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° per year.
Each HI instrument comprises two wide-angle optical cameras - HI-1 and HI-2 have 20° and 70° fields-of-view which
are off-pointed from the Sun direction by 14.0° and 53.7° 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.