Measurement Of The Instrumental Polarization Of A High Resolution Ultraviolet Spectrometer

Diffraction grating spectrometers exhibit a complex dependance on the polarization state of incident light. We have characterized the polarization properties of a high resolution echelle grating spectrometer. Large variations of instrument response with incident polarization state at ultraviolet wavelengths were found. The Imaging Stratospheric Ultraviolet Spectrometer (ISUS) is a high resolution spectrometer for remote sensing of atmospheric trace constituents. Weak line emissions are observed against the bright, partially polarized Rayleigh scattered sky background. To determine the air density from the Rayleigh scattering intensity the effects of polarization on the spectrometer response must be known. It was anticipated that such polarization effects could be significant in the ISUS instrument, which includes a fold mirror, a cross dispersing plane diffraction grating and an echelle grating. Measurements of the sensitivity of ISUS to linearly polarized light at 312.6 nm showed a peak-to-peak variation of 72% as the plane of polarization is rotated. The results of these measurements are presented and nine elements of the sixteen element system Mueller matrix which describes the behavior of ISUS in partially linearly polarized light are measured. The implications of the observed instrumental polarization for remote sensing observations and its impact on a technique for discriminating against the polarized Rayleigh scattered background to improve the measurement sensitivity are discussed. Subject terms: polarization; spectrometers, polarimetry, polarization aberration; instrumental polarization, Mueller calculus.