Sokkia CX Series Operator's Manual page 46

8. FOCUSSING AND TARGET SIGHTING
4. Readjust the focus with the
focussing ring until there is no
parallax between the target image
and the reticle.
Eliminating parallax
This is the relative displacement
of the target image with respect to the reticle when the observer's head is
moved slightly before the eyepiece.
Parallax will introduce reading errors and must be removed before
observations are taken. Parallax can be removed by refocussing the reticle.
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