laterally across the front of the eyepiece. If the crosshairs appears to move with respect to the observed
object, parallax is present.
d. Parallax Removal. Parallax can be removed by turning the focusing ring (3) until the objective
lens is the proper distance for the image to fall in the plane of the vertical crosshair. If the crosshairs
appear fuzzy or dim, readjust the black diopter ring and repeat the process. Check for parallax
periodically throughout the day, since the focal distance of your eyes changes as you tire. However, do
not refocus the crosshairs during the observation of a position.
e. Circle Reading. Both circles are read in the microscope directly alongside the telescope's
reading eyepiece (16). On the outer side of the right-hand support is the changeover knob (13) for the
circle images. To make the horizontal circle visible, turn the knob clockwise as far as it will go; to make
the vertical circle visible, turn the knob in the reverse direction. Simultaneously, on either of the circle
images, the image of the scale of the seconds drum will be visible below the circle image. To bring the
images to sharpness, turn the telescope's edged eyepiece (6). During the reading, the illumination of the
circles should always be bright and uniform. Control the illumination by turning and tilting the
illuminating mirrors (9 and 2). Do not alter the illumination during the angle measurement.
f. Horizontal-Circle Reading. When reading the horizontal circle, set the telescope on the object,
and look in the microscope to view the images of the two diametrically opposite parts of the circle
separated by a fine line. In the middle of the lower circle image is a fixed index mark. Bring the
graduation lines of the upper and lower circle images into coincidence with each other in the immediate
area of this fixed index mark by turning the micrometer milled head (12) on the right-hand bearing
block. This causes the images to move relative to each other. The making of the coincidence must be
done with all possible care, as it conditions the accuracy of the reading. The final movement of the
milled head must always move clockwise.
(1) The unit of graduation of a circle is 4 minutes. When coincidence has been achieved, the
index mark should be either on a graduation line or between two graduations. Read off the whole
degrees from the upright number left of the index mark, and count the graduation intervals from the
numbered degree line to the index mark. Each graduation interval represents 4 minutes. If the index
mark lies in the middle of a graduation interval, count this as only half an interval (of 2 minutes). This
reading is taken to 2 minutes on the circle itself. To this reading, add the reading of the seconds drum
seen in the lower image. The seconds drum is divided into 60 numbered seconds. Each numbered
second is further subdivided into tenths (10 intervals). The numbers represent seconds and tenths of
seconds and are read directly. The drum only shows 60 seconds, and the circle reading always shows 2
minutes equal to 120 seconds, so the drum reading must be doubled. The best way to double the drum
reading is to repeat the coincidence adjustment and read the seconds again, thus obtaining another
reading for the seconds. The two are then added together. The circle reading plus the sum of the two
drum readings give the correct reading. The plate circle of a 2/10-second theodolite is divided into 360,
and each degree into 15 graduations or 4 minutes per graduation. The coincidence
EN0593
3-18
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