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Transit Adjustments - 14071_117
ADJUSTING  THE  LINE  OF  SIGHT

Engineering Aid 2 - Intermediate Structural engineering guide book
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Figure  6-4.—Adjusting  the  plate  bubbles. ADJUSTING THE PLATE BUBBLES.—  The purpose of adjusting the plate bubbles is to make the axis of the plate-level tubes perpendicular to the vertical axis (fig. 6-4). This ensures that when the instrument is set up and leveled, the vertical axis is truly vertical. When this condition is met, horizontal angles are measured in a truly horizontal plane and vertical angle do not incur index error because of an inclined vertical axis. You should make the plate-bubble test every time you set up the instrument for use and always before making 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 and bring each level tube parallel to a set of opposite leveling screws. Bring both bubbles to the center of their tubes by turning the leveling screws (view A, fig. 6-4). 2. Rotate the instrument 180° about its vertical axis. If the bubbles remain centered, no adjustment is necessary. If the bubbles do not remain centered, note the amount of distance that the bubbles move from their center  (view  B,  fig.  6-4)  and  proceed  with  Steps  3 through 5. 3. Bring each bubble half the distance back to the center of its tube by turning the capstan screws at the end of each tube. 4.  Relevel  the  instrument  using  the  leveling  screws and  rotate  the  instrument  again.  Make  a  similar correction if the bubbles do not remain in the center of the  tubes. 5. Check the final adjustment by noting that the bubbles remain in the center of the tubes during the entire revolution about the vertical axis (view C, fig. 6-4). NOTE: You can compensate for out-of-adjustment plate levels by leveling the instrument, rotating it 180° in azimuth, and bringing the bubbles halfway back using the leveling screws. ADJUSTING   THE   VERTICAL   CROSS HAIR.— In a perfectly adjusted transit, the vertical cross hair should lie in a plane that is perpendicular to the horizontal axis. In this way, any point on the hair may  be  used  when  measuring  horizontal  angles  or running  lines. To  make  the  vertical  cross  hair  lie  in  a  plane perpendicular  to  the  horizontal  axis  (fig.  6-5),  you should  follow  the  procedure  below: 1. See that parallax is eliminated. Sight the vertical cross  hair  on  a  well-defined  point;  and  with  all  motions clamped,  move  the  telescope  slightly  up  and  down  on its  horizontal  axis,  using  the  vertical  slow  motion tangent screw. If the instrument is in adjustment, the vertical hair will appear to stay on the point through its entire length. 2. If it does not stay on the point, loosen the two capstan screws holding the cross hairs and slightly rotate the ring by tapping the screws lightly. 3. Sight again on the point. If the vertical cross hair does not stay on the point through its entire length as the telescope is moved up and down, rotate the ring again. Figure 6-5.—Adjusting the vertical cross hair. 6-6







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