Astronomy Principles and Practice Fourth Edition.pdf

342 Modern telescopes and other optical systems
Figure 20.9. A Maksutov–Bouwers camera.
Figure 20.10. The transit telescope.
20.5 The transit telescope
The transit telescope,ormeridian circle, usually takes the form of a small refracting telescope but it
is mounted on only one axis which is horizontal, lying along the east–west direction. In this way, the
telescope is restricted to movement in one fixed plane so that it points in directions which all lie on the
observatory’s meridian. The rotational axis is provided with a declination setting circle. The essential
parts of the transit telescope are illustrated in figure 20.10 and an automated operational system is
depicted in figure 20.11.
In its basic form, an eyepiece is fitted with a special graticule, the central vertical wire
corresponding to the meridian. By setting the telescope to the correct altitude, the passage, or transit,
of a star across the meridian can be timed accurately. This may be done by using the times of passage
across each of the graticule wires in turn and taking a mean value for the transit of the central wire.
At one time, the transit telescope’s function was to supply checks for master clocks which were, in
turn, used to provide civil time. Nowadays, they are used to make regular checks on the rotational
period of the Earth. They are also used to provide fundamental data for the purpose of determining the
coordinates of celestial objects.
As with all experimental procedures, current measurements are made with electronic detectors