Tilt axis

Tilting axis is the horizontal axis of a theodolite or universal instrument around which the telescopic sight rotates when tilting vertically (changing the elevation angle ).

It is mounted on the two supports of the alidade and firmly connected to the vertical circle. If it is not at right angles to the vertical axis of the theodolite substructure, it is called a tilt axis error . With modern theodolites it is only a few arcseconds , but this can still impair the accuracy with steep sightings . It can be eliminated by measuring it by observing each target point in two telescope positions and averaging the measured directional values.

Inclination measurement of the tilting axis of an astronomical universal with a rider level (very high). If the instrument is turned 180 ° or if the instrument is knocked through , the level plays again at almost the same point (α '~ α). The inclination can be read off the scale and is set aside when leveling with the foot screws .

A subsequent reduction in the measured directions is also possible if a rider level is used. If this is read off in both circular positions , the residual influence of the standing axis error does not apply . The latter method is common with measuring accuracies of over 1 ″, especially in astrogeodesy and astrometry .

Large universal instruments such as the Wild T4 and passage instruments also use hanging vials, which are hung directly on two exposed points on the tilt axis. One then speaks of the tilting axis inclination or of the inclination correction .