location of the vertical circle on the standard, which is on the left when the instrument is direct and on
the right when in reverse. Elevating the telescope in a circle left (direct) position increases the circle
reading and in a circle right (reverse) decreases it.
(2) The value read on the vertical circle equals only one-half of the vertical angle added to or
subtracted from 90, and is not the vertical angle added to or subtracted from 90,. However, to compute
the vertical angle, the value of one reading is not doubled, but circle left (direct) and a circle right
(reverse) readings are taken. The smaller one is subtracted from the larger, and the sign of the angle is
determined from the circle left (direct) reading (plus the vertical angle, if above 90,; minus the vertical
angle, if below 90,).
(3) The zenith distance is the vertical angle from a point directly overhead (zenith) to the sighted
point. To compute the zenith distance, the vertical angle is algebraically subtracted from 90 if plus and
added to 90 if minus. A more direct method uses (circle right plus 90) - circle left.
j. Horizontal Collimation Correction. Making corrections to a horizontal collimation error ensures
that the line of collimation (line of sight) of the telescope is perpendicular to the horizontal axis of the
telescope. After setting up the instrument for observation, make a horizontal observation on a clearly
defined object, and read the horizontal circle. Reverse the telescope, and repeat the pointing and reading
on the same point. These two readings should be exactly 180, apart. Any other reading indicates that
the line of sight is out of adjustment, and the difference is equal to twice the collimation error.
Refer to Figure 3-4, page 3-16. To correct the collimation error, turn the micrometer milled head
(12) until the seconds drum shows the mean value found. Restore the coincidence of the circle
graduations by turning the horizontal slow-motion screw (17). Move the vertical crosshair sideways
with the adjusting screws (4) until it is again on the object. The three adjusting screws, one of which is
horizontal, are the pull-action type and are set on the eyepiece 120, apart. To move the crosshair to the
right, loosen the horizontal adjustment screw on the left-hand side of the eyepiece and gently tighten, by
equal amounts, the other two obliquely spaced adjustment screws on the right-hand side of the eyepiece.
Ensure that the screws are not overtightened. To move the crosshair to the left, first loosen, by equal
amounts, the two obliquely spaced adjusting screws on the right, and tighten the left horizontal screw
(Figure 3-4 shows only two of the three screws [17]). Continue this movement until the object appears
exactly on the vertical crosshair. Repeat these measurements and corrections until the collimation error
is brought within the allowable limits for the survey.
k. Vertical Circle Correction. Making corrections to the level of the vertical circle reduces the
index error of vertical-angle observations. Any necessary horizontal collimation correction must be
done before correcting the level of the vertical circle. Refer to Figure 3-4 and perform the following
steps.
EN0593
3-26
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