Long-Term Evolution of the Photometric Calibration of the STEREO Heliospheric Imagers: I. HI-1

The aim of this paper is to determine whether the photometric response of the STEREO HI-1A and HI-1B Heliospheric Imagers has evolved or degraded during the mission to date. This is done using the methodology of Bewsher et al. (Solar Phys. 264, 433, 2010) to calculate the photometric calibration factor, mu, over the four complete STEREO heliocentric orbits between the start of the science mission operations in April 2007 and June 2011, and to evaluate whether this parameter has undergone any significant variation. It is shown that the photometric response of the instruments has remained stable to within similar to aEuro parts per thousand 1%. The value of mu across different parts of the field of view is also determined to evaluate the validity of the calibrated large-scale flatfield determined by Bewsher et al. (Solar Phys. 264, 433, 2010). Based on the results, new revised values for the photometric calibration factor and conversion factors for diffuse sources are presented.