Circular polarimetry is similar to linear polarimetry. Instead of the halfwave plate one uses the quarterwave plate, to convert circularly polarized light of the incoming beam into linearly polarized light. This again enters the spectrograph and is separated into an o- and e-beam on passing the calcite slab. The position angle of the linear polarized vector is defined by the position angle of the quarterwave plate and by the sense of rotation (left or right) of the incoming circularly polarized beam.
Because of its limited diameter, the currently available quarterwave plate (a spare for the Peoples Photometer) allows one to observe point sources only. Two of the apertures in the comb Dekkers just fit into the unobstructed field.
Another complication is that the TV camera cannot see the slit through the quarterwave at all, which hampers acquisition of the target on the slit. Acquire the target with the quarterwave plate out the beam, find a star to guide on (or trust the tracking), and move the quarterwave plate back into the beam. Remember to adjust focus.
Because the quarterwave plate is not exactly quarterwave for all
wavelengths, the circular polarimeter is partly sensitive to linear
polarization, which in the astronomical context generally vastly
exceeds the circular polarization one is trying to measure. If you
suspect linear polarization in your source, you can depolarize it by
inserting the halfwave plate ahead of the quarterwave plate and
setting it into continuous rotation; this should eliminate systematic
errors due to linear polarization, and invert but leave otherwise
intact, the true circular polarization. Note that the normal
setup is with the halfwave after the quarterwave and a request
to invert the order must be made when applying for telescope time; the
inverse order makes TV slit viewing difficult even when just the
halfwave is in the beam, e.g. for linear polarimetry.
The angle offsets are twice as large as in linear polarimetry.
Reduction to degree of circular polarization is identical to that
for linear polarization.
For calibration purposes a very simple circular polarizer is available in the A&G main filter slide. If linear polarization of your source is strong, you must determine to what extent the telescope converts linear to circular. The way to do this is to set up ISIS for circular polarization, but observe a strongly linearly polarized star; obtain 2 complete observations, with a parallactic angle difference of 90 degrees (Fig. 5). External circular polarization is constant, but converted linear will be inverted in the two observations.
There is considerable confusion in the literature about the sign conventions for circular polarimetry. Handedness is defined with reference either to the instantaneous 3D e.m. field configuration, or to the sense of rotation of the electric vector projection as seen by either source or observer. For any of these cases, positive can be associated with left or right handedness. We do not recommend any one of the many versions; just be aware of the confusion and define your observations uniquely, both for yourself and in the paper you write.