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Градуированная дуга, пока пузырек не окажется в центре, быстрый способ, линия обзора, определенная точка, впереди рефлектора, настроить уровень.

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Lesson 12

Dumpy Level

The dumpy level is in adjustment when: (1) the horizontal crosshair is truly horizontal when the instrument is leveled; (2) the axis of the bubble tube is perpendicular to the vertical axis; and (3)the axis of the bubble tube and the line of sight are parallel.

See that the eyepiece and crosshairs are in proper focus. Sight the vertical crosshair on a point. Move your head slightly and slowly sideways and observe if the vertical hair moves off the point. If it does, parallax exists. This indicates imperfect focusing.

To focus the eyepiece, point the telescope at the sky or at some white surface. Turn the eyepiece until the crosshairs appear as distinct as possible. Retest for parallax-after a few trials a position is found d where there is no parallax.

To make the horizontal crosshairslie in a plane perpendicular to the vertical axis, set up the level and sight a definite fixed point in the field of view. Next, turn the telescope about the vertical axis. If the point appears to travel along the crosshair, no adjustment is necessary. If a point leaves the crosshair, loosen the capstan screw that holds the

crosshair ring in place. Turn ring with pressure from fingers or tapping with a pencil and retest until the point remains on the crosshair as the telescope is rotated. Then retighten the capstan screw.

To make the axis of the bubble tubeperpendicular to the vertical axis, center the bubble over both pairs of leveling screws and then bring it to center exactly over one pair of screws. If the telescope is rotated end for end over the same pair of screws, the bubble will remain centered if in adjustment. If not, the amount of movement away from center is double the error adjustment. Bring the bubble back halfway with the adjusting screw

at one end of the bubble tube. Then bring the bubble back to center with the leveling screws. It should remain centered when the telescope is rotated to face the opposite direction. To make the line of sight parallel to the axis of the bubble, perform the twopeg test.First, set two pegs or stakes 60 to 90 m (200 to 300 ft) apart. Then set the instrument midway between them and take a rod reading on each peg with the telescope

bubble centered at each reading (fig. 1-15). The difference in the two rod readings, 1.28 m (4.2ft), gives the true difference in elevation between the pegs. Next, set the instrument at A, preferably the high peg, and read the rod while it is held against or within 1.3 cm (1/2in) of the eyepiece. Note where the center of the eyepiece touches the rod. You may prefer to take this reading backwards through the telescope. This reading, plus or minus the above difference, gives the true reading at B, 2.83 m (9.29ft), which makes the line of sight horizontal. With the horizontal crosshair on this reading on the rod at B, the line of sight is parallel to the axis of the bubble if the bubble is in the center of the tube. If

it is out of adjustment, keep the bubble tube centered and move the horizontal crosshair until the line of sight intercepts the true reading on the rod at B. Then repeat the process to check the adjustments made. If the bubble fails to center on the self-leveling level, bring it halfway toward the center with the leveling screws and the rest of the way by tightening

the most logical adjusting screws until the bubble is precisely centered. Do not loosen any of them. Turn the telescope 180" in azimuth until it is parallel to the same pair of leveling strews. If the bubble does not center, repeat the adjustment.

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Lesson 13

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