Temporary adjustments of theodolites

Temporary adjustments are a set of operations which are performed on a theodolite to make it ready for taking observations. These include its initial setting up on a tripod or other stand, centering, levelling up and focusing of eyepiece.
Initial setting.
The initial setting operation includes fixing the theodolite on a 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 the 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 approximately align with the station mark.
Centering.
Centering means bringing the vertical axis of the theodolite exactly over the station mark.
 Exact centering is done by using the shifting head of the instrument. During this, first the screw-clamping ring of the sliding 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 centering, the screw clamping ring is tightened. This can be done by means of a forced centering plate or tribrach. An optical or laser plummet is normally used for the most accurate setting. The centering and levelling of the instrument is interactive and iterative; a re-levelling may change the centering, so error each is eliminated successively until negligible.
Levelling.
Leveling of an instrument is done to make its vertical axis adjusted with respect to the apparent force of gravity at the station.
For two spirit vials at right angles:
The same principle applies for a bulls-eye level:
Focusing.
To obtain an accurate clear sighting, the cross hairs should be in focus; adjust the eyepiece to do this.
To clearly view the object being sighted focus the objective lens.