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NTT rotating building

The ESO NTT has been the first telescope whose enclosure is designed to allow wind through-flow in the telescope volume (cf. section gif above). It has also become a kind of a standard reference for advanced telescopes in the 4-m class. The same design of telescope and enclosure with minor modifications has been adopted for the 3.5-m GALILEO telescope presently in construction for installation at the observatory of La Palma (Canary Islands) and is considered also for a future South-African telescope.

This enclosure is actually a real building, with several volumes and rooms for telescope, instruments and auxiliary equipment which rotates about the azimuth axis accompanying the telescope. The telescope is housed in a kind of corridor between two walls which separate the closed rooms for the Nasmyth instruments which are air conditioned separately. This corridor is closed on the front and upper sides by 2 large upside-down-L shaped sliding doors while the back a wall includes several louvers allowing through ventilation. A semi-permeable windscreen can be raised across the front side of the slit (fig. gif).

The tests were performed in the turbulent wind tunnel of the Technical University of Aachen with a 1:50 model of the NTT rotating building. The velocity measurements were taken at three locations, indicated in fig. 4.8. The tests were run with several venting configurations of the enclosure

The measurements covered the range of azimuth angles from 0 to 60 with the wind direction. Figs. 4.9 and 4.10 show the main results for point TR45:

The data show a sharp decrease of mean speed for azimuth angles greater than 20 . Without windscreen (configurations O-O-O and O-O-C), this is associated with a large increase of turbulence, particularly for azimuth angles between 20 and 40 where the turbulent fluctuations of the dynamic pressure created by the slit are quite larger than in the free wind flow. The turbulence length scale is respectively 0.8 and 1.4 of the slit width at zero azimuth, which signifies the generation of high frequency turbulence by the slit edges even the telescope faces the wind directly. Fig. 4.11 shows the result of a computation of the pressure spectral density at 6 Hz for a free flow mean speed of 18 m/s, a good indicator of influence on the tracking performance as telescopes have generally their first eigenfrequency between 5 and 8 Hz: the amplification with respect to the free flow conditions reaches one order of magnitude.

This effect is only corrected when the windscreen is raised to full height across the front side of the slit, such that the mean flow velocity, hence the pressure fluctuations on the telescope (cf. equation (gif)) are sharply decreased. On the basis of these results a 20% permeability windscreen was integrated in the final design of enclosure. Observation records of the NTT at La Silla show that in most cases the operator is obliged to raise it when the wind when the wind exceeds about 6 m/s, that is almost half of the time of observation.

 

 

 

 



next up previous contents
Next: Cylindrical enclosure (VLT) Up: Wind loading on Previous: Results



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