Iceland spar and birefringent filter (BF) development

Birefringent crystals for BF are required to provide the necessary linear aperture, wavefront quality, and filter transmission in a given spectral region. Based on the review of foreign and domestic BF developments, the difficulties in using crystals from known deposits are examined. The Institute of Solar-Terrestrial Physics has developed BFs with extreme characteristics, using an innovative method for enriching crystals from East Siberian deposits and new methods for optical treatment and control of BF elements. We present the characteristics of the filters. Further progress in the creation of BFs in our country depends on the availability of necessary synthetic crystals and natural crystal of Iceland spar, which turned out to be a difficult fate in Russia.