Tunable Birefringent Filters Using Liquid Crystal Elements

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Recent advances in liquid crystal technology make it possible to construct tunable birefringent filters operating over the range 3500-20,000 Angstroms, with passbands as narrow as 100 Angstroms. The center wavelength of the filter can be selected electronically, in a few milliseconds, with no moving parts. These filters should be relatively inexpensive to produce ($2-3K). When liquid crystal elements are placed in series with fixed retarders made of birefringent quartz or calcite, one can construct filters with narrower passbands, ranging from 30 Angstroms to as narrow as 0.25 Angstroms. The fixed retarder provides the high resolution while the liquid crystal performs the tuning action. The filter that results is highly selective, and agile. Optical quality in these instruments is determined by the fixed retarders, and only mild restrictions are placed on the liquid crystal elements. In Phase I the performance of individual liquid crystal elements was measured. Under this Phase II award, complete filters of two types will be constructed and tested in ongoing solar and night- time programs.