Circular polarimetry with FAPOL

Circular polarimetry is made with a 1/4 wave plate in the FAPOL unit and a calcite plate in the aperture wheel. (The lambda/4 plate is installed with its optical axis at 45 degrees with respect to the principle plane of the calcite crystal, that is the plane defined by the o- and e- rays.) The field vignetting is as for linear polarimetry limited by the size of the calcites (i.e. 140" in diameter). Verify with staff that the 1/4 wave plate is installed in FAPOL.

The 1/4 wave plate is a retarder which is used to convert elliptically or circularly polarised light into linearly polarised light, and the calcite plate produces the two orthogonally polarized beams. Both the ordinary and extraordinary components of a ray are shifted in phase by the quarter of a wavelength, i.e. the phase delay is 90°. The minimum requirement is to expose at 2 different angles of the 1/4 wave plate: 0° and 90° , but with 4 angles (stepped by 90°) the difference between the transmission of the orthogonally polarized spectra is eliminated in the reductions, since the polarisation modulation of the o- and e- rays have the inverse effect in the 3rd and 4th exposure compared with the 1st and 2nd.

Let O(i) and E(i) be the intensities of the ordinary and extraordinary images obtained through the calcite plate for each of the i=1,2 angles of the 1/4 wave plate. The percentage of circular polarisation (V) and its sign (negative = left handed, positive = right handed) is found as follows:

Q(i) = E(i)/O(i)
QM = Q(1) + Q(2)
V = 50.0*(Q(1)-Q(2))/QM

According to Saviane, Piirola, Bagnulo et al. 2007, ESO Messenger 129, 14-17 the circular polarisation for the following objects are measured with EFOSC2 at the ESO 3.6m telescope to be:

Observing steps