Photometric Calibration of the LASCO-C2 Coronagraph over 14 Years (1996 - 2009)

Abstract

We present a photometric calibration of the SOHO/LASCO-C2 coronagraph based on the analysis of all stars down to magnitude V=8 that transited its field of view during the past 14 years of operation (1996 - 2009), extending the previous work of Llebaria, Lamy, and Danjard (Icarus182, 281, 2006). The pre-processing of the images incorporates the most recent determination of the evolution of the LASCO-C2 performances. The automatic procedure then analyzes some 260 000 images to detect, locate, and measure those stars. Aperture photometry is performed using four different aperture sizes, and the zero points (ZPs) of the photometric transformations between the LASCO-C2 magnitudes for its orange filter and the standard V magnitudes are determined after introducing a correction for the color of the stars. A new statistical method ("bootstrap") is introduced to assess the confidence intervals of the mean yearly value of the ZPs. The correction for finite aperture required to derive the calibration coefficient for the surface photometry of extended sources is based on the reconstructed image of bright saturated stars and a robust model for the growth curve. The global temporal evolution of the sensitivity of LASCO-C2 is compatible with a continuous decrease at a rate of ≈ 0.56 % per year. However, it is better described by two separate linear variations with a discontinuity at the time of the loss of SOHO. After the resumption of normal operations in 1999, the linear decrease of the sensitivity amounts to ≈ 0.35 % per year.