Left panels: variation of the distance from the Sun (top panel) for Encke (black) and ISON (red), and from STEREO-A in AU (bottom panel). Continuous lines mark the time intervals when the comets are visible in the HI-1 FoV, dotted lines the opposite. The blue dashed line in the bottom panel indicates the STEREO A-Sun distance, which is a reference line to visualise when the comets are behind or in front of the plane-of-sky. Right panels: orbital speed of the comets (top panel), and variation of the latitude in the Helio-Centric-Inertial (HCI) coordinate system (bottom panel).