There are three possible methods for selecting the sky background:
The first is to use merely a rough estimate of "average" or "high" or "low" sky background conditions.
The second method is to provide the position of the target and (optionally) an estimate for the heliocentric longitude of the target. This attempts to estimate the zodiacal light at the time of observation for the specific target.
The last option is to explicitly input a value for the sky background.John Biretta <email@example.com>, STScI