How to set up a Theodolite

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.
Temporary adjustments of theodolite include its
setting up, centering, leveling up and elimination of
parallax. Therefore these adjustments can be achieved
in 4 steps:-
Setting
Centering
Levelling
Focusing
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.
Centering
Centering implies that bringing vertical axis of
theodolite immediately over station mark.[1] 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.
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:-
1. Bring one of the level tube parallel to any two of
the foot screws, by rotating the upper part of the
instrument.
2. 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.
3. 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.]
4. Repeat Step 2 and step 3 in the same quadrant till
both the bubble remain central.
5. 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.
Focusing
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 focusing of objective and eye-piece.
Thus, focusing operation involves two steps:
Focusing of eye-piece .
For focusing 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 focusing. [2]
Focusing 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.