Temporary adjustment of theodolite

 Temporary Adjustment of Theodolite:-

Temporary adjustments are a set of operations which are performed on a theodolite to make it ready for taking observations. 

Following operations done for temporary adjustment of transit theodolite:-

1. Setting
2. Levelling
3. Estimation parallax

1. Setting:-

This operation consist of two sub- operation centering and approximate levelling. 

• Centering:-

                              Centering is a operation in which instrument set just over the station with the help of plumb bob.

• Approximate levelling:-

                                                     Approximate levelling with the helloooo of tripod legs. 

                The appropriate levelling is done with reference to a small circular bubble provided on tribrach. 

2. Levelling:-

• After the setting the instrument with centering and approximate levelling, accurate leveling is required to performing operation. 
• It's done with the help of foot screw with reference to the plate level. 
• Levelling is done for the make vertical axis of instrument truly vertical. 
• Two cases rise in levelling for instrument;

1. For three screw head
2. For four screw head

i. For three screw head:-

• Turn the upper plate until the plate level is being parallel to line joining any two foot screw( A & B ).
• Move these foot screw in inward or outward direction from both hand thumb until bubble is in centre. 
• Bubble always move in that direction in which direction left thumb move. 
• Now,  turn the upper plate as plate level make 90 degree from its original position and cover third screw. 
• Now turn this screw and bring bubble in central. 
  
  
  

• Again turn upper plate to its original position through 90 degree. Bring the bubble in center if it is not. 
• And repeat this process until bubble is in center in both condition of plate level. 

ii. For four foot screw head:-

• Torn the upper plate until the plate level is parallel to threw line joining diagonally opposite screws( B & D ).
• Torn these screws with both thumb and bring bubble in centre. 
• Turn the upper plate through 90 degree and make parallel to the other diagonally opposite screws( A & C ).
• Again bring the bubble in centre. 
• Repeat all steps till bubble in center in both position. 

3. Elimination of parallax:-

• Parallax is a condition in which image formed by the objective of telescope is not in the plane of cross hairs.
• Sighting is impossible with telescope until the parallax is eliminated. 
• For accurate sighting, the parallax is eliminated in two steps:-----

       i. Focusing eyepiece for distinct vision of cross hairs. 

       ii. Focusing objective so that the image is formed in plane of cross- hairs.