A fully tunable narrow-band filter system to be used in combination with ZIMPOL for monochromatic imaging vector polarimetry is being developed. It may be used over the whole visible spectrum, from the t.JV at about 395 nm, to the red at about 660 nm, with a band width of 30-50 rnA. The main components are two lithium-niobate Fabry-Perot etalons made of Y-cut crystals, which means that the channel spectra and tuning parameters are different for the two orthogonal states of polarization (ordinary and extraordinary rays). This allows the finesse to be dramatically enhanced by a double-pass configuration, a possibility that is not available to other Fabry-Perot systems. Tuning can be achieved in three ways: temperature tuning to center the pass band within the selected fine tuning range, voltage tuning for rapid fine tuning, and tilt tuning as an additional, though normally not used, possibility. The filter system will allow us to explore the spatial structuring of the polarization signatures in the Second Solar Spectrum, including vector mapping of the Hanle and Zeeman effects in any spectral line between the UV and red part of the spectrum.
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