Until 1990, most spectropolarimetry at the AAT was carried out with a polarimeter module designed around a Pockels cell modulator built at ROE. This system is described by McLean et al. (1984 MNRAS 209, 655), and its use is discussed in detail in AAO UM 24: Spectropolarimetry at the AAT. In essence, this spectropolarimeter module can be used with any instrument mounted at Cassegrain, and with either the IPCS or (less efficiently) a CCD as detector.
In 1990, a new rotating wave plate module built at Hatfield Polytechnic was commissioned for spectropolarimetry with the RGO spectrograph and FORS, using a CCD as detector. This is now the system of choice for most linear polarization work, since it is more sensitive than the Pockels cell system. Separate CCD frames taken with the wave plate in four positions (0°, 45°, 22.5°, 67.5°) are combined to obtain polarimetry. The whole sequence of exposures is carried out automatically under computer control, and the data can be reduced at the telescope in pseudo-real time. The analyser system remains the calcite block used with the Pockels cell system, and thus the spatial coverage remains the same as before. (There are two calcite blocks available: the thick calcite is normally used with the 25cm camera and gives a separation of about 12' along the slit, while the thin calcite gives a smaller separation and is normally used with the 82cm camera).
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