In-orbit verification, calibration, and performance of the Heliospheric Imager on the STEREO mission

The Heliospheric Imager (HI) forms part of the SECCHI suite of instruments aboard the two NASA STEREO spacecraft which were launched successfully from Cape Canaveral AFB on 25 Oct 2006 (26 Oct UTC). Following lunar swingby's on 15 Dec and 21 Jan respectively, the two spacecraft were placed in heliocentric orbits at approximately 1 AU - one leading and one lagging the Earth, with each spacecraft separating from the Earth by 22.5 degrees per year. Each HI instrument comprises two wide-angle optical cameras - HI-1 and HI-2 have 20 degrees and 70 degrees fields-of-view which are off-pointed from the Sun direction by 14.0 degrees and 53.7 degrees respectively, with the optical axes pointed towards the ecliptic plane. In this way the cameras will for the first time provide stereographic images of the solar corona, and in particular of Coronal Mass Ejections (CMEs) as they propagate outwards through interplanetary space towards the Earth and beyond. The wide-field coverage of HI enables imaging of solar ejecta from 15 to about 330 solar radii whilst the other SECCHI instruments (2 coronagraphs and an EUV imager) provide coverage from the lower corona out to 15 solar radii. This paper briefly reviews the design and performance requirements for the instrument. The various activation, checkout and calibration activities before and after opening the instrument's protective cover or door (instrument 'first-light') are then described and it is shown that the instrument has met the design requirements, including CCD and camera imaging performance, correction for shutterless operation of the cameras, straylight rejection and thermal requirements. It is demonstrated from observations of a CME event on 24-25 Jan 2007 that the instrument is capable of detecting CMEs at an intensity of 1% of the coronal background. Lessons learnt during the design, development and in-orbit operation of the instrument are discussed.