Averted vision

Averted vision is a controversial technique for viewing faint objects visually. 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 does not appear in the academic literature of vision or astronomy.

It is claimed this technique is very useful to astronomers viewing large but faint nebulae and star clusters. It is not useful for viewing point sources or resolving adjacent stars. By developing the technique, some observers report a gain of up to three or four magnitudes (15:1 to 40:1) but no documentation to support these values has been presented. Other's 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.

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

It is claimed, 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@. 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.

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. 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.

@ This situation should result in a major loss in visual sensitivity along the line of sight at night, but it does not.

ee also

* Peripheral vision
* Scotopic vision
* Purkinje effect

External links and references

*Anstis, S. M. (1974) A chart demonstrating variations in acuity with retinal position. Vision Research vol 14, pp 589-592
* [http://sightresearch.net/pdf/3description.pdf "Fulton, J. T. (2005) Section 3.2.2, Chapter 3, Processes in Biological Vision"]
* [http://vegas.astronomynv.org/Tutorials/avertedvision.htm "Just What is Averted Vision, Anyway?"]
* [http://www.geocities.com/CapeCanaveral/Lab/3230/techniques.html Techniques for Good Observing]
* [http://www.seds.org/messier/more/m041_ari.html M41 possibly recorded by Aristotle]