Normally TurPol is equipped with the 
/2 retarder, which
provides measurements of linear polarization only. In order to measure
circular & linear polarisation simultaneously, or to do circular
polarimetry, the 
/2 plate must be replaced by the 
/4
plate.
To do this, you have to remove the 4 small bolts (3mm Allen key) from
the upper part of the back cover of the polarimeter, disconnect the
motor cable and slide out the module in which the wave plate is
mounted. Note that to be able to slide the module out, you have to
also remove two of the big instrument mounting bolts on that
side. Take the module into the control room or electronics room to be
able to work on a table.
Unscrew and remove the 
/2 plate from its holder and put the
/4 plate instead (or vice--versa, of course). The orientation
of the 
/4 wave plate optical axis must be such that in the starting
position, defined by just actuating the micro-switch, the optical axis
marked with the golden lines in the cylindrical wave plate mounting
must be at 45
to the sides of the rectangular black aluminium
plate supporting the unit. This is most easily done by rotating the
large gear clockwise from this starting position by an amount
corresponding to two full rotations of the small gear (which has a
fiducial mark to be able to do this). In this position, the wave plate
mark should be placed parallel to the sides of the unit, or
``vertical'' when you hold the module with the small gear downwards.
The orientation of the 
/2 plate should also be parallel to
the sides as above, but with the big gear in the starting position
defined by the micro-switch. The surface of the wave plate mount with the
optical axis mark should look upwards, i.e. be visible from the side
of the big gear.
Changing the wave plate should be done by NOT staff only.