Averted vision
Encyclopedia
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 been done quantifying 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
.
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, and is done by simply moving the telescope back and forth slightly to move the object around in the field of view. It is based on the fact that the eye has evolved to be more sensitive to motion.
(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. 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 shown 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.
Peripheral vision
Peripheral vision is a part of vision that occurs outside the very center of gaze. There is a broad set of non-central points in the field of view that is included in the notion of 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 been done quantifying 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
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...
(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
Aristotle
Aristotle was a Greek philosopher and polymath, a student of Plato and teacher of Alexander the Great. His writings cover many subjects, including physics, metaphysics, poetry, theater, music, logic, rhetoric, linguistics, politics, government, ethics, biology, and zoology...
while observing the star cluster now known as M41
Messier 41
Messier 41 is an open cluster in the Canis Major constellation. It was discovered by Giovanni Batista Hodierna before 1654 and was perhaps known to Aristotle about 325 BC. M41 lies about four degrees almost exactly south of Sirius...
.
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, and is done by simply moving the telescope back and forth slightly to move the object around in the field of view. It is based on the fact that the eye has evolved to be more sensitive to motion.
Physiology
Averted vision works because there are no rodsRod cell
Rod cells, or rods, are photoreceptor cells in the retina of the eye that can function in less intense light than can the other type of visual photoreceptor, cone cells. Named for their cylindrical shape, rods are concentrated at the outer edges of the retina and are used in peripheral vision. On...
(cells which detect dim light in black and white) in the fovea
Fovea
The fovea centralis, also generally known as the fovea , is a part of the eye, located in the center of the macula region of the retina....
: a small area in the center of the eye. The fovea contains only cone
Cone cell
Cone cells, or cones, are photoreceptor cells in the retina of the eye that are responsible for color vision; they function best in relatively bright light, as opposed to rod cells that work better in dim light. If the retina is exposed to an intense visual stimulus, a negative afterimage will be...
cells, which serve as bright light and color detectors and are not as useful during the 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 shown 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.