Polarization Properties of a Grating Spectrograph

Abstract

The 13.7-m Czerny-Turner spectrograph at the McMath solar telescope is evaluated with regard to polarization properties. Let I || = the transmissivity of the spectrograph for incident light linear polarized parallel to the entrance slit and I perpendicular = transmissivity for light polarized perpendicular to the entrance slit. The ratio I ||/I perpendicular was measured photoelectrically as a function of wavelength in six diffraction orders. Values of this ratio vary from 0.3 to 20. It is shown that spectrograph transmission as a function of polarization may cause large photometric and radiometric errors. This fact is not unique to this particular instrument. Two mechanisms appear to contribute to the polarizance of the grating. These are: (1) the Rayleigh or Wood's anomalies in which polarizance maxima and minima occur and (2) a vector wave interaction introduced because groove dimension is nearly the same as the wavelength. For lambda > groove depth it is found that the wavelength of a peak polarizance (lambda(p)) is given by lambda(p) = 0.7d costheta, where d is the ruling separation and theta is the grating angle. Photoelectric scans of the solar spectrum were made across several of the Wood's anomalies to obtain their profiles.