In order to measure circular polarisation, the half-wave retarder plate is replaced by the quarter-wave retarder. By requesting the quarter-wave retarder in your NIRF (see the NOT web pages), NOT Staff will prepare Turpol by installing this retarder.
With the quarter-wave retarder plate installed, there are two observing modes available:
Circular polarimetry. Select from the MODE menu. The 
/4
plate is rotated in 90 
steps, and therefore the 'Wheel' light
is illuminated during every second sub--integration. The REDUCE menu
functions for circular polarisation reduction are available as for linear
polarimetry.
Simultaneous circular and linear polarimetry. After the
installation of the 
/4 plate, the observations are carried out
in the Linear polarisation mode, where the retarder is being rotated
in 22.5 
steps. The efficiency in this mode is about 0.7 for
circular, and 0.5 for linear polarisation measurements. The off-line
reduction program cirli (see Chapter 4 on Data Reduction) takes these
factors into account and lists the corrected circular and linear
polarisation data in the respective output files. However, the
on--line reduction for linear polarisation under the REDUCE menu
assumes the use of the 
/2 plate, and therefore the linear
polarisation values listed on the screen have to be multiplied by a
factor of 2 to get the true values.