Permanent adjustment of Vertical Cross Hair of telescope of transit |
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The vertical cross hair should lie in a plane perpendicular to the horizontal axis. Test : To conduct a test for this, first the instrument is to be temporarily adjusted on any station and sight a well defined point object at a distance of about 100m. Get the object point bisected on the vertical cross hair . Keeping both the upper and lower plate main screw clamped, swing the telescope in vertical direction. If the point appears to move continuously on the vertical hair, the cross hair lies in a plane perpendicular to the horizontal axis (Figure 21.2a).
Adjustment : If the point object appears to depart from the cross hair (Figure 21.2b), loosen two adjacent capstan screws and twist the cross-hair ring in the telescope tube, so that the point appear on the vertical cross hair. Tighten the two screws. Carry out the test again and make adjustment, if required, until the point traverses the entire length of the hair as the telescope is swung in vertical direction. |