ADJUSTING THE TELESCOPE LEVEL

Figure 6-8.—Adjusting the telescope level. 1. Sight the vertical cross hair on some high point, A, at least 30° above the horizontal and at a distance of 200 feet, such as the tip of a church steeple or other well-defined object, and clamp the plates. 2. Depress the telescope and mark a second point, B, at about the same level as the telescope. 3. Plunge the telescope, unclamp the lower plate, and rotate the instrument about its vertical axis. 4. Sight on the first point, A. 5. Clamp the lower plate and depress the telescope. If the vertical cross hair intersects the second or lower point, B, the horizontal axis is in adjustment. In this case, point B is coincident with point D in both direct and reverse positions of the telescope. 6. If not, mark the new point, C, on this line and note the distance, BC, between this point and the original point. 7. Mark point D exactly midway of the distance BC. CD is the amount of correction to be made. 8. Adjust the horizontal axis by turning the small capstan screw in the adjustable bearing at one end of the horizontal axis until point C appears to have moved to point D. 9. Repeat this test until the vertical cross hair passes through the high and low points in the direct and inverted position of the telescope. 10. Check all previous adjustments. NOTE: When you cannot immediately correct the above condition, you can compensate by repeating any survey procedure with the telescope reversed and then use the average of the results. ADJUSTING THE TELESCOPE LEVEL.— To be able to use a transit for direct leveling and to measure vertical angles without index error, you must ensure that the axis of the telescope level is parallel to the line of sight. To adjust the telescope level of the transit, use the same two-peg method that we discussed previously for the engineer’s level. The only difference is that you must level the telescope carefully before each reading. After computing the reading that should be made on the far rod (fig. 6-3), you set the horizontal cross hair on the computed reading using the vertical slow motion screw. Then you move one end of the spirit level vertically by means of the adjusting nuts until the bubble is centered in the tube (fig. 6-8). NOTE: As with the engineer’s level, you should compensate for the above maladjustment by careful balancing of all backlights and foresights. ADJUSTING THE VERTICAL CIRCLE VERNIER.— For vertical angles to be measured without index error caused by displacement of the vertical circle vernier, the vernier should read zero when the plate bubbles and telescope bubbles are properly leveled. To make the vertical circle vernier read zero when the instrument is leveled (fig. 6-9), you should perform the following steps: 1. With the plate bubbles leveled, bring the telescope bubble to the center of the tube and read the vernier of the vertical circle. 2. If the vernier does not read zero, loosen the capstan screws holding the vernier and move the index until it reads zero on the vertical circle. 3. lighten the screws and read the vernier with all the bubbles in the center of their tubes to make sure that Figure 6-9.—Adjusting the vertical circle vernier. 6-8