It may be useful to note that the VLT is just one case of the extraordinary development of new large astronomical telescopes which is now taking place. At the time of writing this report, there are, beside the VLT, several other ongoing projects: Other large telescope projects are still in an earlier planning and development phase, such as the Columbus project for a telescope with a double "binocular" mount with two 8-m mirrors, and several national projects in Germany, Spain and England.

99#99 is often called airmass in the astronomical jargon.

See footnote at page gif.

Including the total seeing contribution.

The 2.2-m building includes a cooled floor while the 3.6-m now has an active air cooling system in the dome.

The SuperIMAQ program [ESO] was written by Ph. Diericks of ESO to compute the PSF and all image quality parameters of a telescope mirror, accounting for all diffraction effects, surface wavefront errors and seeing by the method described in [Diericks].

Mirror seeing is treated extensively in section gif.

See for instance [Incoprera], p. 506.

The wavefront of light is reflected by the primary mirror toward the secondary mirror and then again downwards to an instrument focus (see fig. gif).

The DIMM is special 35-cm telescope for measuring seeing located in open air on a 6-m high tower (cf. also page gif).

see footnote at page gif.

In that case the DIMM was located 2 meters above the ground, 40 m south (downwind) of the NTT.

See fig. gif

See for instance [Incoprera], p. 506.

We recall that seeing contributions along the line of sight add up with the power 5/3, while all other contributions add quadratically to the total seeing (cf. equations (gif) and (gif)).

Only the lower atmospheric pressure of an astronomical mountain site was considered here (770 mb with respect to 1000 mb for the wind tunnel).

One may recall that turbulence frequencies are increased inversely to the geometrical scale: as a comparison the velocity spectra are generally measured up to 200 Hz.

This is the case at the Paranal site of the VLT ([Sarazin 90]).

The mirror cooling system should ideally allow the mirror to remain always slight colder than ambient air without overcooling it, in order to avoid water condensation. During the observations, the mirror can only be cooled from the back side and, because of its thermal inertia, there will be inevitably instances when the air temperature will drop during the night to a lower temperature than the front surface.

Lorenzo Zago, zago@elgc.epfl.ch, Sun Feb 26 22:57:31 GMT+0100 1995