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The Four-Laser Guide Star Facility: Design Considerations and System Implementation

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The Four-Laser Guide Star Facility: Design Considerations and System Implementation Adv. Opt. Techn. 2014; 3(3): 345–361 Research Article Domenico Bonaccini Calia, Wolfgang Hackenberg, Ronald Holzlöhner*, Steffan Lewis and Thomas Pfrommer The Four-Laser Guide Star Facility: Design considerations and system implementation Abstract: Ground-based optical telescopes, in particular object, can be measured with wavefront sensors and ana- large ones, require adaptive optics to overcome the atmos- lyzed by an adaptive optics (AO) system [1, 2]. A correction pheric seeing limit due to turbulence. Correcting the dis- signal is sent to a deformable mirror to correct and flatten torted wavefront necessitates bright stars in the field of the wavefront on atmospheric timescales that are on the view. The sky coverage can be greatly increased by using order of one to several milliseconds. artificial sodium laser guide stars in addition to natu- There is a lack of sufficiently bright stars in the sky that ral guide stars. We describe the underlying physics and can serve as natural guide stars (NGS) for AO. Therefore, technical considerations relevant to such systems before one has to resort to artificial beacons that can be gener- discussing the design of the four-laser guide star facility ated by powerful lasers. The telescope includes wavefront (4LGSF) which is currently under development for the sensors that either image the Rayleigh scattering of those ESO Very Large Telescope (VLT) on Cerro Paranal, Chile. lasers (Rayleigh LGS) or else the resonant scattering from The focus is upon the justification of the requirements and alkali atoms in the mesosphere, typically sodium (sodium their technical solution. LGS). The resolution of large telescopes without AO is Keywords: adaptive optics; adaptive optics facility; ELT; limited by atmospheric turbulence (seeing-limited). In laser guide star; 4LGSF; mesospheric sodium. this regime, the image contrast relative to the sky back- ground at constant exposure time becomes almost inde- pendent of the telescope diameter. When imaging the DOI 10.1515/aot-2014-0025 Received March 25, 2014; accepted May 9, 2014 AO-corrected light from point-like distant objects in the diffraction limit, the dependence of the size of the Airy disk on the telescope aperture results in a concentration of light in the image and a contrast varying in proportion 1 Introduction to the square of the telescope aperture [3]. Consequently, a number of large telescopes including Unit Telescope 4 Laser guide star (LGS) adaptive optics can enable large of the Very Large Telescope (VLT) are equipped with laser ground-based telescopes to observe near the diffraction guide star adaptive optics systems, and the next gen- limit in the optical and infrared wavelength regimes any- eration of extremely large 30–40 m-class telescopes are where in the sky. Optical turbulence in the lower 15–20 km being designed for adaptive optics from the start. Laser of the atmosphere distorts incoming light from a science guide stars can greatly extend the sky coverage of these object. The emitted wavefront from a sufficiently bright telescopes. guide star within the field of view of the telescope, whose The idea of using adaptive optics with artificial light passes through the same turbulence as the science guide stars, produced in the mesospheric sodium layer, was first reported in classified publications by Happer *Corresponding author: Ronald Holzlöhner, European Southern and MacDonald [4] and later independently by Foy and Observatory (ESO), Karl-Schwarzschild-Str. 2, 85748 Garching, Laberie [5] for public astronomical use. The latter paper Germany, e-mail: [email protected] prompted the U.S. Air Force to declassify their laser guide Domenico Bonaccini Calia, Wolfgang Hackenberg, Steffan Lewis star AO work, performed in the 1980s and 1990s [6–8]. and Thomas Pfrommer: European Southern Observatory (ESO), Karl- Schwarzschild-Str. 2, 85748 Garching, Germany These early publications on LGS-AO technologies and the results marked the start of a more widespread investiga- www.degruyter.com/aot tion and deployment of this technique for astronomy. © 2014 THOSS Media and De Gruyter 346 D. Bonaccini Calia et al.: Four-Laser Guide Star design and implementation The first non-military sodium laser guide star systems from space that ablate in the thermosphere and subse- for astronomy were truly experimental systems, installed quently recombine with electrons from the ionosphere in the 1990s [9–11]. Built on this initial experience, there is [15]. The average sodium abundance in meteorites is a range of sodium LGS systems used for routine astronomi- about 0.6%, and while drifting downwards, the increas- cal science operation today, most employing a single laser ing air density at around 80 km allows sodium combin- beam and most of them using short pulses; see [12] for a ing with oxygen radicals from ozone molecules to form summary. A notable sodium LGS system with multiple sodium hydroxyl and other molecules. This lower end beams (five) is the GeMS multiple LGS system at Gemini of the atomic sodium layer is usually defined by a sharp South, commissioned in 2012/2013 [13]. Another multi- boundary, which points to a fast reaction rate when the LGS system, the four-laser guide star facility (4LGSF) cur- air density reaches a critical limit [15]. The sodium dissi- rently under development for installation on UT4 of the pation rate is also strongly affected by the MLT tempera- VLT [14], is the subject of this article. tures, which are in the range of 180–200 K representing The remainder of this paper is organized as follows: the coldest region in the atmosphere. Because the tem- Section 2 discusses the laser beam of guide star systems perature of the MLT varies seasonally and with latitude, and its interaction with the sodium layer. Section 3 this lower boundary varies in extreme cases by 10 km, but includes an overview of existing LGS laser source types, usually lies between 80 km and 85 km [16]. Section 4 presents the design of the 4LGSF, and finally, Figure 1 shows an example of the sodium layer cross Section 5 contains an outlook of future LGS technology. section as detected by a high-resolution lidar (light detection and ranging) system, located near Vancouver, Canada, at the University of British Columbia (UBC) Large 2 Optical interactions affecting the Zenith Telescope [17]. The horizontal axis shows the time evolution, while the vertical axis is the height, starting at laser guide star 75 km above mean sea level (MSL). The color represents the detected raw photon counts; see [16] for more details 2.1 Sodium layer and guide star geometry on the sodium layer. The lasers of an LGS AO system excite the atomic The upper mesosphere and lower thermosphere (MLT) sodium, resulting in resonant scattering of photons at a below the ionosphere extending from 80 to 120 km pro- wavelength of 589.159 nm. The large product of abundance vides favorable conditions for atomic metals to exist in and scattering cross section makes sodium the technically relatively high abundance, yet at still traceable fractions most interesting alkali in the MLT. (order 10–8 compared to the number of air molecules at this The detected aberrations of the returned LGS wave- height). Such elements are deposited by micrometeorites front that have been induced by propagating through --- UBC LIDAR at LZT (49 N, 123 W) on 20100725 --- 110 1200 105 1000 100 800 MSL (km) 95 ve 600 90 Height abo 400 85 80 200 75 0 45678910 UTC (h) Figure 1 Example of a sodium layer detected with the UBC (UTC–9h) lidar during 1 night. D. Bonaccini Calia et al.: Four-Laser Guide Star design and implementation 347 the lower atmosphere enable the AO system to acquire a the sodium layer and fluctuates rapidly, adding an error real-time snapshot of the turbulence and mitigate it. The term in the reconstruction of the wavefront. low frequency (timescale seconds) focus term of the tur- Figure 2 shows an example of an elongated LGS bulence is usually sensed with the help of one or several plume generated by the 5 W sodium LGS on UT4 of the NGS, while the high frequency terms are sensed with the VLT, observed at a distance of 2.3 km from the launch LGS signal. telescope. The angular extent of the LGS plume in this If the variations in the sodium centroid are sufficiently example is about 14 arcmin. large on small timescales (subseconds to milliseconds), Because the sodium layer at about 90 km height is not they cause a performance loss of the AO system, because at virtual infinity like the science objects, the light cone, the usually fainter NGS signal takes longer to provide the captured by the primary mirror, does not probe the full true focus information. In the wavefront sensor signal, cylinder in the turbulent atmosphere that the starlight variations of the sodium centroid cannot be distinguished has to pass to reach the telescope pupil [22]. This effect is from focus variations arising from atmospheric turbu- called the cone-effect and can be overcome by deploying lence. The resulting performance degradation of LGS AO more than one laser guide star. Projecting multiple LGS in systems may be relevant for extremely large telescopes, an asterism around the science field allows the AO system as the focus indetermination grows with the square of the to cover the entire cylinder, i.e., determine the 3D atmos- telescope aperture D [18] pheric turbulence and mitigate it with the use of several 2 deformable mirrors, each one conjugate to a turbulent D cos χ σ Focus = ∆a, (1) layer at a different altitude [23, 24]. 16 3(aa− ) 2 0 Multiple lasers can be launched from the side of the primary mirror or from behind the secondary mirror.
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