Figure 11-13.-Coincidence- type level.
shown in figure
11-13. The TELESCOPE
LEVEL, mounted below the telescope, uses a
prism system and a 450 mirror for leveling
operations. When the telescope is plunged to the
reverse position, the level assembly is brought to
the top.
TELESCOPE.— The telescope of a theodolite
can be rotated around the horizontal axis for
direct and reverse readings. It is a 28-power
instrument with the shortest focusing distance of
about 1.4 meters. The cross wires are focused by
turning the eyepiece; the image, by turning
the focusing ring. The reticle (fig. 11-14) has
horizontal and vertical cross wires, a set of
vertical and horizontal ticks (at a stadia ratio
of 1:100), and a solar circle on the reticle
for making solar observations. This circle covers
31 min of arc and can be imposed on the
sun’s image (32 min of arc) to make the pointing
refer to the sun’s center. One-half of the vertical
line is split for finer centering on small distant
objects.
Figure 11-14.-Theodolite reticle.
The telescope of the theodolite is an inverted
image type. Its cross wires can be illuminated by
either sunlight reflected by mirrors or by battery
source. The amount of illumination for the
telescope can be adjusted by changing the position
of the illumination mirror.
TRIBRACH.— The tribrach assembly (fig.
11-15), found on most makes and models, is a
detachable part of the theodolite that contains the
leveling screw, the circular level, and the optical
plumbing device. A locking device holds the
alidade and the tribrach together and permits
interchanging of instruments without moving the
tripod. In a “leapfrog” method, the instrument
(alidade) is detached after observations are
completed. It is then moved to the next station
and another tribrach. This procedure reduces the
amount of instrument setup time by half.
CIRCLES.— The theodolite circles are read
through an optical microscope. The eyepiece is
located to the right of the telescope in the direct
position, and to the left, in the reverse. The
microscope consists of a series of lenses and
prisms that bring both the horizontal and the
Figure 11-15.-Three-screw leveling head.
11-22
