LESSON NOTE ON TEMPORARY ADJUSTMENT OF THEODOLITE Part C

TEMPORARY ADJUSTMENT OF THEODOLITE

Temporary adjustment are the adjustment which are required to be made at each setting of the instrument before taking observation. These adjustments are also known as station adjustments.

The following three adjustments are required:

1. Setting up and centring,

2.Levlling

3.Elimination of parallax.

Setting up and Centring

This involves setting the theodolite exactly over the station mark or on the station peg. It is done by the following steps:

• The plumb bob is suspended from a small hook attached to the vertical axis of the theodolite
•     The instrument is placed over the station mark with the telescope at a convenient height and with the tripod legs set well apart.
• Two legs of the tripod are set frimly into the ground and the third leg is moved radially to bring plumb bob exactly over the station mark. Then the third leg is also pushed into the ground.
• The instrument is roughly centered over the station mark and then by means of the shifting head, the plumb bob is brought excatly over the station mark.

 

Levelling

The procedure of levelling with three foot screws is as follows:-

•    Turn the upper plate  untill the longitudinal axis of the plate level is roughly parallel to a line joining any two of the levelling screws.
•     Hold these two levelling screws between the thumb and the first finger and turn them uniformly so that the thumbs move either towards each other or away from each other until the bubble is central.
• Turn the upper palte 90, i.e untill the axis of the level passes over the position of the third levelling screw C.
• Turn this levelling screw C untill the bubble is central.
• Return the upper plate through 90 to its original position. Rotate screws A and B inwards or outwards till the bubble is central.
• Repeat the above steps,till the bubble is central in both the positions.

Elimination of parallax

It consists of focusing of eyepiece and the objective.

Focussing the eyepiece This operation is done to make the cross-hairs appear clearly visible. 

The following steps are involved:-

1.     The telescope is directed towards the sky or a sheet of white paper held in front of the objective.

2.     The eyepiece is moved in or out until the cross-hairs appears clear and distinct.

Focussing the objective This operation is done to bring the image of the object in the plane of the cross-hairs. The following steps are involved:-

1. The telescope is directed towards the object.

2. The focusing screw is turned until the image appear clear and sharp.