SOME AIR-FLOW PROPERTIES OF TELESCOPE ENCLOSURES ESTIMATED FROM WATER-TUNNEL TESTS

The traditional telescope enclosure shape is a cylinder topped by a hemispherical dome. Studies of seeing conditions inside telescope domes indicate a need to provide substantial air ventilation to prevent image degradation due to thermal variation of the air within the dome. Although the traditional enclosure shape was initially chosen for the Japan National Large Telescope, studies of the hydrodynamic properties of this style compared to two alternative styles indicate that changes may be desirable. Water-tunnel tests were made using 1:500 scale models of the enclosures. Flow patterns outside and inside the enclosures were observed using streams of dye or bubbles. For all enclosure styles, an exterior component of flow was found that was "uplifted" over the top of the enclosure and divided from the original main flow stream direction at different levels depending on the model characteristics. We also found that an external horizontal guiding vane significantly affected the location of the stagnation point. It is important that the stagnation point be above the turbulent boundary layer at the site. Enclosures with flat or near-flat roofs appear to be superior. The time to flush out the enclosure was measured and found to vary with enclosure style, orientation of the viewing slit, and ventilation provisions. Sidewalls on either side of the telescope were found to be beneficial and also vents more or less orthogonal to the slit opening improved flushing conditions.