Astronomy Principles and Practice Fourth Edition.pdf

Zenith tubes 343
Figure 20.11. The Carlsberg Meridian Telescope with CCD detector system attached for the automatic monitoring
of stellar transits. Both positions and brightnesses of over 100 000 stars are regularly recorded each night.
under computer control. The operation of the Carlsberg Meridian Telescope W20.7 has progressively
improved since its establishment on La Palma in 1984, at that time being one of the world’s first
automatic telescopes. The instrument was programmed to automatically set itself to the correct
altitudes of about 500 stars in turn and measure the time of passage and their magnitudes. More
recently, the photoelectric scanning-slit micrometer has been replaced by a CCD detector operating in
a drift-scan mode. Between 100 000 and 200 000 stars are now measured each night and the limiting
magnitude of the system is ∼17.
20.6 Zenith tubes
Very accurate positions of the latitude of an observatory are obtained by using zenith tubes of which
there are several designs. Their advantages over the transit telescope for this particular purpose are
that the instrument does not suffer from flexure, as it is always used in a near vertical position, and
the effects of atmospheric refraction are very small. Regular positional measurements by this type of
instrument allow the motions of the Earth’s poles to be studied.
The zenith tube or telescope consists of a small refractor which is limited in movement so that
only the zenith field can be observed. In its basic form, the measurements are made using a micrometer
eyepiece but there are several more modern versions using photographic and electronic recording
techniques which provide improved accuracy.
A single determination of an instrument’s position requires measurements of the difference in
zenith distance of two stars when they are placed on the observer’s meridian—absolute zenith distances
are not required. The telescope is first directed to a position which is just south of the zenith and the
micrometer eyepiece adjusted so that the transit of a star is made to occur along the horizontal wire. By
turning the telescope through 180 ◦ about a vertical axis, transits occurring just north of the zenith can
be observed. When a suitable star appears in the field, the horizontal wire is adjusted by the micrometer