HIGH-EFFICIENCY SUPERACHROMATIC POLARIMETRY OPTICS FOR USE IN OPTICAL ASTRONOMICAL SPECTROGRAPHS

A design for an optical spectropolarimeter consisting of a Pancharatnam half-wave plate and a modified Glan-Taylor polarizing beam splitter retrofitted to an existing astronomical spectrograph is described. The wave plate has a retardance of 180-degrees +/-2-degrees over the entire design wavelength range from 3200 angstrom to 1.1-mu-m. The calcite polarizing beam splitter has a beam separation of one beam diameter, and a low length-to-width ratio of 1.035, making efficient use of the limited space available in most spectrographs. Cassegrain CCD spectrographs on the Mount Locke 2.7-m and Mount Palomar 5-m telescopes have been retrofitted with spectropolarimeters of the type described. The combination of these "superachromatic" polarimeter parts with the high-efficiency CCD detectors now used in these and many other astronomical spectrographs should lead to a great expansion of spectropolarimetry in many branches of astronomical research.