Study of the atmospheric refraction in a single mode instrument - Application to AMBER/VLTI

International audienceThis paper presents a study of the atmospheric refraction and its effect on the light coupling efficiency in an instrument using single-mode optical fibers. We show the analytical approach which allowed us to assess the need to correct the refraction in J- and H-bands while observing with an 8-m Unit Telescope. We then developed numerical simulations to go further in calculations. The hypotheses on the instrumental characteristics are those of AMBER (Astronomical Multi BEam combineR), the near infrared focal beam combiner of the Very Large Telescope Interferometric mode (VLTI), but most of the conclusions can be generalized to other single-mode instruments. We used the software package caos (Code for Adaptive Optics Systems) to take into account the atmospheric turbulence effect after correction by the ESO system MACAO (Multi-Application Curvature Adaptive Optics). The opto-mechanical study and design of the system correcting the atmospheric refraction on AMBER is then detailed. We showed that the atmospheric refraction becomes predominant over the atmospheric turbulence for some zenith angles z and spectral conditions: for z larger than 30° in J-band for example. The study of the optical system showed that it allows to achieve the required instrumental performance in terms of throughput in J- and H-bands. First observations in J-band of a bright star, alpha Cir star, at more than 30° from zenith clearly showed the gain to control the atmospheric refraction in a single mode instrument, and validated the operating law

VLTI-AMBER observations of Eta Carinae with high spatial resolution and spectral resolutions of 1,500 and 10,000

We present the first interferometric NIR observations of the LBV ? Carinae with high spectral resolution. The observations were carried out with three 8.2 m VLTI Unit Telescopes in the K-band. The raw data are spectrally dispersed interferograms obtained with spectral resolutions of 1,500 (MR-K mode) and 12,000 (HR-K mode). The observations were performed in the wavelength range around both the He I 2.059 ?m and the Br? 2.166 ?m emission lines. The spectrally dispersed AMBER interferograms allow the investigation of the wavelength dependence of the visibility, differential phase, and closure phase of ? Car. In the K-band continuum, a diameter of 4.0+/-0.2 mas (Gaussian FWHM) was measured for ? Car's optically thick wind region, whereas the Br? and He I emission line regions are larger. If we fit Hillier et al. model visibilities to the observed AMBER visibilities, we obtain 50% encircled-energy diameters of 4.3, 6.5 and 9.6 mas in the 2.17 ?m continuum, the He I, and the Br?emission lines, respectively. In the continuum near the Br? line, an elongation along a position angle of 128° +/- 15° was found, consistent with previous VLTI/VINCI measurements. We find good agreement between the measured visibilities and the predictions of the radiative transfer model of Hillier et al. For the interpretation of the non-zero differential and closure phases measured within the Br? line, we present a simple geometric model of an inclined, latitude-dependent wind zone. Our observations support theoretical models of anisotropic winds from fast-rotating, luminous hot stars with enhanced high-velocity mass loss near the polar regions