Averted vision

Averted vision is a technique for viewing faint objects which involves not looking directly at the object, but looking a little off to the side, while continuing to concentrate on the object. This technique is very useful to astronomers, as it oftentimes allows them to see especially faint or otherwise invisible objects. By developing the technique, some observers report a gain of up to three or four magnitudes. 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.

A similar technique that employs the same principle is called scope rocking, and is done by simply moving the telescope back and forth slightly to move the object around in the field of view.

Physiology
In the simplest sense, averted vision works because there are 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 only cone cells, which serve as bright light and color detectors and are not as useful during the night. The density of the rod cells usually reaches a maximum around 20 degrees off the center of vision. However, due to the way the cells are connected to the nervous system, the most sensitive portion of the eye is usually 8 to 16 degrees away from the center. This is because photoreceptor cells closer to the center have more ganglion cells connecting to them, and thus more nerve connections.

It is also important to note that 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. 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.

External links and references

 * "Just What is Averted Vision, Anyway?"
 * Techniques for Good Observing
 * M41 possibly recorded by Aristotle