Sokkia 130R Series Operator's Manual page 37

4. Readjust the focus until there is no
parallax
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.
8. FOCUSSING AND TARGET SIGHTING
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