The SECCHI field of view is different for the two spacecraft (Ahead and Behind). The Ahead spacecraft sees the same background that the earth will see some days in the future and the Behind spacecraft sees the background as seen by the earth some time in the past. The maps show only the background stars and do NOT show the planets, asteroids and comets for each date. You will have to consult a current starchart or almanac. Since the starmaps are made from an earth based star map and the STEREO spacecraft are deep space probes far from earth there is significant parallax for solar system objects such as planets, comets and asteroids. The important parameter to take into account is the separation angle from earth for each spacecraft.
The maps are only approximate since the STEREO spacecraft are constantly moving and so are the background stars so an object at the edge of the field of view may have moved out of the field during the day. You can see how much of the field is affected by looking at consequeutive days to see how far the stars move each day.
The star maps have right ascension (RA) and declination (Dec) grid with the Sun's path along the red line (the ecliptic). SECCHI COR2 is always pointed at the Sun and thus what it sees will follow the Sun on the ecliptic. The ecliptic is at a 23.5 degree angle to the RA and Dec coodinate system.
The COR2 field of view is about 15 degrees represented by the white circle on the map. The HI-1 field of view is about 20 degrees from 3.98 - 23.98 degrees from sun center. The HI-2 field of view is about 70 degrees with about a 5 degree overlap with the HI-1 field of view. (18.68- 88.68 degrees).
For now, choosing HI just puts the COR2 map center at the center of the HI-1 field of view. Since Xephem generates maps on a sphere, there is some distortion for large fields of view. So it may not be useful to use this tool to generate the HI-1 or HI-2 field of view.