X-shooter is the first 2nd generation instrument of the ESO Very Large Telescope (VLT). It is a very efficient, single-target, intermediate-resolution spectrograph that was installed at the ...Cassegrain focus of UT2 in 2009. The instrument covers, in a single exposure, the spectral range from 300 to 2500 nm. It is designed to maximize the sensitivity in this spectral range through dichroic splitting in three arms with optimized optics, coatings, dispersive elements and detectors. It operates at intermediate spectral resolution (R ~ 4000−17 000, depending on wavelength and slit width) with fixed échelle spectral format (prism cross-dispersers) in the three arms. It includes a 1.8″ × 4″ integral field unit as an alternative to the 11′′ long slits. A dedicated data reduction package delivers fully calibrated two-dimensional and extracted spectra over the full wavelength range. We describe the main characteristics of the instrument and present its performance as measured during commissioning, science verification and the first months of science operations.
Context. A new challenging adaptive optics (AO) system, called multi-object adaptive optics (MOAO), has been successfully demonstrated on-sky for the first time at the 4.2 m William Herschel ...Telescope, Canary Islands, Spain, at the end of September 2010. Aims. This system, called CANARY, is aimed at demonstrating the feasibility of MOAO in preparation of a future multi-object near infra-red (IR) integral field unit spectrograph to equip extremely large telescopes for analysing the morphology and dynamics of high-z galaxies. Methods. CANARY compensates for the atmospheric turbulence with a deformable mirror driven in open-loop and controlled through a tomographic reconstruction by three widely separated off-axis natural guide star (NGS) wavefront sensors, which are in open loop too. We compared the performance of conventional closed-loop AO, MOAO, and ground-layer adaptive optics (GLAO) by analysing both IR images and simultaneous wave-front measurements. Results. In H-band, Strehl ratios of 0.20 are measured with MOAO while achieving 0.25 with closed-loop AO in fairly similar seeing conditions (r0 ≈ 15 cm at 0.5 μm). As expected, MOAO has performed at an intermediate level between GLAO and closed-loop AO.
Context. To find evidence one day of life on extra solar planets, one will have to directly detect photons of the exoplanet to obtain spectra and to look for specific spectroscopic features. One ...possible technique is dark fringe interferometry with several telescopes in space. This type of interferometry requires an achromatic p phase shift in one arm of an interferometer. We have already presented a concept of a quasi-achromatic phase shifter that is made of two cellular mirrors in which each cell position and phase shift is specific, so that the behavior of the null depth as a function of the wavelength is flat within a broad range. Aims. We want to experimentally validate this concept of an achromatic phase shifter and measure its performance in two different cases: a transmissive mask, which is made in bulk optics that are machined with ion etching and a reflective one, which is made by using a commercial segmented deformable mirror that is properly controlled. Methods. We assembled a dedicated optical bench, nicknamed DAMNED, to assess the concept and characterize its performance in the visible and to determine the limitations of this phase shifter. We analyze its performance by comparing the experimental results with a numerical instrument model. Results. We tested several transmissive masks and a reflective one. We reached an attenuation of about 2 x 10 super(-3) with a white source (DeltaLambda = 430 to 830 nm) that proved to be the actual achromatic behavior of the phase shifter, despite its modest value. Extrapolated to mid-IR, its performance would be within typical specifications of a space mission as Darwin.
ABSTRACT
The definition and optimization studies for the Gaia satellite spectrograph, the ‘radial velocity spectrometer’ (RVS), converged in late 2002 with the adoption of the instrument baseline. ...This paper reviews the characteristics of the selected configuration and presents its expected performance. The RVS is a 2.0 × 1.6 degree integral field spectrograph, dispersing the light of all sources entering its field of view with a resolving power R=λ/Δλ= 11 500 over the wavelength range 848, 874 nm. The RVS will continuously and repeatedly scan the sky during the 5‐yr Gaia mission. On average, each source will be observed 102 times over this period. The RVS will collect the spectra of about 100–150 million stars up to magnitude V≃ 17–18. At the end of the mission, the RVS will provide radial velocities with precisions of ∼2 km s−1 at V= 15 and ∼15–20 km s−1 at V= 17, for a solar‐metallicity G5 dwarf. The RVS will also provide rotational velocities, with precisions (at the end of the mission) for late‐type stars of σvsin i≃ 5 km s−1 at V≃ 15 as well as atmospheric parameters up to V≃ 14–15. The individual abundances of elements such as silicon and magnesium, vital for the understanding of Galactic evolution, will be obtained up to V≃ 12–13. Finally, the presence of the 862.0‐nm diffuse interstellar band (DIB) in the RVS wavelength range will make it possible to derive the three‐dimensional structure of the interstellar reddening.
Context. Coronagraphic techniques are required to observe substellar mass companions close to nearby bright stars by direct imagery. Phase mask coronagraphs are particularly interesting because they ...give access to the innermost regions. While the principle of the first such concept was validated experimentally a decade ago, the achieved brightness attenuation was too low to be conclusive, probably due to the imperfect thickness profile of the mask. Aims. We have manufactured and tested a second-generation Roddier & Roddier coronagraph in preparation for the development of more elaborate phase mask designs, planned to be used in the future European Extremely Large Telescope. Methods. A monolithic phase mask was made by ion beam machining. Experimentally obtained coronagraphic images were compared with simulated images. Results. Good agreement with theory was obtained. A peak attenuation of 216 was achieved, and a contrast of ~10-5 was measured at 5.7 $\lambda/D$. The results exploring contrasts obtained at different distances from the star for different mask dimensions are particularly interesting, confirming predictions made in the literature.
Objectives: The purpose of this study is to determine, on a quantitative basis, the kinetics of ionization of pure and alloyed titanium in various electrolytic media including commercial dental gels ...containing fluoride ions (Fluogel).
Methods: The experimental method is a linear sweep voltammetric analysis, making use of a rotating disc electrode. Several successive analyses of different titanium samples are carried out, particularly as a function of the nature of the electrolytic solutions. The current corrosion
I
cor is measured by the determination of linear polarization resistence
R
p.
Results:
R
p and
I
cor measurements by the voltammetric and analysis confirm the excellent resistance of titanium in physiological solutions; they also show that a fluoride-containing dental gel, which is a fluoridated and acidic preparation, is significantly harmful with respect to the corrosion process of titanium.
Conclusion: The results of this study must draw our attention towards the use of prophylactic gels in which pH is acidic. Indeed titanium suffers an important corrosion process in fluoridated acidic media. Recommendations for the prescription and for the optimized composition of the dental gels could be derived.
Enantiopure trans-ethynyl N-tert-butanesulfinylaziridines (R(S))-6 were prepared in good to excellent yields by the condensation of the racemic allenylzinc species 1 derived from ...3-chloro-1-trimethylsilylpropyne onto the corresponding enantiopure N-tert-butanesulfinimines (R(S))-5. The absolute stereochemistry of enantiopure N-tert-butanesulfinylaziridines (R(S))-6 was shown to be (R(S),2R,3R) and results from a chelate-type transition state in which the zinc atom of allenylzinc 1 is coordinated by both the nitogen and the oxygen atoms of the imine. Further removal of the N-tert-butanesulfinyl auxiliary of alkyl 3-substituted and 3,3-disubstituted ethynyl N-tert-butanesulfinylaziridines (R(S))-6 could be achieved by treatment with HCl in MeOH affording the corresponding deprotected aziridines (2R,3R)-9 and (2R)-9 respectively as enantiomerically pure compounds.
Substituted vinylidene zinc carbenoids undergo a Fritsch−Buttenberg−Wiechell rearrangement. The migratory aptitude of the two groups R/R1 was studied by 13C labeling experiments and depends on the ...degree of substitution in R and R1 groups and on the presence of an oxygen atom in the allylic position.
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