Photometric Calibration of the STEREO SECCHI COR2 Coronagraph and Comparisons with SOHO LASCO

The Sun-Earth Connection Coronal and Heliospheric Investigation (SECCHI) COR2 coronagraphs aboard the Solar TErrestrial RElations Observatory (STEREO) spacecraft have been observing the solar corona since their launched on 25 October 2006. We present the photometric calibration of the COR2 coronagraphs based on the analysis of the ever-present star field in the background of the data. We detail the pre-processing of the images before the star measurements are made. We used aperture photometry to calculate the total flux of each star measured by the COR2 instruments. We compare the calibrated COR2 data to the calibrated images from SOHO LASCO coronagraphs.