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R. Clark, F. Hill, C. Toner (NSO)
GONG+ is now taking magnetograms every minute. It also takes daily calibration observations that include east and west limb measurements recorded in a non imaging mode, but with the magnetogram modulator still active. There are large-scale spatial artifacts due to modulator inhomogenieties, and we need to remove them.
East and west limb calibration observations are subjected to the normal calibration procedures using known 60 day averaged calibration data. These observations are taken with calibration lenses inserted in the optical path to switch conjugal planes in the optical system, so that the magnetogram modulator and optical elements at other pupil planes in the instrument are imaged onto the CCD. Spatial features in the resulting pupil plane magnetogram images are dominated by instrumental nonuniformities. Averaging these over a long period allows the determination of a modulator non uniformity correction which can be aplied to normal GONG observations to give a zero point corrected magnetogram.
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Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.