ) natural guide star (NGS) is required. Exactly how bright and nearby depends on the local atmospheric conditions; the above values correspond to the typical regime in which the wavefront sensor is not photon-limited and the off-axis Strehl ratio at 2.2 µm is not too severely reduced (e.g., Le Louarn et al 1998). In the absence of a nearby NGS, higher-order wavefront corrections using a laser guide star (LGS) can suffice, provided a moderately bright (V ∼ 16) reference star lies relatively close (∆θ ≤ 90 ) to the science target for determination of the lowest-order tiptilt correction.…”

Section: Introductionmentioning

confidence: 94%

“…The phase error due to such focus anisoplanatism increases with mirror diameter as D 5/6 (e.g., Le Louarn et al 1998, and references therein). Until the advent of multiple-LGS systems that can correct for this effect (e.g., Tallon & Foy 1990), AO systems on 8 m-class telescopes will therefore reach the highest Strehl ratios only when operating in NGS mode.…”

Section: Introductionmentioning

confidence: 99%

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Galaxies in southern bright star fields

Baker

Davies

Lehnert

et al. 2003

A&A

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Abstract. As a prerequisite for cosmological studies using adaptive optics techniques, we have begun to identify and characterize faint sources in the vicinity of bright stars at high Galactic latitudes. The initial phase of this work has been a program of K s imaging conducted with SOFI at the ESO NTT. From observations of 42 southern fields evenly divided between the spring and autumn skies, we have identified 391 additional stars and 1589 galaxies lying at separations ∆θ ≤ 60 from candidate guide stars in the magnitude range 9.0 ≤ R ≤ 12.4. When analyzed as a "discrete deep field" with 131 arcmin 2 area, our dataset gives galaxy number counts that agree with those derived previously over the range 16 ≤ K s < 20.5. This consistency indicates that in the aggregate, our fields should be suitable for future statistical studies. We provide our source catalogue as a resource for users of large telescopes in the southern hemisphere.

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Section: Introductionmentioning

confidence: 94%

Section: Introductionmentioning

confidence: 99%

Galaxies in southern bright star fields

Baker

Davies

Lehnert

et al. 2003

A&A

View full text Add to dashboard Cite

show abstract

“…Moreover, the following discussion, analytical computations, and simulations only apply to AO systems using unresolved sources. Extended references that can be resolved by the combination of WFS and telescope, such as asteroids (Ribak & Rigaut 1994), extragalactic nuclei (Le Louarn et al 1998), laser guide stars (Foy & Labeyrie 1985), and even, for ELTs, some natural guide stars (Weiner et al 2000), are not treated here. For the variety of cases that we consider in this work, in open loop, the spots focusing on the wavefront derivative sensitive element of the two systems (i.e.…”

Section: Approach and Assumptionsmentioning

confidence: 99%

Expected gain in the pyramid wavefront sensor with limited Strehl ratio

Viotto

Ragazzoni

Bergomi

et al. 2016

A&A

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Context. One of the main properties of the pyramid wavefront sensor is that, once the loop is closed, and as the reference star image shrinks on the pyramid pin, the wavefront estimation signal-to-noise ratio can considerably improve. This has been shown to translate into a gain in limiting magnitude when compared with the Shack-Hartmann wavefront sensor, in which the sampling on the wavefront is performed before the light is split into four quadrants, which does not allow the quality of the focused spot to increase. Since this property is strictly related to the size of the re-imaged spot on the pyramid pin, the better the wavefront correction, the higher the gain. Aims. The goal of this paper is to extend the descriptive and analytical computation of this gain that was given in a previous paper, to partial wavefront correction conditions, which are representative for most of the wide field correction adaptive optics systems. Methods. After focusing on the low Strehl ratio regime, we analyze the minimum spatial sampling required for the wavefront sensor correction to still experience a considerable gain in sensitivity between the pyramid and the Shack-Hartmann wavefront sensors. Results. We find that the gain can be described as a function of the sampling in terms of the Fried parameter.

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“…Sky coverage is poor with only natural guide stars (NGS). The use of LGS improves the performances, but full sky coverage is still not achieved, especially at visible wavelengths [2]. The main problem is the undetermination of the wavefront tip-tilt.…”

Section: Introductionmentioning

confidence: 99%

End-to-end model for the Polychromatic Laser Guide Star project (ELP-OA)

Meilard

Foy

Tallon

et al. 2010

1st AO4ELT Conference - Adaptive Optics for Extremely Large Telescopes

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Abstract. In order to optimize parameters for the ELP-OA project, we have developped an end-to-end model from the laser emission to the computation of the tip-tilt Strehl ratio. ELP-OA aims at measuring the wavefront tip-tilt from the laser guide star alone, without any natural source. It relies on 2-photon excitation of mesospheric sodium, achieved by two laser chains (at 569nm and 589nm) delivering 34W average power each. The modeled twin laser beam is launched with a three aperture interferometer that produces an interferometric pattern in the mesosphere after the passage through a Kolmogorov phase screen. The backscattered flux is computed with an optical Bloch equations code [1]. The result of this excitation is an emission at different wavelengths. The wavelengths retained are 330nm, 569nm and 589nm. An image of this polychromatic LGS (PLGS) is computed, as it will be observed at the 1.52m telescope at the Observatoire de Haute-Provence (OHP), through a Kolmogorov phase screen again. Then we simulate the adaptive optics device (adapted from ONERA's BOA AO) at the telescope except for tip-tilt. Air refraction index variations due to atmospheric turbulence create a dispersion between 330nm and 569nm, and the tip-tilt value is derived from this dispersion. This dispersion between 330nm and 569nm is measured using a correlation method and a sub-pixel interpolation. The calculation of the relative position between two simultaneous images at 330nm and 569nm gives a position error, from which the Strehl ratio for tip-tilt is deduced. At this time, we have got Strehl ratios up to approximatively 35% at 550nm, for tilt corrected r 0 = 10cm and a projector baseline of 70cm.

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Laser Guide Star for 3.6- and 8-m telescopes: Performance and astrophysical implications

Cited by 37 publications

References 17 publications

Galaxies in southern bright star fields

Galaxies in southern bright star fields

Expected gain in the pyramid wavefront sensor with limited Strehl ratio

End-to-end model for the Polychromatic Laser Guide Star project (ELP-OA)

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