The half-wave plate positions are controlled through the MEDM GUI running on one of the computers in the Hall A Counting House. The half-wave plates have only two positions: IN (in the laser beam path) and OUT (pulled out of the laser path). The speed, acceleration, deceleration and number of steps can be defined for the half-wave plate motor, but the defaults should be fine except in special circumstances. The GUI allows insertion and removal of the half-wave plates:
To move the half-wave plates IN, click the ``Go'' button under ``Move Half-Wave Plates IN''.
To move the half-wave plates OUT, click the ``Go'' button under ``Move Half-Wave Plates OUT''.
After the motion is finished, you can get the position of the motor by clicking on the ``Get Position'' button. This button shouldn't be pressed while the motor is moving because the motor likes to perform one operation at a time.
The motor that drives the half-wave plates isn't 100% reliable, so the plates should be checked on the camera monitor to make sure they are in the correct position after a move. If you see a problem with the motion of the motor (i.e. it's driving too far forward or about to drive off the table) then press the Emergency Stop ``HALT'' button.