We present an optical design of a near-IR (at Fel15648.5 A) birefringent filter system that is used to form a magnetograph to obtain chromatic images of the sun and measure the solar magnetic field at the Big Bear Solar Observatory (BBSO). This magnetograph consists of an interference prefilter, a polarization analyzer, a wavelength-tunable birefringent filter, and a Fabry-Perot Etalon. This system is expected to achieve a clean narrow pass band of 1/8 A. It can also be tuned across the spectral line to obtain line profiles of 2-D fields of view. During the design of the tunable near-IR birefringent filter and the testing of its components, we found that the value of the birefringent index μ(n_(e)-n_(o)) of calcite at a spectral range near 15648.5 A is different from the value quoted in the literature. Because the birefringent index is important for the design of optical components, the birefringent index of calcite and its thermal coefficient were measured. Test results of the four pairs of calcite plates used in the birefringent filter are presented.
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