Figure 6-1.—Adjusting the level tube.ADJUSTING THE LEVEL TUBE.— The verticalaxis of rotation of the instrument is the basis for alladjustments to the engineer’s level. When theinstrument is set up and leveled the vertical axis ofrotation and the longitudinal axis of the level tube shouldbe perpendicular to one another. If they are notperpendicular, then the vertical axis cannot be madetruly vertical. Adjustment of the level tube makes theaxis of the level tube perpendicular to the vertical axis.To check and adjust the level tube, you should followthe procedures below:1. Setup the instrument and approximately levelthe bubble over each pair of opposite leveling screws.Then carefully center the bubble over one pair of screws,as shown in view A, figure 6-1.2. Rotate the instrument 180°. If the bubbleremains centered, then the level tube is in properadjustment. If the bubble does NOT remain centerednote the movement of the bubble away from center(view B, fig. 6-1).3. Bring the bubble half the distance back to thecenter of the tube by turning the capstan nuts at one endof the tube (view C, fig. 6-1).4. Relevel with the leveling screws (view D, fig.6-1) and rotate the instrument again. Repeat Step 3above if the bubble does not remain at the center of thetube.5. Check the final adjustment by noting that thebubble remains in the center of the tube during the entirerevolution about the vertical axis.NOTE: When the level tube is out of adjustment,you can compensate for it by releveling the instrumentbefore each sighting.ADJUSTING THE HORIZONTAL CROSSHAIR.— For the horizontal cross hair to lie in a trulyhorizontal plane when the instrument is leveled, thehorizontal cross hair must be perpendicular to thevertical axis. To make the horizontal cross hair (fig. 6-2)lie in a plane perpendicular to the vertical axis, youshould perform the following steps:1. With the instrument carefully leveled, sight oneend of the horizontal cross hair on a well-defined pointat least 250 feet away. Turn the telescope slowly aboutthe vertical axis, using the slow motion screw. If thecross hairs are in adjustment, the horizontal cross hairwill stay on the point through its entire length.2. If it does not stay on the point, loosen twoadjacent reticle capstan screws and rotate the reticle bylightly tapping two opposite screws.3. Sight on the point again. If the horizontal crosshair does not stay on the point through its entire length,rotate the ring again.4. Repeat this process until the condition issatisfied.NOTE: To compensate for the above maladjust-ment, you should use only that part of the horizontalcross hair that is closest to the vertical hair for allsightings.ADJUSTING THE LINE OF SIGHT.— For aperfectly adjusted level, the line of sight is parallel to theaxis of the level tube. When the level meets thiscondition, the line of sight will generate a trulyhorizontal plane when the instrument is rotated. WhenFigure 6-2.—Adjusting the horizontal cross hair.6-4
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