Optical Spectroscopy with Very Large Telescope: Instrumental Development and Data Interpretation. The case of the Rocky-Planets Finder ESPRESSO and Spectral Characterization of BL Lac Objects.

This PhD thesis is focused on one of the main research area in modern Astrophysics: the spectroscopy. Within the current framework of observational astronomy, I carried out the Thesis aiming at the figure of Instrument Scientist who acts as a junction between astronomer and design engineers joining the science and technology in order to contribute to a more consolidated and optimised design of new instrumentations for large experiments. For this reason, in this Thesis I will present two main parts of my PhD activities that have characterised my entire doctoral training. In particular, I will first introduce ESPRESSO, the Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observations, which will be a new generation extra stable spectrograph at the European Southern Observatory (ESO) Very Large Telescope (VLT). ESPRESSO will be a spectrograph mainly optimised for finding rocky planet orbiting main sequence G stars in the habitable zone through the Radial Velocity (RV) method. I will outline my contribution to ESPRESSO in Chapter 2 where I illustrate simulations on the expected performances of the detectors used by a main part of the instrument: the Front End Unit. This subsystem is responsible to stabilise the field and pupil of ESPRESSO through active optics and to collect the light from the four telescope and inject it through fiber feeding the spectrograph. I also outline the design of the Exposure Meter of the instrument which is in charge to continuously monitoring the exposure in order to evaluate a critical value, called Mean Time of Exposure (MTE), which is used in order to correct the measured Radial Velocity for the relative Earth motion at the time of observation. For the first time, the exposure will be monitored chromatically allowing a better characterisation of the behaviour of the MTE and of the overall observation simultaneously. In Chapter 3 I focus on pure astrophysical research based on spectroscopy of a particular class of Active Galatic Nuclei: the BL Lacertae Objects. These objects are characterised by a strong non-thermal emission which arises from the accreting nucleus. For this reason, spectral features (when present) are extremely diluted making the determination of their redshifts a challenging task. With the adoption of FORS2 spectrograph, in low resolution, at 10mt class VLT I completed a survey of these objects started few years ago and I derived from the complete sample a conspicuous number of properties of the class, including the characterisation of their line-of-sight Mg II absorber systems. Moreover, I will outline the results obtained from observation of a small subsample of strong FERMI γ-rays (and in certain cases VHE emitters) source with ESO X-SHOOTER which combines medium resolution (R ∼ 4000) and large spectral range. In particular, for the source PKS 0048-097 the determination of its redshift for the first time allowed to deeply investigate its Spectral Energy Distribution and the close environment of the object. The spectra of the other sources in the sample are also illustrated and explicated for particular interesting objects.