The optic nerve in your eye is positioned just about directly behind the pupil. When you look directly at an object, the image is focused on the inner part of the eye in the area of the nerve. Since there are fewer rods and cones in this spot, your direct vision of an object is less acute than your peripheral vision, where the object is focused in another part of the eye. Thus, when observing a faint object, you can see more detail if you focus your gaze just to one side of where the object lies. This is called averted vision.

The following is a typical example of the use of averted vision. In the image of Saturn, the satellites Rhea, Dione, and Tethys are shown; the magnitudes are 9.8, 10.2, and 10.3, respectively. Due to the brightness of Saturn, these satellites are often difficult to see in a small telescope.

If you aim your gaze on the ball of the planet or on the opposite side from the satellites, one or more of them may pop into view. In addition to the bright satellite Titan, under favorable sky conditions a small telescope will generally show Rhea, along with Dione and Tethys, but these last two require averted vision. In scopes of 8-inches or larger, they're almost always visible directly.