The roll angle system measures the momentary roll angle of the rotation spacecraft. From the spacecraft looking towards the Sun, the spacecraft is rotating clockwise. Therefore, the momentary rotation vector is pointing more or less in the direction of the z-axis of the imaging coordinate system. After correcting for the off-pointing (i.e. align the z-axis to point towards the Sun center), the roll angle is defined to be the angle between the Solar fixed y-axis, which is pointing from the Sun center towards the north celestial pole (see Fig. 1), and the imaging y-axis. The angle is defined to be increasing in time and is measured in radians.