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General presentation

Modern astronomy makes use of optical telescopes for the observation of sky objects in the wavelength range from ultraviolet to infrared, that is from about 300 nm to 30 m . Although the recent years have seen the development of space astronomy from automatic telescopes carried by satellites, the huge cost of these satellites and some of their inherent limitations will mean that ground based telescopes will be still for many decades and perhaps centuries the main instruments of astronomers. Indeed this last decade has seen the start of a number of new projects for telescopes larger than any in operation today, which, thanks to the high quality of optical systems and their electronics, aim at being almost as performing as satellite instruments, with a much lower cost.

Two main developments are presently being pursued in this field. One consists in developing larger primary mirrors, with diameters from 6 to 10 meters, which constitute a significant technological leap with respect to the 4-m class mirrors used in the best telescopes built until the mid 80s. The advantages sought with larger mirrors are the light collecting performance which is proportional to the mirror area and the improvement of the theoretical resolution which is proportional to the ratio between wavelength and mirror diameter.

The other development line aims at decreasing the disturbances caused by the atmospheric environment on the optical performance of a telescope. This objective is sought on the one hand by locating new telescope in high mountain sites with favorable atmospheric characteristics, and on the other hand by reducing local effects, mainly of thermal origin, caused in and by the observatory itself. Ground based telescopes must of course observe through the atmosphere, which has two main consequences on the quality of observations. The first consequence is a degradation due to the turbulent variations of the index of refraction. This causes the image of a star to appear as a moving disk with angular size which is often quite larger that the theoretical limit size due to diffraction from the telescope optics. This degradation is called the seeing and is often quantified as the angular apparent diameter of a star image. Although contributions to this seeing effect come from all atmospheric layers, frequently one of the largest disturbances is generated close to the telescope itself by differences of temperature between air and telescope structures. This effect tends to disappear when the telescope is exposed to wind, which, however, creates then another problem.

The second disturbing effect is caused by vibrations of the telescope due to the wind mechanical turbulence. During the observation the telescope must track the star with very high accuracy. If the telescope is even partly exposed, the wind will tend to shake it. The guiding accuracy required even for short integration times is such that the telescope oscillations due to the wind load cannot be absorbed by the structural rigidity of the telescope alone and must be corrected by an active control loop. There is nonetheless a limit to the amplitude and frequency bandwidth of the wind disturbance that can be corrected by the control system. Moreover on the large telescopes of the newest generations the primary mirrors are much thinner and less stiff, for a number of reasons, than was the case in predecessor telescopes, and therefore may also have the figure of their optical surface deformed by the wind loads.

Thus the wind flow has a twofold action on the overall telescope performance: on the one hand it improves the seeing quality but on the other hand it degrades the guiding accuracy; conversely if the telescope is well shielded from the wind, guiding will be very accurate but seeing will inevitably worsen. It is important to note that both seeing and guiding inaccuracies have similar effects on the image quality of the telescope inasmuch they both cause an enlargement of the apparent size of a star image recorded during the exposure time.

The engineer designing the enclosure of a telescope has therefore a difficult job in finding a compromise in the exposure of the telescope to the wind, which will maximize the overall quality of the observation. Telescope enclosures are a very particular type of buildings, which must fulfill an unusual set of requirements. In general there will be different technical solutions, depending on a variety of parameters such as the size and type of the telescope, its geographic and meteorologic location, the type of observation which is aimed at and various other requirements concerning maintenance, access and operation.

It is important to underline that the problem, such that it is set nowadays, is quite new. Until a few years ago, the design of a telescope enclosure was deemed to a quite straightforward matter and was essentially based on conventional ideas which in fact were hiding a basic ignorance of the interaction between a telescope and its local atmospheric environment, particularly with respect to the seeing problem. It was only when particular circumstances proved that some of the concepts traditionally used for the design of telescope enclosures were counterproductive, that more systematic research work was undertaken on the subject.

This dissertation intends to describe and summarize in a comprehensive manner several studies, both theoretical and experimental, on the effects of seeing and wind turbulence associated with various types and configurations of telescope enclosures. The knowledge acquired on these effects will then be integrated into new procedures for a global evaluation of telescope performance and will provide improved design guidelines for the project of future telescopes and their enclosures.

One may note that this work is of interdisciplinary nature. While its main objective is to provide civil engineers with a better understanding of the design drivers of telescope enclosures, this research englobes notions, experiments and methods in the fields of optics, wind aerodynamics, atmosphere physics and structural engineering.

Most of the work presented in this dissertation was performed in the framework of the development of the new Very Large Telescope (VLT) observatory by the European Southern Observatory (ESO). The VLT observatory will be constituted by four 8-m telescopes which will be able to combine their light beams into a single image. When completed near the year 2000, the VLT will be the largest astronomical observatory in the worldgif. Because the VLT project was started without preconceived ideas, the work covered a wide spectrum of conditions and design solutions. As a consequence this work should have applications well beyond the scope of the VLT project, and has the ambition to become a reference for the design of telescope enclosures.

next up previous contents
Next: Scope and summary Up: Introduction Previous: Introduction

Lorenzo Zago,, Sun Feb 26 22:57:31 GMT+0100 1995