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1. Introduction

Wollaston prisms are extensively used in focal plane astronomical instruments working at visual/ultraviolet wavelengths (e.g. Fosbury et al. 1989). Their main advantage compared to other polarizers/analyzers is that they produce simultaneous images in two perpendicular polarization states, and this minimizes the effect of transparency variations, i.e. the photometric errors propagating into the polarimetric measurement. The use of Wollastons in the IR was so far much more limited though excellent polarization images obtained with this technique can be found in the literature (e.g. Packham et al. 1996). In this paper we present a study of the optical properties of infrared birefringent crystals aimed at defining convenient materials to manufacture Wollaston prisms with large beam separation and small lateral chromatism.

The second aim of this paper is to determine the long wavelength transmission cutoff of Calcite at cryogenic temperatures. Surprisingly, no such information exists in the literature and, to the best of our knowledge, all available data are still based on the room temperature work of Nysander (1909) who found that CaCOtex2html_wrap_inline1159 becomes opaque to the ordinary ray at tex2html_wrap_inline1161 tex2html_wrap_inline1163m.

In Sect. 2 we briefly review the basic concepts of Wollaston prisms and analyze the optical performances of birefringent crystals, including several materials recently developed for non-linear applications. In Sect. 3 we present and discuss new measurements of the transmittance of Calcite at cryogenic temperatures. In Sect. 4 we draw our conclusions.

Copyright by the European Southern Observatory (ESO)