Search

Correction of telluric absorption lines is important during the data reduction of spectroscopic observations of astronomical targets. The popular tool molecfit creates synthetic telluric transmission spectra based on the LBLRTM radiative transfer code, the HITRAN database, available atmospheric profiles (temperature, humidity, pressure variation with altitude), and instrumental line spread functions. Significantly improved accuracy is reached by using atmospheric profiles obtained at the same time and location as the science observations. Provided some conditions are met, the addition of a radiometer providing temperature and humidity profiles every few minutes as well as regular twilight spectra of telluric stars processed by molecfit to measure the atmospheric abundance of a few molecules allow blind telluric line correction, i.e., one can provide the transmission spectrum of the atmosphere for each observation. As a by-product, spectrographs in Chilean observatories can become one of the rare monitoring tools of the abundance of greenhouse gases in South America.

Molecfit is a general tool to correct for telluric absorption lines based on synthetic modelling of the Earth's atmospheric transmission. It combines a publicly available radiative transfer code, a molecular line database, atmospheric profiles, and various kernels to model the instrument line spread function. As a result, the standard deviation of the residuals after correction of unsaturated telluric lines is frequently better than 2% of the continuum. In this presentation, I will briefly describe the tool and the current developments: (a) integration within the ESO pipelines, (b) various improvements, (c) deployment of radiometers at Paranal for precise measurements of the atmospheric temperature and humidity profiles. In particular, in some specific conditions, molecfit is able to blindly correct telluric lines to a high accuracy.

n/a

Replacing the unused ADC by a polarizing filter would transform the VST into the first large polarimetric optical survey telescope, without affecting the current capabilities of the VST+OmegaCAM system. Scientific cases include: mapping the Milky-Way and Magellanic clouds magnetic fields, surveys of quasar polarization, identification and variability of polarized brown dwarfs, polarimetric characterization and mapping of solar-system objects (incl. the moon, comets, and asteroids), detection of polarized signal around the L4 and L5 Lagrange points, a census of magnetic white dwarfs and Herbig Ae/Be stars, etc. The design of the VST limits the technical implementation for the polarimetric unit to a single-beam system: therefore, we will develop data-driven calibration methods to achieve a sub-percent polarimetric accuracy. The proposed implementation is of low cost and requires no major development, as polarizing filters of the needed size are readily available. In particular, it does not require major modification neither of VST, nor of OmegaCAM. In fact, the currently offered capabilities of the VST+OmegaCAM are preserved as the polarizing filter can be removed from the optical beam. Would this project be accepted, efforts will be needed on the design of the polarimeter unit to optimize accuracy and execution times, on the design of the calibration plan, and on the extension of the data reduction pipeline to extract the polarization signals.

"Molecfit is a tool to correct for telluric absorption lines based on synthetic modelling of the Earth's atmospheric transmission. \nWe will present the tool, its applicability, its limitations, examples of applications at ESO and at other observatories, as well as recent and future developments."

Spectroscopy of QSO absorption lines provides essential observational input for the study of nucleosynthesis and chemical evo- lution of galaxies at high redshift. But new observations may indicate that present chemical abundance data are biased due to deficient spectral resolution and unknown selection effects: Recent high-resolution spectra reveal the hitherto unperceived chemi- cal nonuniformity of a molecule-bearing damped Lyman-alpha (DLA) system, and the still ongoing H/ESO DLA survey produces convincing evidence for the effect of dust attenuation. We present a revised analysis of the H2-bearing DLA complex toward the QSO HE 0515-4414 showing nonuniform differential depletion of chemical elements onto dust grains, and introduce the H/ESO DLA survey and its implications. Conclusively, we aim at starting an unbiased chemical abundance database established on high-resolution spectroscopic observations. New data to probe the temperature-redshift relation predicted by standard cosmology and to test the constancy of fundamental constants will be potential spin-offs.