It is generally acknowledged among astronomers that the telescopes which have enclosures that allow for wind ventilation have a low dome seeing. Unfortunately here also the evidence is mostly anedoptical and few actual data exist. Even at the NTT which is possibly the best example of a "ventilated" telescope in operation, multifold reasons and technical problems have prevented comprehensive measurements of the local seeing phenomenon over long periods of time.
We will therefore present here only some individual measurements, obtained in collaboration with S. Ortolani of Padoua Observatory and M. Sarazin of ESO, which nonetheless illustrate the performance of the NTT with respect to seeing. Fig. shows a comparison of seeing sequences taken at the NTT [Ortolani 94] and the corresponding measurements at the DIMM telescope. The agreement is very good and shows that the NTT experiences virtually only the natural seeing. One should note that the sequence of May 16 is one of exceptionally low natural seeing. In both cases the wind was weak: during the May 16 sequence it was between 0 and 3 m/s, while during the May 17 sequence it was 45 m/s. The primary mirror was about 1 to 2 degrees K colder than ambient air.
Figure: Comparison of seeing measurements at the NTT and at the DIMM.
However the NTT does not behave always so favorably. On a previous occasion some seeing measurements which let appear the presence of mirror seeing ([Ortolani 93]) were taken during a night in which the primary mirror was significantly warmer than ambient air (fig. ). Here the wind was 67 m/s but was facing the back of the telescope building.
The effect of opening/closing the louvers at the back of the telescope was investigated during a test run: the flaps were alternatively opened and closed every about ten minutes. Although the seeing measured by the DIMM was very variable, the difference of seeing between the two flaps conditions was very noticeable - see fig. . The wind speed was about 7 m/s and when the flaps are closed the ventilation of the telescope volume is much reduced and the seeing increases immediately. This conclusion joins much anedoptical evidence from NTT observers complaining that raising the windscreen (see fig. ) to maintain a good guiding performance often degrades the seeing quality.
In conclusion, even with the limitations due to the scarcity of objective data, the experience of the NTT brings the following elements of knowledge:
Figure: Seeing at the NTT versus mirror-air temperature difference.
Figure: Effect of flaps on the seeing at the NTT.