Astron. Astrophys. Suppl. Ser. 131, 531-538
J. Surán
Geodetical Observatory Pecný, Ondrejov 244, Czech Republic
Received November 26, 1997; accepted March 10, 1998
A "floating mirror horizon" could substitute for the plate
with mercury now used with astronomical instruments. It would have a greater and
more stable reflectivity (by about 25%) than the present surface of just mercury,
and have a distinctly better definition of images. With astrolabe instruments both
images of a star would have practically the same brightness. There would be no need
to clean the surface of the mercury, so the particular instrument would be
permanently ready for use. Theory and design of the new device are presented in this
paper, along with tests from an experimental model. Regarding conditions of the
mirror's flotation, it is shown that its optimum shape is that of a cylindrical
plate of minimum (necessary) thickness. For some typical dimensions (with a
diameter, say, of 10 or 20 cm, and the metacentric height of the mirrors being
between 20-40 cm), its conditions of stability are comparable to those of
transoceanic ships. The whole appliance has to consist of three basic parts: a basin
with mercury; a floating mirror; and a centring device. The latter, while enabling
the mirror its free floating, maintains with adequate accuracy its centring in
relation to the centre of the mercury basin. This reduces to negligible (say, 0.01")
the amount of concentric deformation by capillary forces of the surface of the
mercury. It also enables preservation of proper orientation of the mirror -- of its
sloping line -- with respect to the axis of observation and rotation of the
particular instrument (of an astrolabe, PZT, etc.); and it also effectively dampens
vibrations of the mirror when the instrument is rotated and set to a particular
object a star, for example. In general, the surface of a floating mirror is
inclined at an angle to a horizontal surface. This effect can be eliminated by
simultaneous rotation of the mirror and the instrument (of an astrolabe, e.g.), or
by the mirror's rotation through (with a PZT or meridian circle). The
autocollimation method of observing a small experimental model of the floating
mirror horizon showed that it possessed the expected properties.
Key words: instrumentation: miscellaneous -- methods: observational -- astrometry
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