Figure 6-4.—Adjusting the plate bubbles.ADJUSTING THE PLATE BUBBLES.— Thepurpose of adjusting the plate bubbles is to make the axisof the plate-level tubes perpendicular to the vertical axis(fig. 6-4). This ensures that when the instrument is setup and leveled, the vertical axis is truly vertical. Whenthis condition is met, horizontal angles are measured ina truly horizontal plane and vertical angle do not incurindex error because of an inclined vertical axis.You should make the plate-bubble test every timeyou set up the instrument for use and always beforemaking any other tests and adjustments of the transit.Make this test and adjustment using the following steps:1. Rotate the instrument about the vertical axis andbring each level tube parallel to a set of opposite levelingscrews. Bring both bubbles to the center of their tubesby turning the leveling screws (view A, fig. 6-4).2. Rotate the instrument 180° about its verticalaxis. If the bubbles remain centered, no adjustment isnecessary. If the bubbles do not remain centered, notethe amount of distance that the bubbles move from theircenter (view B, fig. 6-4) and proceed with Steps 3through 5.3. Bring each bubble half the distance back to thecenter of its tube by turning the capstan screws at theend of each tube.4. Relevel the instrument using the leveling screwsand rotate the instrument again. Make a similarcorrection if the bubbles do not remain in the center ofthe tubes.5. Check the final adjustment by noting that thebubbles remain in the center of the tubes during theentire revolution about the vertical axis (view C, fig.6-4).NOTE: You can compensate for out-of-adjustmentplate levels by leveling the instrument, rotating it 180°in azimuth, and bringing the bubbles halfway backusing the leveling screws.ADJUSTING THE VERTICAL CROSSHAIR.— In a perfectly adjusted transit, the verticalcross hair should lie in a plane that is perpendicular tothe horizontal axis. In this way, any point on the hairmay be used when measuring horizontal angles orrunning lines.To make the vertical cross hair lie in a planeperpendicular to the horizontal axis (fig. 6-5), youshould follow the procedure below:1. See that parallax is eliminated. Sight the verticalcross hair on a well-defined point; and with all motionsclamped, move the telescope slightly up and down onits horizontal axis, using the vertical slow motiontangent screw. If the instrument is in adjustment, thevertical hair will appear to stay on the point through itsentire length.2. If it does not stay on the point, loosen the twocapstan screws holding the cross hairs and slightly rotatethe ring by tapping the screws lightly.3. Sight again on the point. If the vertical cross hairdoes not stay on the point through its entire length as thetelescope is moved up and down, rotate the ring again.Figure 6-5.—Adjusting the vertical cross hair.6-6
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