OPTICAL-MATERIALS FOR NEAR-INFRARED WOLLASTON PRISMS

The optical characteristics of birefringent materials transmitting in the near IR (0.9 - 2.5 mu m) are analyzed. Wollaston prisms with large beam separation and virtually free from lateral chromatism - e.g. wit h fields of view > 1 arcmin and image elongation < 0.3 '' - could be m anufactured using AgGaS2 or LiYF4 (YLF). These materials are used for non linear applications and may therefore find another interesting app lication in IR focal plane instruments. From the optical point of view the best material is AgGaS2 but YLF may be practically more convenien t because of its lower refractive index (n similar to 1.45), better th ermo-mechanical properties and lower price. New measurements of the tr ansparency of Calcite at room and cryogenic temperatures are also pres ented, these show significant absorption (similar or equal to 0.4 cm(- 1)) of the ordinary ray already at lambda = 2 mu m and very strong ban ds (absorption coefficient a > 3 cm(-1)) beyond 2.25 mu m. The absorpt ion does not decrease significantly when the crystals is cooled, CaCO3 should not be therefore used in instruments working beyond 2.0 mu m.