Temporary Adjustments of a Digital Theodolite.

Object:

Temporary adjustment of Digital or Electronic Theodolite.

Procedure:

Temporary adjustments of theodolite:

Temporary adjustments or station adjustments are those that are taken over the station at the time of taking observation or those which are taken at every instrument setting and preparing to take observations with the instrument.

Such adjustments include the following steps:

Setting the theodolite over the station:

Fig: 2.1 Digital theodolite

1. It consists of placing the instrument over the required station by a plumb bob or by optical plummet.
2. The tripod stand is placed over the required station and the theodolite is lifted from the box and fixed on top of the stand using a wing nut or according to the fixing arrangement provided along the instrument.
   

Approximate leveling by tripod stand:

1. The approximate leveling is done with the help of tripod legs either concerning a small circular bubble provided on tribrach is done by eye judgment.
   
2. The tripod legs are placed well apart and firmly fixed on the ground.
   
3. Two legs of the tripod stand are kept firmly on the ground and the third is moved in or out, clockwise or anticlockwise so that the bubble is approximately at the center of its run.
   

Centering:

1. Centering is the process of setting the instrument exactly over the station.
   
2. It should be confirmed at the time of approximate leveling that the plumb bob suspended from the hook under the vertical axis lies almost over the station peg.
   
3. After that, with the help of a shifting head (moveable capstan nut), the centering is done accurately so that the plumb bob exactly over the nail of the station peg.
   

Levelling:

1. After having centered and approximately leveled the instrument, accurate leveling is done with the help of foot screws and concerning the plate levels.
   
2. Its purpose is to make the vertical axis truly vertical.
   
3. The manner of leveling the instrument by the plate levels depends upon whether there are three leveling screws or four leveling screws.
   

Three screw head:

1. Turn the upper plate until the longitudinal axis of the plate level is roughly parallel to a line joining any two (such as a and b) of the leveling screws.
   
2. Hold these two leveling screws between the thumb and first finger of each hand and turn them uniformly so that the thumbs move either towards each other or away from each other until the bubble is central. It should be noted that the bubble will move in the direction of movement of the left thumb.
   
3. Turn the upper plate through 90˚, i.e. until the axis of the level passes over the position of the third leveling screw c.
   
4. Turn this leveling screw until the bubble is central.
   
5. Return the upper plate through 90˚ to its original position and repeat step (2) till the bubble is central.
   
6. Turn back again through 90˚ and repeat step 4.
   
7. Repeat steps (2) and (4) till the bubble is central in both the positions.
   
8. Now rotate the instrument through 180˚. The bubble should remain the center of its run, provided it is in correct adjustment. The vertical axis will then be truly vertical. If not, it needs permanent adjustment.
   

Elimination of parallax:

Parallax is a condition arising when the image formed by the objective is not in the plane of the cross-hairs. Unless parallax is removed, accurate sighting is impossible. Its elimination is done in two steps:

Focusing the eye-piece:

• 1. In order to focus the eye-piece for the distinct vision of the cross-hairs, point the telescope towards the sky (or hold the sheet of white paper in front of the objective).
     
  2. Move the eye-piece in or out till the cross-hairs are seen sharp and distinct.
     

Focusing on the objective:

• 1. The telescope is then directed towards the object to be sighted and the focusing screw is turned till the image appears clear and sharp.
     
  2. The image so formed is in the plane of cross-hairs.