VASAO is an ambitious project that explores new conceptual direction in the field of astronomical adaptive optics. In
the era of 8 meter and larger telescopes, and their instrument costs and telescope time pressure, there is a natural niche
for such ground-breaking conceptual development in the 4 meter class telescope. The aim of VASAO is to provide
diffraction limited imaging in the visible with 100% sky coverage; the challenge (but potential rewards) arises from the
simultaneity of these requirements. To this end, CFHT is conducting a feasibility study based on the polychromatic
guide star concept (Foy et al., 1995 [4]) coupled with a high order curvature AO system, presented in this paper.
A number of experiments have been started (or carried out) to study the challenges and limits of the techniques involved
in an operational setting; these include the FlyEyes detector, and a polychromatic tip-tilt test on natural stars.
Because such a project straddles such a fine line between facility instrument and experimental facility, careful thought
has to be given to the balance between modes of operations and potential astrophysical targets.
A European Laser Guide Star (LGS) test facility is proposed for the 4.2m William Herschel Telescope (WHT) on La
Palma. It will test the next-generation Adaptive Optics (AO) LGS technologies to aid risk mitigation of Extremely Large
Telescope (ELT) LGS AO systems. In particular, critical scaling of current LGS AO technologies to ELT dimensions
will be tested. For example, experiments addressing increased spot elongation, cone effect and the order of correction
required.
A pan-European consortium proposes to construct test facility infrastructure on the WHT for a number of risk mitigating
experiments. The infrastructure includes the construction of a Nasmyth platform based controlled environment 'Ground-based
Adaptive optics Innovative Laboratory' (GRAIL), an experimental test environment 'Testbed integration facility'
(TIF) and some common-experiment equipment such as the Common Re-Imaging AO System.
Experiments that are proposed for this facility cover the areas of laser technologies, spot elongation, LGS wavefront
sensing, parallel launch concepts, Multi-Object AO, atmospheric characterisation, co-phasing and real-time control
system risk mitigation.
The differential atmospheric tip-tilt can be measured using a Polychromatic Laser Guide Star. A two photon excitation
has been proposed. It consists in exciting the 4D5/2 level of mesospheric sodium atoms with two identical lasers operating
at 589 nm and 569 nm. With two modeless lasers of 2×15W at the mesosphere level, this scheme will produce a returned
flux at 330 nm of about 4×104 photons/s/m2. Thanks to our modeless laser, we propose a new method which consists in
exciting directly the 4P3/2 sodium level with one photon excitation, using a single laser operating at 330 nm. This
solution was previously rejected probably because of strong saturation problems using single longitudinal mode lasers.
We show that 1 W modeless laser at 330 nm can produce the same returned flux at 330 nm. This solution will save at
least 400 k€ of equipment. Moreover, our new method is very promising in terms of simplicity but also in terms of flux
because the returned flux above will probably be not sufficient for getting a good Strehl ratio. We propose very efficient
solid state laser systems for the production of tens of watts at 330 nm.
Building on an extensive and successful experience in Adaptive Optics (AO) and on recent developments made in its funding nations, the Canada-France-Hawaii-Telescope Corporation (CFHT) is studying the VASAO concept: an integrated AO system that would allow diffraction limited imaging of the whole sky in the visible as well as in the infrared. At the core of VASAO, Pueo-Hou (the new Pueo) is built on Pueo, the current CFHT AO bonnette. Pueo will be refurbished and improved to be able to image the isoplanetic field at 700 nm with Strehl ratios of 30% or better, making possible imaging with a resolution of 50 milliarcseconds between 500 and 700nm, and at the telescope limit of diffraction above. The polychromatic tip-tilt laser guide star currently envisioned will be generated by a single 330nm mode-less laser, and the relative position of the 330nm and 589nm artificial stars created on the mesosphere by the 330nm excitation of the sodium layer will be monitored to provide the atmospheric tip-tilt along the line of sight, following the philosophy developed for the ELP-OA project. The feasibility study of VASAO will take most of 2006 in parallel with the development of a science case making the best possible use of the unique capabilities of the system, If the feasibility study is encouraging, VASAO development could start in 2007 for a full deployment on the sky by 2011-2012.
A theoretical time-dependent analysis of a high-average power copper HyBrID laser is proposed, pointing out the time-varying properties of beam quality parameters and brightness. Numerical data are compared to experimental measurements performed on a 80 W average power copper HyBrID laser. A significant improvement of the beam quality with time is put in evidence.
The main aim of this paper is to demonstrate potential materials and fibres for 589 nm and 569 nm high-power laser sources, which can be developed by using rare-earth ions (Eu3+, Sm3+) as dopants in glass fibres. In this paper, we also discuss the advantages of using high-power excitation sources: for example the NIR semiconductors at 980 nm and 800 nm and high-power Yb-fibre laser based systems for high-power upconverted lasers. In this context the spectroscopic properties of Eu3+-Er3+ and Sm3+- based glass and fibre systems are also discussed in bulk oxide glasses and in fibre geometries for designing lasers at 589 nm and 569 nm for adaptive optics.
We briefly recall the principle of the polychromatic laser guide star, which aims at providing measurements of the tilt of incoming wavefronts with a 100% sky coverage, We describe the main results of the feasibility study of this concept undertaken within the ELP-OA porgramme. We finally summarize our plans for a full demonstrator at Observatoire de Haute-Provence.
We describe the current status of the ELP-OA project in which we try to demonstrate in practice that it is possible to measure the tilt of a wave front using only a polychromatic laser guide star and no natural guide star. The first phase of ELP-OA, consisting of feasibility experiments, has recently been completed successfully. This paper provides an overview over the results of this first phase and over the continuation of the ELP-OA project.
Adaptive optics at astronomical telescopes aims at correcting in real time the phase corrugations of incoming wavefronts caused by the turbulent atmosphere, as early proposed by Babcock. Measuring the phase errors requires a bright source located within the isoplanatic patch of the program source. The probability that such a reference source exists is a function of the wavelength, of the required image quality (Strehl ratio), of the turbulence optical properties, and of the direction of the observation. It turns out that the sky coverage is disastrously low in particular in the visible wavelength range where, unfortunately, the gain in spatial resolution brought by adaptive optics is the largest. Foy and Labeyrie have proposed to overcome this difficulty by creating an artificial point source in the sky in the direction of the observation relying on the backscattered light due to a laser beam. This laser guide star (hereinafter referred to as LGS) can be bright enough to allow us to accurately measure the wavefront phase errors, except for two modes which are the piston (not relevant in this case) and the tilt. Pilkington has emphasized that the round trip time of the laser beam to the mesosphere, where the LGS is most often formed, is significantly shorter than the typical tilt coherence time; then the inverse-return-of-light principle causes deflections of the outgoing and the ingoing beams to cancel. The apparent direction of the LGS is independent of the tilt. Therefore the tilt cannot be measured only from the LGS. Until now, the way to overcome this difficulty has been to use a natural guide star to sense the tilt. Although the tilt is sensed through the entire telescope pupil, one cannot use a faint source because $APEX 90% of the variance of the phase error is in the tilt. Therefore, correcting the tilt requires a higher accuracy of the measurements than for higher orders of the wavefront. Hence current adaptive optics devices coupled with a LGS face low sky coverage. Several methods have been proposed to get a partial sky coverage for the tilt. The only one providing us with a full sky coverage is the polychromatic LGS (hereafter referred to as PLGS). We present here a progress report of the R&D program Etoile Laser Polychromatique et Optique Adaptative (ELP-OA) carried out in France to develop the PLGS concept. After a short recall of the principles of the PLGS, we will review the goal of ELP-OA and the steps to get over to bring it into play. We finally shortly described the effort in Europe to develop the LGS.
Adaptive optics at astronomical telescopes aims at correcting in real time the phase corrugations of incoming wavefronts caused by the turbulent atmosphere, as early proposed by Babcock. Measuring the phase errors requires a bright source, which is located within the isoplanatic patch of the program source. The probability that such a reference source exists is a function of the wavelength of the observation, of the required image quality (Strehl ratio), of the turbulence optical properties, and of the direction of the observation. Several papers have addressed the problem of the sky coverage as a function of these parameters (see e.g.: Le Louarn et al). It turns out that the sky coverage is disastrously low in particular in the short (visible) wavelength range where, unfortunately, the gain in spatial resolution brought by adaptive optics is the largest. Foy and Labeyrie have proposed to overcome this difficulty by creating an artificial point source in the sky in the direction of the observation relying on the backscattered light due to a laser beam. This laser guide star (hereafter referred to as LGS) can be bright enough to allow us to accurately measure the wavefront phase errors, except for two modes which are the piston (which is not relevant in this case) and the tilt. Pilkington has emphasized that the round trip time of the laser beam to the mesosphere, where the LGS is most often formed, is significantly shorter than the typical tilt coherence time; then the inverse-return- of-light principle causes deflections of the outgoing and the ingoing beams to cancel. The apparent direction of the LGS is independent of the tilt. Therefore the tilt cannot be measured only from the LGS. Until now, the way to overcome this difficulty has been to use a natural guide star to sense the tilt. Although the tilt is sensed through the entire telescope pupil, one cannot use a faint source because approximately equals 90% of the variance of the phase error is in the tilt. Therefore, correcting the tilt requires a higher accuracy of the measurements than for higher orders of the wavefront. Hence current adaptive optics devices coupled with a LGS face low sky coverage. Several methods have been proposed to get a partial or total sky coverage for the tilt, such as the dual adaptive optics concept, the elongation perspective method, or the polychromatic LGS (hereafter referred to as PLGS). We present here a progress report of the R&D program Etoile Laser Polychromatique et Optique Adaptative (ELP-OA) carried out in France to develop the PLGS concept. After a short recall of the principles of the PLGS, we will review the goal of ELP-OA and the steps to get over to bring it into play.
KEYWORDS: Laser guide stars, Telescopes, Sodium, Stars, Global system for mobile communications, Oscillators, Wavefronts, Adaptive optics, Calibration, Photometry
We present results from measurements of the return flux from a polychromatic sodium laser guide star produced in Pierrelatte, France during the PASS-2 experiment. In the experiment, photometry of light at 330, 569, 589, and 589.6 nm emitted by mesospheric sodium under two-color laser excitation (569 and 589 nm) was performed. The variation of oscillator and laser configurations as well as simultaneous measurements of the atmospheric coherence length and the mesospheric sodium density permit a comparison of the results with atomic physics models. Using the results, we can determine the setup that produces the maximum return flux from the polychromatic laser guide star. The knowledge gained will be used to aid the ELP- OA project, which has as its goal the design, testing, and implementation of an adaptive optics system that uses a polychromatic laser guide star for wave front tilt measurements.
PASS-2 is an experiment designed to perform photometry of the polychromatic laser guide star. The tilt of an atmospherically distorted wave front coming from an astronomical object cannot be determined with a monochromatic laser guide star. If it is possible to produce a laser guide star that emits light at different wavelengths, however, the tilt can be determined from the measurable differences between the tilts at the different wavelengths. This is the concept of the polychromatic laser guide star. The PASS-2 experiment is a step towards an implementation of an adaptive optics system that uses a polychromatic laser guide star for the wave front tilt measurement. The goal of the experiment is to validate the feasibility of a polychromatic laser guide star adaptive optics system and to determine the laser parameters that produce the optimal return flux from the polychromatic laser guide star. To this end, the return flux from the polychromatic laser guide star at 330 and 589.6 nm will be measured as a function of laser parameters, atmospheric conditions, and the density of the mesospheric sodium layer.
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