Photometric Calibration of the Lasco-C3 Coronagraph Using Stars |
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Authors: | A F Thernisien J S Morrill R A Howard D Wang |
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Institution: | (1) Universities Space Research Association, 10211 Wincopin Circle, Suite 620, Columbia, MD, 21044, U.S.A.;(2) Naval Research Laboratory, Code 7660, 4555 Overlook Avenue SW, Washington, DC, 20375, U.S.A.;(3) Interferometrics, 14120 Parke Long Court, Suite 103, Chantilly, VA, 20151, U.S.A. |
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Abstract: | The LASCO-C3 coronagraph on SOHO, launched in December 1995, has been collecting images of the corona and background star
fields in a regular manner since 1996. This instrument contains a number of broadband filters with various passbands in the
range between 400 and 1100 nm. The filter used most often has been the Clear filter (400–900 nm) but there are four other
filters with about 100 nm passbands that are also used periodically. Preliminary calibration of the C3 optical system was
done before flight and a number of techniques that use star intensities or magnitudes and position have been applied during
flight. In order to understand the long-term behavior of the C3 instrument, we have recently performed an analysis of LASCO
data that examines the observed intensities of a set of moderately bright stars whose spectra is known from 13 color photometry.
Using these star spectra and the observed count rates we have derived the photometric calibration factors of the C3 coronagraph
for all five color filters with an absolute precision of about ± 7%. Observations with the Clear filter have been used to
look for long-term trends in the instrument sensitivity. The observations indicate a very slight decrease in the instrument
sensitivity of about 3.5% over the 8 years studied here. |
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