Atmospheric refractivity effects on mid-infrared ELT adaptive optics

We discuss the effect of atmospheric dispersion on the performance of a mid-infrared adaptive optics assisted instrument on an extremely large telescope (ELT). Dispersion and atmospheric chromaticity is generally considered to be negligible in this wavelength regime. It is shown here, however, that with the much-reduced diffraction limit size on an ELT and the need for diffraction-limited performance, refractivity phenomena should be carefully considered in the design and operation of such an instrument. We include an overview of the theory of refractivity, and the influence of infrared resonances caused by the presence of water vapour and other constituents in the atmosphere. `Traditional' atmospheric dispersion is likely to cause a loss of Strehl only at the shortest wavelengths (L-band). A more likely source of error is the difference in wavelengths at which the wavefront is sensed and corrected, leading to pointing offsets between wavefront sensor and science instrument that evolve with time over a long exposure. Infrared radiation is also subject to additional turbulence caused by the presence of water vapour in the atmosphere not seen by visible wavefront sensors, whose effect is poorly understood. We make use of information obtained at radio wavelengths to make a first-order estimate of its effect on the performance of a mid-IR ground-based instrument. The calculations in this paper are performed using parameters from two different sites, one `standard good site' and one `high and dry site' to illustrate the importance of the choice of site for an ELT.Comment: 11 pages, to be published in SPIE Proceedings vol. 7015, Adaptive Optics Systems, eds. N. Hubin, C.E. Max and P.L. Wizinowich, 200

Refractive Index of Humid Air in the Infrared: Model Fits

The theory of summation of electromagnetic line transitions is used to tabulate the Taylor expansion of the refractive index of humid air over the basic independent parameters (temperature, pressure, humidity, wavelength) in five separate infrared regions from the H to the Q band at a fixed percentage of Carbon Dioxide. These are least-squares fits to raw, highly resolved spectra for a set of temperatures from 10 to 25 C, a set of pressures from 500 to 1023 hPa, and a set of relative humidities from 5 to 60%. These choices reflect the prospective application to characterize ambient air at mountain altitudes of astronomical telescopes.Comment: Corrected exponents of c0ref, c1ref and c1p in Table

VLT/NACO astrometry of the HR8799 planetary system. L'-band observations of the three outer planets

HR8799 is so far the only directly imaged multiple exoplanet system. The orbital configuration would, if better known, provide valuable insight into the formation and dynamical evolution of wide-orbit planetary systems. We present L'-band observations of the HR8799 system obtained with NACO at VLT, adding to the astrometric monitoring of the planets HR8799b, c and d. We investigate how well the two simple cases of (i) a circular orbit and (ii) a face-on orbit fit the astrometric data for HR8799d over a total time baseline of ~2 years. The results indicate that the orbit of HR8799d is inclined with respect to our line of sight, and suggest that the orbit is slightly eccentric or non-coplanar with the outer planets and debris disk.Comment: 5 pages, 4 figures, 1 table, accepted for publication in A\&A. Updated version includes minor changes made in the proof

Simple DFT-LSDA modeling of the molecular-like aspects of ultra-thin film properties

Ordered ultra-thin films (UTF`s) are atomic n-layers (n = 1,2,3,...) with translational symmetry in-plane and molecular-like inter-planar spacings. Though commonly used (especially at relatively large n-values) as models of crystalline surfaces, they are intrinsically interesting and of growing technological significance as the basic building blocks of multi-layer electronic devices. Predicting the structure and properties of even a simple diatomic 1-layer means addressing aspects of molecular binding (and boundary conditions) in the context of an extended, periodically bounded system. At the level of refinement provided by the local spin density approximation to Density Functional Theory, the baseline standard of today`s predictive, chemically specific solid-state calculations, a number of technical and fundamental issues arise. The authors focus on treatment of the isolated atoms, on basis sets, and on numerical precision, as illustrated by the Fe atom and BN 1- and 2-layer calculations. Computational requirements are illustrated by a brief summary of recently completed calculations on crystalline sapphire, {alpha}-Al{sub 2}O{sub 3}, which used the same code

Representations of spectral coordinates in FITS

Greisen & Calabretta describe a generalized method for specifying the coordinates of FITS data samples. Following that general method, Calabretta & Greisen describe detailed conventions for defining celestial coordinates as they are projected onto a two-dimensional plane. The present paper extends the discussion to the spectral coordinates of wavelength, frequency, and velocity. World coordinate functions are defined for spectral axes sampled linearly in wavelength, frequency, or velocity, linearly in the logarithm of wavelength or frequency, as projected by ideal dispersing elements, and as specified by a lookup table.Comment: 25 pages, 5 figure

A new concept for the combination of optical interferometers and high-resolution spectrographs

The combination of high spatial and spectral resolution in optical astronomy enables new observational approaches to many open problems in stellar and circumstellar astrophysics. However, constructing a high-resolution spectrograph for an interferometer is a costly and time-intensive undertaking. Our aim is to show that, by coupling existing high-resolution spectrographs to existing interferometers, one could observe in the domain of high spectral and spatial resolution, and avoid the construction of a new complex and expensive instrument. We investigate in this article the different challenges which arise from combining an interferometer with a high-resolution spectrograph. The requirements for the different sub-systems are determined, with special attention given to the problems of fringe tracking and dispersion. A concept study for the combination of the VLTI (Very Large Telescope Interferometer) with UVES (UV-Visual Echelle Spectrograph) is carried out, and several other specific instrument pairings are discussed. We show that the proposed combination of an interferometer with a high-resolution spectrograph is indeed feasible with current technology, for a fraction of the cost of building a whole new spectrograph. The impact on the existing instruments and their ongoing programs would be minimal.Comment: 27 pages, 9 figures, Experimental Astronomy; v2: accepted versio