Temporary adjustments of theodolite

The temporary adjustments are the operations done at every set-up of the theodolite, before any angle is observed. They are done in this order:

1. Setting up the theodolite over the station

• Spread the tripod legs so that the head is at about chest height and roughly level, with the plumb bob (or optical plummet) near the station mark.
• Fix the instrument on the tripod head with the central fixing screw.
• Adjust the legs (move one leg in or out, or shift all the legs together) until the plumb bob is vertically above the station mark. This is centering. Fine centering is done by loosening the fixing screw and sliding the instrument on the tripod head.

2. Levelling up

The vertical axis is made truly vertical using the plate levels and the three foot screws.

1. Turn the upper plate until the plate level is parallel to the line joining any two foot screws.
2. Turn those two screws together in opposite directions (thumbs moving towards or away from each other) until the bubble is central.
3. Turn the instrument through 90 degrees so that the level points towards the third foot screw, and bring the bubble central using that third screw only.
4. Return to the first position and repeat. Continue until the bubble stays central in every position of the instrument (check by turning it through 180 degrees and through a full turn).

Levelling can disturb the centering slightly, so check the plumb bob again.

   Foot screws: A, B, C         Step 1: level || AB
                                Step 2: turn 90 deg,
                                use C only
        A ---- B                Repeat until bubble stays
          \  /                  central everywhere
            C

3. Elimination of parallax

Parallax is the apparent movement of the image relative to the cross hairs when the eye is moved. It is removed in two steps:

1. Focusing the eyepiece: point the telescope at the sky or hold a sheet of white paper in front of it, and turn the eyepiece until the cross hairs appear sharp and black.
2. Focusing the objective: sight the object and turn the focusing screw until the image is sharp and lies exactly in the plane of the cross hairs.

If the cross hairs and the image do not move relative to each other when the eye is moved slightly, the parallax has been removed.

After these three adjustments the instrument is ready for observation.