A decade of VLTI technical development

Abstract

Following the successful VLTI 'First Fringes' obtained in 2001 with the siderostats and with the 8m telescopes and based on the results from the commissioning phase, it is now possible to review with a critical eye the development approach followed over the last ten years and to draw a few conclusions.We first recall this approach that aimed at minimizing the risk of not meeting the stringent requirements imposed by interferometry. This approach is based on the elaboration of exhaustive error budgets, an extensive set of analyses and early tests with feedback on subsystem specs, the performance characterization at subsystem level with identification of improvements when needed; and finally the commissioning of the complete VLTI at system level.To illustrate this process, we provide practical examples taken from the project's history. We focus on two areas that have been considered among the most critical ones over the entire project life, namely: the turbulence inside the interferometer arms ('internal seeing') and the mechanical vibrations ('OPD stability').For these two areas, we will finally compare the performances predicted during the development phase with those obtained at Paranal and we will draw conclusions.

Atmospheric and internal turbulence measured on the Very Large Telescope Interferometer with VINCI

“…The standard deviation of the internal piston over this 20 min sequence is less than 2 microns. Koehler 10 reports similar complementary results obtained on the optical path leading to UT1 and UT3.…”

Active Optics—Advances of Cycloid-like Variable Curvature Mirrors for the VLTI Array

“…In addition to the on-axis cophasing by delay-lines, cat's eye mirrors, i.e., VCMs realizing the cophasing of the field of view up to 3 arcsec. Dérie et al [13], Koehler [14], and Gonté et al [15] developed the delay-line telescope systems with the implementation of 16 VCMs.…”

An active fiber sensor for mirror vibration metrology in astronomical interferometers

“…Several passive and active solutions have been proposed and applied to tackle this problem. Among the passive methods, the removal of vibration sources, usage of vibration-damping optical mounts and of integrated photonic components for interferometric beam combination have demonstrated their effectiveness in improving the stability of astronomical interferometers (Köhler, Lévêque & Gitton 2003;Coudé Du Foresto, Ridgway & Mariotti 1997; Le Bouquin et al 2004). Despite these construction solutions, very high performance optical interferometry still requires active vibration compensation systems.…”