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.