### d. Line of Sight

#### (1) Description

The LOS should be perpendicular to the HA to allow straight lines to be prolonged by rotating the telescope in a vertical plane.

#### (2) Check

The check is performed by Double Centering. To minimize pointing errors, use sights at least 200 feet long. The instrument is set up and leveled, then a sight is taken on a distinct target or point, Figure G-21.

 Figure G-21 Step 1

Angle α is error in the LOS-HA perpendicular condition.
The telescope is rotated about the HA, Figure G-22(a), to set point, B, opposite the instrument from point A, Figure G-22(b).

 (a) Reverse Telescope (b) Set Foresight Point Figure G-22 Step 2

If in adjustment, the instrument and points A and B will be colinear. Otherwise the angle β is twice the error, angle α.

The instrument is rotated around the VA, Figure G-23(a), and point A is sighted again, Figure G-23(b).

 (a) Rotate Instrument (b) Backsight Figure G-23 Step 3

The telescope is rotated about the HA, Figure G-24(a), to set another point C opposite the instrument from point A, Figure G-24(b).

 (a) Reverse Telescope (b) Set Foresight Point Figure G-24 Step 4

If in adjustment, points B and C will coincide.

If the LOS is not perpendicular to the HA, the plane in which a vertical angle is measured is offset from the instrument's VA. The further away the point, the greater the offset. Think of the LOS as tracing out a cone as the telescope is rotated, Figure G-25.

The cone is centered on the instrument and its axis coincides with the instrument's HA.

#### (3) Compensation

##### (a) Procedure

Set the correct colinear point, D, halfway between points B and C, Figure G-26.

 Figure G-26 Procedural Compensation

Because the instrument was reversed twice, the distance between points C and D is four times the error. To correct the condition, use the crosshair adjusting screws. They are located immediately forward of the eyepiece, usually under a protective cover. Use the horizontal screw(s) to move the crosshairs 1/4 the distance from point C in the direction of point B, Figure G-27.