12 July 2008 In-orbit calibration of the polarization flat fields of the SOHO-LASCO coronagraphs
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Since 11 years SOHO-LASCO coronagraphs are producing a unique set of the Sun corona images in the 2-32 solar radius range. For the first time a complete set of coronal calibrated images in WL (polarized and unpolarized) for the full solar cycle is available. The telescopes are equipped with 3 polarizers at -60,0 and 60 degrees, one all pass channel and a set filters. Ground calibrations were completed with in orbit calibrations. To control the evolution of sensivity for each bandpass and for each polarizer, the LASCO-C2 and LASCO-C3 coronographs were provided with an internal system of calibration in orbit. The measures obtained in 1996 and 2003 have been used to determine the CCD flat field for each filter bandpass, the gain constant (ADU to phe- conversion) and the polarizers transmittance map. The solar corona itself was also used to control the local response. Spacecraft rotations by 45 and 90 complete the test, and allowed for a ultimate but relevant global correction of the polarized images.
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A. Llebaria, A. Llebaria, P. Lamy, P. Lamy, } "In-orbit calibration of the polarization flat fields of the SOHO-LASCO coronagraphs", Proc. SPIE 7010, Space Telescopes and Instrumentation 2008: Optical, Infrared, and Millimeter, 70101I (12 July 2008); doi: 10.1117/12.789321; https://doi.org/10.1117/12.789321

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