Straylight-Rejection Performance of the STEREO HI Instruments

The SECCHI Heliospheric Imager (HI) instruments on-board the STEREO
spacecraft have been collecting images of solar wind transients, including coronal mass
ejections, as they propagate through the inner heliosphere since the beginning of 2007.
The scientific use of the images depends critically on the performance of the instruments
and its evolution over time. One of the most important factors affecting the performance
of the instrument is the rejection of straylight from the Sun and from other bright objects
located both within and outside the HI fields of view.
This paper presents an analysis of the evolution of the straylight-rejection performance
of the HI instrument on each of the two STEREO spacecraft over the three first years of
the mission. The straylight level has been evaluated and expressed in mean solar brightness
units, in which such scientific observations are usually quoted, using photometric conversion
factors.