A large part of the present work was performed in the framework the a project for a new generation telescope named the Very Large Telescope (VLT). It is then useful to illustrate briefly this project and by the same occasion to introduce an essential nomenclature of the terms and concepts relative to large optical telescopes which are used throughout this report.
The European Southern Observatory (ESO) is an intergovernmental organization constituted by eight European countries, including Switzerland, for the purpose of providing and managing astronomical observatories in the southern hemisphere. ESO owns the observatory of La Silla, in the Chilean Andes, which includes 15 telescopes of different size and characteristics, among which is the 3.5-meter New Technology Telescope (NTT) which is deemed to be presently the best telescope in operation.
In 1984 ESO started the VLT project for the VLT, a new observatory on the site of Cerro Paranal, which will be constituted by four unit telescopes, each with a primary mirror with a diameter of 8 meters. A schematic plan view of the future observatory is shown in fig. .
The telescope and its main components are schematically illustrated in figs. and . The mounting is altitude-azimuth. The primary mirror is only 17 cm thick and is supported by a so-called active optics system which consists in 150 combined hydraulic-electromechanic actuators capable of correcting the mirror shape for gravity and other low frequency deflections by applying controlled forces. The secondary mirror is provided with a fast tilt control called field stabilization for precision guiding. For observations in the infrared wavelength, a chopping motion of the secondary mirror allows to measure the difference between the signal and sky background, but in this case the field stabilization may not be as effective.
The telescope is protected by an enclosure of cylindrical shape (fig. ). The upper part co-rotates with the telescope and has a large upside-down-L shaped slit, closed with two sliding doors. The two slit doors are supported on two protuberances of the dome, which also integrate a set of pneumatically activated bars that constitutes a wind screen with different levels of wind permeability across the slit. In the lower fixed part there are 5 large doors (one 10x4m and four 5x4m) while in the rotating upper part there are about 150 1.5x0.5m openings equipped with louvers.
Figure: Main components of the VLT 8-m unit telescope.
Figure: Schematic plan view of the VLT observatory
Figure: Unit enclosure of the VLT
Figure: Optical schematics of an astronomical telescope