Retractable enclosure

The test results presented in the two previous sections illustrate well the main problem with ventilated telescope enclosure: the natural wind ventilation desired to get rid of dome seeing effects is associated to high frequency turbulence produced by the slit edges, which then affects the guiding performance of the telescope. An alternative envisaged to overcome this problem consisted a retractable enclosure which leaves the upper part of the telescope completely exposed to open air during the observations. Even if the overall pressure fluctuations on the telescope are larger than in the previous enclosures, their frequency is much lower and the perturbations can be corrected dynamically by the guiding control loop.

Wind tunnel tests with a model a retractable enclosure were performed at the EPFL/LASEN boundary layer wind tunnel as part of the feasibility studies of the VLT project (see section ). The telescope is exposed to open air for the observations, except for the lower part which is enclosed in a protecting recess. The model was at the 1:80 scale and included a very detailed model of the telescope structure. The edge of the enclosure was horizontal and located 16 mm (model scale) above the altitude axis. The tests were conducted with a mean speed of 16 m/s and a rms of 1.8 m/s, which gives a free flow turbulence intensity of 0.11 .

  
Figure 4.16: Model of the retractable enclosure in the LASEN wind tunnel

The profiles of mean speed and turbulence intensity were measured along a vertical line 1.9 cm (model scale) in front of the edge of the telescope pointing at zenith and are shown in fig. 4.17. Note that the height is given in the model scale (1:80 with respect to the 8-m telescope).

These measurements show the telescope wind environment as divided in two regions: the top of the telescope is essentially in the undisturbed free flow while the lower telescope structure and the primary mirror are located in a region with recirculating flow.

  [IMAGE ]
Figure 4.17: Vertical profile of mean speed, turbulence intensity and turbulence length scale (full scale) in front of the telescope pointing at zenith. The height is given in the model scale (1:80 with respect to the 8-m telescope).