17 December 1998 Grating stray light analysis and control in the UVCS/SOHO
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Abstract
The Ultraviolet Coronagraph Spectrometer (UVCS) of the Solar and Heliospheric (SOHO) mission has been developed for spectroscopic and polarimetric determinations of temperatures, densities and flow velocities in the extended solar corona. The instrument consists of a visible light (VL) polarimeter, and two ultraviolet (UV) spectrometers, optimized for the H I Lyman (alpha) (Ly-(alpha) ) line ((lambda) 1216 angstrom) and for the O VI doublet ((lambda) (lambda) 1032/1037 angstrom). The stray-light profile of the Ly-(alpha) holographic grating has been measured, in a laboratory set-up, in both the spatial and spectral directions. The observed profile has been found to be a combination of two components: the scattering from the grating surface, and the Fraunhofer diffraction due to the vignetting of the grating. An analytical expression for the scattering component of the grating point spread function (PSF) has been derived from a simple model of the grating surface roughness. The stray-light profile generated by the analytical expression of the grating PSF gives a good fit of stray-light profile measured in the laboratory. This instrument function has been used in the analysis of in-flight UVCS observations of the profile of electron scattered Ly- (alpha) from the solar corona. These observations have resulted, for the first time, in the most direct measurement of the coronal electron temperature.
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Silvano Fineschi, Larry D. Gardner, John L. Kohl, Marco Romoli, Giancarlo C. Noci, "Grating stray light analysis and control in the UVCS/SOHO", Proc. SPIE 3443, X-Ray and Ultraviolet Spectroscopy and Polarimetry II, (17 December 1998); doi: 10.1117/12.333614; https://doi.org/10.1117/12.333614
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