MONOPTEC provided four acrylic models for the tests. Three had circular openings, at (in degrees) 0, 30, and 60 zenith angles. As a control, a fourth model had a slot from zenith to 60 degrees, to model a conventional slotted dome. The opening widths were limited to 40% of the dome diameter. While the FSD can easily accomodate openings to 50% of overall diameter, the slotted sphere would not have been realistic at such a large opening width.
Wind tunnel testing can be arranged for observatory projects. In general, the NASA tests can be interpreted over a range of dome scales, with the Reynolds number being the determinant.
It is useful to perform boundary layer testing of observatory projects. Topographic studies can identify possible local wind effects. These tests can include thermal components to identify diurnal cycles at the site.

ABSTRACT WIND-TUNNEL STUDY OF AN OBSERVATORY
WITH A CIRCULAR APERTURE
Gregory G. Zilliac, NASA-Ames Research Center
and
Ethan Cliffon, MONOPTEC

"We see the opening of an era: it is an era of seeking beyond
the confines of our atmosphere; may it be also an era of
awakening to the countries of our earth."
Bertrand De Jouvenel, Circa 1970s
Publications of the Astronomical Society of the Pacific
(Vol. 103, No. 669, November 1991)

The undesireable effects of the wind on the image quality obtained at astronomical facilities can be manifested in several ways. First, the image can vibrate as a result of wind shaking the telescope structure. Second, image spread occurs when the mirror-to-air interface is not in thermal librium. Last, optical wave front distortions cause image spread at the dome interior-to-exterior air interface as a result of temperature gradients.

This report describes results of a wind-tunnel test of a new concept in observatory dome design, the fixed shutter dome, which is intended to protect telescopes from wind shake. The concept is an evolution of the split-sphere topology first seen on the 2-meter telescope at Pic-du-Midi Observatory (France). In order to close larger apertures, the new design allows an upper spherical segment to rotate under a shell of slightly larger diameter which is fixed to a lower rotatable sergment. This design is struturally more efficient than convential designs.

From an aerodynamic standpoint, the new dome configuration is similar in overall shape to conventional observatory domes, with the exception of the telescope viewing aperture. The new design consists of a circular aperture of reduced area in contrast to conventional domes with rectangular or slotted openings. Wind-tunnel results of a side-by-side comparison of the new dome with a conventional dome demonstrate that the mean and fluctuating velocity through the aperture and in the center of the dome configuration are lower than those of conventional domes, thus reducing the likelihood of telescope flow-induced vibration.