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4 February 2004 Lessons learned from the SOHO/LASCO-C2 calibration
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The LASCO-C2 coronagraph onboard the SOHO solar probe have been providing for the last seven years an unprecedented long sequence of coronal images at high cadence (about 75 images/day). The LASCO-C2 calibrations included the determination of the geometric characteristics (attitude, distortion) as well as the photometric and photopolarimetric responses. Such calibrations needed resort to a complementary set of approaches including optical-modelling, pre-flight measures and in-orbit measures and monitoring. In this paper we discuss about the specific contribution of each of them, the example of radiometric calibration of LASCO-C2 is dominated by the strong vignetting induced by the occultors. The occultors fully mask the extended circular area centered on the Sun image. Due to operational constraints the vignetting function has been obtained using a complementary set of approaches: 1) ray tracing, 2) the geometric convolution of diaphragms, 3) the measure of uniform sources in laboratory, 4) the measures in orbit of the stars and F-corona. Finaly the relationship of radiometry with geometric calibrations, strylight calibration and the log term stability monitoring is discussed.
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Antoine Llebaria, Philippe L. Lamy, and Maurice V. Bout "Lessons learned from the SOHO/LASCO-C2 calibration", Proc. SPIE 5171, Telescopes and Instrumentation for Solar Astrophysics, (4 February 2004);

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