Nikon DTM-502 Series Instruction Manual page 168

Only when the setting "HA corr." is set to OFF
while you complete updating the trunnion axis
parameter, a question screen to change the setting
will be shown.
Press the [Yes] softkey to change the setting
([ENT] doesn't work here).
================== Trunnion axis compensation ===================
The trunnion axis error, which is reported when the vertical axis and the trunnion
axis (horizontal axis) are not perpendicular to each other, and the sighting axis
error, which is from the sighting and the trunnion axes, can be compensated by the
Tr-axis and ACH parameters on this instrument.
<Plane view>
<Front view>
ACH
Tr-axis
These parameters are applied to horizontal angle readings.
Since the amount of the compensation is changed in proportion to its vertical angle,
while the compensation is set to ON (HA corr.=ON) you can see a slight movement
in horizontal angle even if you clamp the tangent screw.
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