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 of200 feet, such as the tip of a church steeple or otherwell-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 thetelescope. If the vertical cross hair intersects the secondor lower point, B, the horizontal axis is in adjustment.In this case, point B is coincident with point D in bothdirect and reverse positions of the telescope.6. If not, mark the new point, C, on this line andnote the distance, BC, between this point and the originalpoint.7. Mark point D exactly midway of the distanceBC. CD is the amount of correction to be made.8. Adjust the horizontal axis by turning the smallcapstan screw in the adjustable bearing at one end of thehorizontal axis until point C appears to have moved topoint D.9. Repeat this test until the vertical cross hairpasses through the high and low points in the direct andinverted position of the telescope.10. Check all previous adjustments.NOTE: When you cannot immediately correct theabove condition, you can compensate by repeating anysurvey procedure with the telescope reversed and thenuse the average of the results.ADJUSTING THE TELESCOPE LEVEL.— Tobe able to use a transit for direct leveling and to measurevertical angles without index error, you must ensure thatthe axis of the telescope level is parallel to the line ofsight. To adjust the telescope level of the transit, use thesame two-peg method that we discussed previously forthe engineer’s level. The only difference is that you mustlevel the telescope carefully before each reading. Aftercomputing the reading that should be made on the farrod (fig. 6-3), you set the horizontal cross hair on thecomputed reading using the vertical slow motion screw.Then you move one end of the spirit level vertically bymeans of the adjusting nuts until the bubble is centeredin the tube (fig. 6-8).NOTE: As with the engineer’s level, you shouldcompensate for the above maladjustment by carefulbalancing of all backlights and foresights.ADJUSTING THE VERTICAL CIRCLEVERNIER.— For vertical angles to be measuredwithout index error caused by displacement of thevertical circle vernier, the vernier should read zero whenthe plate bubbles and telescope bubbles are properlyleveled. To make the vertical circle vernier read zerowhen the instrument is leveled (fig. 6-9), you shouldperform the following steps:1. With the plate bubbles leveled, bring thetelescope bubble to the center of the tube and read thevernier of the vertical circle.2. If the vernier does not read zero, loosen thecapstan screws holding the vernier and move the indexuntil it reads zero on the vertical circle.3. lighten the screws and read the vernier with allthe bubbles in the center of their tubes to make sure thatFigure 6-9.—Adjusting the vertical circle vernier.6-8
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