Temporary adjustments of theodolites

Temporary adjustments are set of operations which are required to be done on an instrument (theodolite) in order to make it ready for taking observations.^[1] Temporary adjustments of theodolite include its setting up, centering, levelling up and elimination of parallax. Therefore these adjustments can be achieved in 4 steps:-

Setting
Centering
Levelling
Focussing^[2]

Setting

The setting operation includes fixing the theodolite with tripod along with approximate levelling and centering over the station mark. For setting up the instrument, the tripod is placed over the station with its legs widely spread so that the centre of the tripod head lies above the station point and its head approximately level (by eye estimation). The instrument is then fixed with the tripod by screwing through trivet. The height of the instrument should be such that observer can see through telescope conveniently. After this, a plumb bob is suspended from the bottom of the instrument and it should be such that plumb bob should point near to the station mark.^[3]

Centering

Centering implies that bringing vertical axis of theodolite immediately over station mark.^[4] To do this the following procedure is followed:-

First, the approximate centring of the instrument is done by moving the tripod legs radially or circumferentially as per need of the circumstances.

It may be noted that due to radial movement of the legs, plumb bob gets shifted in the direction of the movement of the leg without seriously affecting the level of the instrument. On the other hand, when the legs are moved side ways or circumferentially, the plumb does not shift much but the level gets affected. Sometimes, the instrument and the tripod have to be moved bodily for centring. It must be noted that the centering and leveling of instrument is done recursively. Finally, exact centring is done by using the shifting head of the instrument. During this, first the screw-clamping ring of the shifting head is loosened and the upper plate of the shifting head is slid over the lower one until the plumb bob is exactly over the station mark. After the exact centring, the screw clamping ring gets tightened.^[5]

Levelling

Leveling of an instrument is done to make the vertical axis of the instrument truly vertical. For accurate levelling the following steps are strictly followed:-

Bring one of the level tube parallel to any two of the foot screws, by rotating the upper part of the instrument.
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.
The bubble of the other level tube is then brought to the centre of the level tube by rotating the third foot screw either inward or outward. [In step 1 itself, the other plate level will be parallel to the line joining the third foot screw and the centre of the line joining the previous two foot screws.]
Repeat Step 2 and step 3 in the same quadrant till both the bubble remain central.
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.

Otherwise, repeat the whole process starting from step1 to step5.^[6]

Focussing

To obtain the clear reading, the image formed by the objective lens should fall in the plane of diaphragm and the focus of eye-piece should also be at the plane of diaphragm. This is being carried out by removing parallax by proper focussing of objective and eye-piece. Thus, focussing operation involves two steps:

Focussing of eye-piece.

For focussing of the eye piece, point the telescope to the sky or hold a piece of white paper in front of telescope. Move the eye-piece in and out until a distinct sharp black image of the cross-hairs is seen. This confirms proper focussing. ^[7]

Focussing of object glass.

It is done for each independent observation to bring the image of the object in the plane of cross hairs. It includes following steps of operation: First, direct the telescope towards the object for observation. Next, turn the focusing screw until the image of the object appears clear and sharp as the observer looks through properly focused eye-piece. If focusing has been done properly, there will be no parallax i.e., there will be no apparent movement of the image relative to the cross hairs if the observer moves his eye from one side to the other or from top to bottom.^[8]

References