The Ba II 4554 /Hβ imaging polarimeter for the Dutch Open Telescope

In order to expand the high-resolution, multi-wavelength imaging capabilities of the Dutch Open Telescope (DOT), an additional polarimetric channel based on a 80mÅ tunable Lyot filter for Ba II 4554 and Hβ has been designed and constructed. The large atomic mass and the resulting steep line wings, make Ba II 4554 particularly suitable for the creation of photospheric Dopplergrams and Stokes-V magnetograms. The line also yields a significant degree of linear (scattering) polarization for observations near the limb of the Sun, which is modified by both horizontal and vertical weak-field topologies through the Hanle effect and hyperfine-structure level crossing. The polarimeter is based on liquid crystal variable retarders (LCVRs) as polarization modulators in combination with the Lyot filter's entrance polarizer. The tunability of the LCVRs is exploited to enable specific wavelength calibration, selection of the reference frame of linear polarization, and optimization of instrumental polarization cross-talk, which for the DOT is constant in time. With the future Ba II 4554 photospheric magnetograms, we expect to be able to discern magnetic structures of about 150km with field strengths down to 100 G, and that Hanletype observations can be performed at a resolution of about 1″. The range of applicability of Hβ imaging polarimetry has to be explored after installation.