4.5 Checks on permanent adjustments
It is usually impossible to make corrections to permanent adjustments in the field, and often unwise to attempt them back at base either, unless clear instructions are given in the instrument manual. However, it is necessary at least to know when an instrument needs to be sent back to the maker for adjustment, and to give the correct diagnosis of the problem. The following guidelines should assist in this; the simpler adjustments are described first. 11 A small amount of thought and practice will usually show how this is done on any particular instrument.
Bubble errors
If there is a significant error in the plate bubble when levelling the instrument up (see Section 4.3, under Setting up, step 8), this can be removed by levelling the instrument carefully, then adjusting the plate bubble so that it lies in the centre of its glass. In a level instrument with no bubble error, the instrument can be rotated about its vertical axis and the bubble will always return to the centre point. If the instrument has been levelled on a tribrach and the cup bubble on the tribrach is not central, then this indicates a bubble error on the tribrach. Again, this can be removed by levelling the instrument and then adjusting the tribrach bubble. If there is a significant difference between the CL and 360° CR readings on a vertical observation, this indicates a bubble error in the alidade bubble. Putting this right is more advanced. The procedure is to make a vertical observation and calculate the mean of the CL and 360° CR readings. With the instrument observing the target in CL, adjust the alidade bubble until, with the bubble central, the average reading is obtained. Then reobserve on CR and check that 360° CR gives the same angle.
Plummet errors
If the plummet (laser or optical) rotates with the instrument, it is easy to see whether there is a plummet error by simply rotating the instrument. If the line of collimation makes a circle then there is an error; this can be removed by keeping the instrument still and adjusting the line of collimation of the plummet to point at the centre of the circle. Details of how to do this should be given in the instruments manual. If the plummet does not rotate (e.g. it is fixed to a tribrach), then errors are harder to detect. One simple method is periodically to lay the instrument on its side on a bench, with the tribrach attached and the vertical axis clamped. Sight through the plummet and mark the point on the wall on the line of collimation. Unclamp the vertical axis, rotate through approximately 120°, and repeat; then rotate through a further 120°, and repeat again. If the three marks are in different places, the plummet has an error. Correcting such an error generally requires a special instrument, or professional servicing.
Reticle errors
Reticle errors should only be corrected by an instrument maker, but are relatively easy to diagnose. To check the vertical reticle, sight on a suitable target with some part of the reticle, then rotate the telescope about the trunnion axis and see whether all parts of the vertical reticle align with the target. The horizontal reticle is similarly checked, by rotating the telescope about the vertical axis.
Collimation errors
Collimation errors in the horizontal plane can be detected by sighting on a target at a similar height to the instrument and taking a horizontal angle reading; then transitting the instrument and taking the reading again. If the two readings do not differ by exactly 180°, then this is due to Collimation error. As explained above, this error is relatively
