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Figure  11-14.-Theodolite  reticle.

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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



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