At each set up of a level instrument, temporary adjustment is required to be carried out prior to any staff observation.
Temporary Adjustment of a Level
At each set up of a level instrument, temporary adjustment is required to be carried out prior to any staff observation. It involves some well defined operations which are required to be carried out in proper sequence.
The temporary adjustment of a dumpy level consists of (1)Setting , (2)Leveling and (3) Focusing .
During Setting, the tripod stand is set up at a convenient height having its head horizontal (through eye estimation). The instrument is then fixed on the head by rotating the lower part of the instrument with right hand and holding firmly the upper part with left hand. Before fixing, the leveling screws are required to be brought in between the tribrach and trivet. The bull's eye bubble (circular bubble), if present, is then brought to the centre by adjusting the tripod legs.
Next, Leveling of the instrument is done to make the vertical axis of the instrument truly vertical. It is achieved by carrying out the following steps:
Step 1: The level tube is brought parallel to any two of the foot screws, by rotating the upper part of the instrument.
Step 2: The bubble is brought to the centre of the level tube by rotating both the foot screws either inward or outward. (The bubble moves in the same direction as the left thumb.)
Step 3: The level tube is then brought over the third foot screw again by rotating the upper part of the instrument.
Step 4: The bubble is then again brought to the centre of the level tube by rotating the third foot screw either inward or outward.
Step 5: Repeat Step 1 by rotating the upper part of the instrument in the same quadrant of the circle and then Step 2.
Step 6: Repeat Step 3 by rotating the upper part of the instrument in the same quadrant of the circle and then Step 4.
Step 7: Repeat Steps 5 and 6, till the bubble remains central
in both the positions. Step 8: By rotating the upper part of the
instrument through 180 ° , the level tube is brought parallel to first
two foot screws in reverse order. The bubble will remain in the centre if the
instrument is in permanent adjustment.
Focusing is required to be done in order to form image through objective lens at the plane of the diaphragm and to view the clear image of the object through eye-piece. This is being carried out by removing parallax by proper focusing of objective and eye-piece. For focusing the eye-piece, the telescope is first pointed towards the sky. Then the ring of eye-piece is turned either in or out until the cross-hairs are seen sharp and distinct. Focusing of eye-piece depends on the vision of observer and thus required whenever there is a change in observer.
For focusing the objective, the telescope is first pointed towards the object. Then, the focusing screw is turned until the image of the object appears clear and sharp and there is no relative movement between the image and the cross-hairs. This is required to be done before taking any observation.