Wide-field Solc-type birefringent filter

A 24-element wide-field Solc birefringent filter (WFSBF) has been produced and tested for the first time. WFSBF is a second unit to the 3-unit BF designed for imaging solar magnetic fields in the FeI 6173 A spectral line. WFSBF passband full width at half maximum is 0.2 A. The optical stage of the Solc BF is apodized. The main passband has a two-peak profile, to increase BF transmittance in spectral line wings. The neighbouring passbands at 2 A are cut off by the first BF unit. To measure magnetic fields, a narrower passband of the third BF unit is to scan the spectral line wings in the ±0.05A positions, in accordance with two-peak positions of the Solc filter profile. The wide field of view (FOV) of the Solc filter was reached with composite birefringent stages of 24 positive artificial paratellurite and 24 negative natural calcite crystals. FOV of a composite stage is eleven times larger than that of the only-calcite one. The calculated passband is compared to the experimental one. Technological aspects of the manufacture as well as devices for plate orientation are discussed.