FOCAS polarimetric optical components
Retarders (wave plates)In order to measure both linear and circular polarization of the light from the target, FOCAS has a half-wave plate and a quarter-wave plate. Both are superachromatic plates, made from quartz crystal plates and magnesium fluoride plates cemented between parallel plates of fused silica Suprasil. Each plate unit consists of four fan-shaped pieces. The effective wavelength region is 3500-12000 Angstroms (Figure 1). They were manufactured by B. Halle Nachfl. The half-wave and quarter-wave plates are mounted on both ends of the wave plate slide, and we cannot use both simultaneously.
Figure 1. Wavelength dependence of the retardance (upper) and the orientation of the effective optical axis (lower) for the half-wave plate (left) and the quarter-wave plate (right), respectively.
Beam splitter (Wollaston prism)The crystal quartz Wollaston prism, also manufactured by B. Halle Nachfl., is used to split the incident beam into two orthogonally polarized beams (ordinary and extraordinary). The separation angle between o- and e- beams is 32'.17 at 6330 angstroms, and is slightly wavelength-dependent. To avoid blending of the ordinary and extraordinary rays of 6330 angstrom, the spatial opening of each slit (or slot) has to be narrower than 10.3 mm and the gap between neighboring slits (or slots) has to be wider than 10.3 mm. We have prepared some standard masks for various fields of polarimetry, which have eight slits (or slots) of 10.0 mm length aligned at equal intervals of a 11.5 mm length gap. Please refer to the page on available focal plane masks.