Averted vision is a technique for viewing faint objects which uses peripheral vision. It involves not looking directly at the object, but looking a little off to the side, while continuing to concentrate on the object. This subject is discussed in the popular astronomy literature but only a few rigorous studies have quantified the effect. However, the technique is based on well-known properties regarding the structure of the eye.
It is claimed this technique is most useful to astronomers for viewing large but faint nebulae and star clusters. By developing the technique, some observers report a gain of up to three or four magnitudes (15:1 to 40:1). Others report no appreciable improvement.
There is some evidence that the technique has been known since ancient times, as it seems to have been reported by Aristotle while observing the star cluster now known as M41. This technique of being able to see very dim lights over a long distance has also been passed down over hundreds of generations of sailors whose duties included standing lookout watches, making one better able to spot dim lights from other ships or shore locations at night.
It also matters whether you avert right or left. The most effective direction is that which places the object on the nasal side of the vision. This avoids the possibility the object will be imaged on the blind spot at approximately 15 degrees on the temporal side of the line of sight. So, for right-eyed observers it is best to shift to the right, and for left-eye observers it is best to shift to the left. Some people also claim that it is better to avert up instead of down. The best thing to do is practice and find the best location for one's own eyes.
A similar technique is called scope rocking, which is done by simply moving the telescope back and forth slightly to move the object around in the field of view. This technique is based on the fact that the visual system is more sensitive to motion than to static objects.
Physiology
Averted vision works because there are virtually no rods (cells which detect dim light in black and white) in the fovea: a small area in the center of the eye. The fovea contains primarily cone cells, which serve as bright light and color detectors and are not as useful during the night. This situation results in a decrease in visual sensitivity in central vision at night. Based on the early, but controversial, work of Osterberg (1935), the density of the rod cells usually reaches a maximum around 20 degrees off the center of vision. Fulton (2005) presents an alternate interpretation of Osterberg's analysis. Due to the way the cells are connected to the nervous system, the area of most sensitivity, but reduced acuity, is closer to the center than this.
The resolution of the eye, its ability to resolve fine detail, falls off rapidly beyond 0.6 degrees from the line of sight as illustrated by Anstis (1974). It is four times poorer at 10 degrees radius as it is within the 0.6 degree radius from the line of sight.