Parts of the Instrument and Levelling Head
Adjusting the levelling head
Aim - to level the tribrach so that the telescope rotates in a horizontal plane.
Levelling an instrument with a linear bubble tube.
Align
the bubble tube parallel to two foot screws.
Facing the instrument rotate both foot screws in opposite directions at the same time. Either thumbs moving inwards or thumbs moving outwards.
The bubble moves in the direction of the left thumb.
When
the bubble has settled in the exact centre of the tube the instrument
is level on this axis.
Rotate the telescope through 90° so the bubble tube is aligned with the third levelling screw.
The
first two screws are level so they must not be touched.
Using the left hand adjust the third levelling screw to centre the bubble. Again the bubble will move in the same direction as the left thumb.
With
the bubble centred the instrument should be level.
As a check rotate the instrument through 90°. If necessary re-level the original two foot screws and repeat the rotation to check the third one.
You may have to do two sets of adjustments before the instrument is level and the bubble remains in the centre of the tube as the telescope is rotated.
The tube is engraved with calibration marks to show the centre. The bubble must be no more than one space on the calibration scale off the centre for the compensator to work.
Now check your knowledge:
