Custom Search
|
|
 
|
||
|
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 suns
image (32 min of arc) to make the pointing refer
to the suns 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. vertical circle images into a single field of view. In the DEGREE-GRADUATED SCALES (fig. 11-16), the images of both circles are shown as they would appear through the microscope of the 1-min theodolite. Both circles are graduated from 0 to 360 with an index graduation for each degree on the main scales. This scales graduation appears to be superimposed over an auxiliary that is graduated in minutes to cover a span of 60 min (1). The position of the degree mark on the auxiliary scale is used as an index to get a direct reading in degrees and minutes. If necessary, these scales can be interpolated to the nearest 0.2 min of arc.The vertical circle reads 0 when the theodolites telescope is pointed at the zenith, and 180 when it is pointed straight down. A level line reads 90 in the direct position and 2700 in the reverse. The values read from the vertical circle are referred to as ZENITH DISTANCES and not vertical angles. Figure 11-17 shows how these zenith distances can be converted into vertical angles.
Figure 11-16.-Degree-graduated scales.
Figure 11-17.-Converting zenith distances into vertical angles (degrees).In the MIL-GRADUATED SCALES (fig. 11-18), the images of both circles are shown as they would appear through the reading micro-scope of the 0.2-mil theodolite. Both circles are graduated from 0 to 6,400 mils. The main scales are marked and numbered every 10 mils, with the
Figure 11-18.-Mil-graduated scales.
Figure 11-19.-Vertical angles from zenith distances (mils). last zero dropped. The auxiliary scales are graduated from 0 to 10 roils in 0.2-mil increments. Readings on the auxiliary scale can be interpolated to 0.1 mil. The vertical circle reads 0 mil when the telescope is pointed at the zenith, and 3,200 mils when it is pointed straight down. A level line reads 1,600 roils in the direct position and 4,800 roils in the reverse. The values read are zenith distances. These zenith distances can be converted into vertical angles as shown in figure 11-19. |
 
|
|
|
||
