There are two tests and adjustments necessary for an automatic level:
(1) To ensure that the line of collimation of the telescope is horizontal, within the limits of the bubble, when the circular bubble is central.
(2) The two-peg test for collimation error.
Alternative two-peg test
(1) Circular bubble:
Although the circular bubble is relatively insensitive, it nevertheless plays an important part in the efficient
functioning of the compensator:
(1) The compensator has a limited working range. If the circular bubble is out of adjustment, thereby resulting in excessive tilt of the line of collimation (and the vertical axis), the compensator may not function efficiently or, as it attempts to compensate, the large swing of the pendulum system may cause it to stick in the telescope tube.
(2) The compensator gives the most accurate results near the centre of its movement, so even if the bubble is in adjustment, it should be carefully and accurately centred.
(3) The plane of the pendulum swing of the freely suspended surfaces should be parallel to the line of sight, otherwise over- or under-compensation may occur. This would result if the circular bubble were in error transversely. Any residual error of adjustment can be eliminated by centring the bubble with the telescope pointing backwards, whilst at the next instrument set-up it is centred with the telescope pointing forward. This alternating process is continued throughout the levelling.
(4) Inclination of the telescope can cause an error in automatic levels, which does not occur in tilting levels, known as ‘height shift’. Due to the inclination of the telescope the centre of the object lens is displaced vertically above or below the centre of the cross-hair, resulting in very small reading errors, but which cannot be tolerated in precise work.
From the above it can be seen that not only must the circular bubble be in adjustment but it should also be accurately centred when in use. To adjust the bubble, bring it exactly to centre using the footscrews. Now rotate the instrument through 180◦ about the vertical axis. If the bubble moves off centre, bring it halfway back to centre with the footscrews and then exactly back to the centre using its adjusting screws.
(2) Two-peg test:
This is carried out exactly as for the tilting level. However, the line of sight is raised or lowered to its correct reading by moving the cross-hair by means of its adjusting screws. If the instrument is still unsatisfactory the fault may lie with the compensator, in which case it should be returned to the manufacturer.