Abstract

The SUMER (solar ultraviolet measurements of emitted radiation) instrument on the SOHO (solar and heliospheric observatory) satellite is sensitive to the state of polarization of the incident radiation primarily due to two optical elements, the scan mirror and the holographic grating. The angle of incidence of light striking the scan mirror varies from roughly 73.3 to 81.6 degrees (with respect to the mirror normal), which causes the mirror reflectance to be sensitive to the state of polarization of the incident radiation. Therefore, the measurement and characterization of this polarization sensitivity as a function of wavelength was performed using the engineering model optics (scan mirror and grating) and synchrotron radiation, which is nearly 100% linearly polarized, from the SUPERACO (Super Anneau de Collisions d'Orsay) positron storage ring in Orsay, France. The polarization sensitivity or modulation factor of the SUMER instrument was found to be between 0.4 to 0.6, depending on the wavelength and the angle of incidence of light striking the scan mirror, and agrees with the calculated polarization properties based on the measured optical constants for silicon carbide (SiC).

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