Photometric calibration of the LASCO-C2 coronagraph for Solar System objects

We present a photometric calibration of the SOHO/LASCO-C2 coronagraph appropriate to Solar System objects based on the extensive analysis of all stars down to magnitude V = 8 which transited its field-of-view during the past nine years of operation (1996–2004). An automatic procedure was developed to analyze some 143,000 images, and to detect, locate and measure those stars. Aperture photometry was performed using three different aperture sizes and the zero points of the photometric transformations between the LASCO-C2 magnitudes for its three filters (orange, blue and red) and the standard V magnitudes were determined after introducing a correction for the color of the stars. The calibration coefficients for the surface photometry of extended sources were then derived from the zero points. An analysis of their temporal evolution indicates a slight decrease of the sensitivity of LASCO-C2 at a rate of ∼ 0.7 % per year.