Enter heliocentric longitude of target, if known, in decimal degrees (dd.d). If not known, just input 90 degrees, which is typical for HST observations. Valid range is 0 to 360 degrees. Needed for background estimation.
Most HST observations are made when the target is roughly 90 degrees from the sun. However, scheduling constraints can force the observation into other sun angles.
The brightest backgrounds result when sources in the ecliptic plane are observed close to the sun. If you are concerned about the worst-case sky background, you may wish to try setting "sun angle" to 50 degrees, which is the HST solar avoidance angle. Ecliptic longitudes less than 50 degrees are possible for sources far from the celestial equator, but for these sources the zodiaical light is already moderately low.
The lowest backgrounds occur when sources are farthest from the sun, i.e., "sun angle" = 150 to 180 degrees.
Note that "Sun Angle" is ignored if "Low Sky" is checked.John Biretta <email@example.com>, STScI