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
Ethan Cliffon, MONOPTEC
"We see the opening of an era: it is an era of seeking
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)
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.