Line cross parallax

The crosshair parallax (or crosshair parallax ) is an error that occurs when measuring with optical and electro-optical measuring instruments, mostly with theodolites and leveling devices. The same error can also occur with other telescopes, for example in astrometry and measuring microscopes . Line cross parallax occurs when the line cross plate does not coincide with the image plane of the real intermediate image, that is, when it does not coincide with the focal point of the eyepiece .

Detecting the parallax error

The line cross parallax is recognized by moving the eye slightly to and fro in front of the eyepiece. If the crosshair also moves in front of the targeted point, there is crosshair parallax.

Eliminating a line cross parallax

Line cross parallaxes have a negative effect on the measurement results achieved. It must be removed before the measurement because it is not possible to take it into account afterwards.

1. Rotating telescope against a bright, diffuse background
2. Focusing the cross
3. Bringing the image into focus (focusing)

This is to be done by every observer. This must be checked in the meantime for longer measurements.

When taking measurements in tunnels, mines or at dusk, make sure that the reticle can be illuminated so that the reticle parallax can be recognized and eliminated.