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Just What is Averted Vision, Anyway?

by Jeff Medkeff

Astronomers often employ an observing technique called "averted vision", the art of looking slightly to the side of a faint object being studied. This works because, we are told, there are more rods slightly off the optical axis of our eyes. But there is a great deal more to it than that, and with some understanding of the physiology of the eye, it will be seen that there are right and wrong ways to use averted vision.

It is true that the density of rods peaks well outside the center of vision. Since the rods are the eye's faint light detectors, it stands to reason that this peculiarity of physiology is what makes averted vision work. The density of the rods at a point 20 degrees off the center of vision reaches about 160,000 rod cells per square millimeter. This is a greater density than the peak density of the cones - the eye's bright light and color detectors - on the fovea (the center of vision), where cones only reach about 140,000 cells per square millimeter.

The point of greatest density of the rods does not correspond to the point of greatest sensitivity, however. The area of greatest sensitivity has been shown to vary considerably from observer to observer, but it is never as far as 18 degrees from the center of vision. The reason for this has to do with the manner in which the retinal cells are "wired" to the brain.

In the fovea, each cone is connected to a single ganglion cell, which in turn is hooked up to a nerve fiber that eventually joins the optic nerve. As we move away from the fovea, each ganglion cell starts to service several cones or rods. Eighteen degrees from the fovea, 100 rods might be connected to a single ganglion cell. At some point on this line extending outward from the fovea, the number of rods per ganglion cell is such that the eye operates at peak sensitivity. For most people, this point is somewhere between 8 and 16 degrees from the fovea.

But so far we have only been considering the sensitivity of the eye as a function of an image's angle from the fovea. One might suppose that it makes a difference if we avert our vision to the left or right, up or down, or at some angle. And it does matter. The most effective direction to avert our eyes is that required to place the object on the nasal side of our vision. Simplified, this means if you are a right-eyed observer, you shift your eyes to the right; if a left-eye observer, you shift your gaze to the left. Whichever eye you use, you avert your gaze in that direction.

By using this most efficient portion of the retina, you will experience a gain of some four magnitudes or more over your direct vision! The effect of this is not insignificant. It means the detection or not of many stars and most details in deep sky objects.

It is important not to avert your vision the opposite direction - that is, if right eyed, you should not use averted vision by shifting your gaze to the left. This will place the image on the blind spot, right where the optic nerve connects to the retina. Nothing will be seen in such a circumstance, no matter how bright!

This poses an interesting dilemma for binocular observers and for those who use binocular viewing attachments on their telescopes. Averting one eye to its optimal position puts the image on, or nearly on, the blind spot on the other eye. This is counterproductive; the advantage of the binocular system is its use of two eyes. Inadvertently disabling one eye makes no sense. The solution is simple, and astronomers have been saying it for centuries: look up!

The second most efficient direction to avert your gaze is upward - look in the direction of the top of your head, so that the image is below your center of vision. The area of the retina in use here is somewhat less sensitive than the optimal horizontal location, but only slightly so. Doing this does not put the image in the blind spot of either eye, and considering the gains to be had from binocular vision, this will likely prove as efficient (or more so) under such conditions as using the optimal monocular method.

If you choose to avert your gaze downward, you will find your averted vision slightly less sensitive again. In actuality, the retina is every bit as sensitive here as it is if you avert your vision upward, but it is sensitive over a much smaller area. Thus, it is harder to consistently rest the image on the "sweet spot".

Some observers will notice that their most sensitive areas are slightly to the side and down, or in other ways not exactly as eye physiology would suggest. In my case, I find averting to the right and slightly up (I am right eyed) is best for me. There are large variations in the way our eyes are made up - in fact, our retinas are even more distinctive than our fingerprints. Almost nothing can be said categorically about vision, but we can say what will apply in the majority of cases. It is well known that experienced observers see much more detail, and many fainter objects, than beginners. I believe that this is caused in part by the observer learning about the individual characteristics of his or her eyes over the course of many nights of observations.

Next time you are out with your binoculars or telescope, take some time to explore these different areas of your vision. It might be quite apparent what is the most promising averted vision method for you. And if it happens to be something other than what medical science predicted, don't let that stop you from doing it your way. They are, after all, your eyes, and only you know what you can see with them.



Jeff Medkeff has been an amateur astronomer for over 20 years. He is an enthusiastic and prolific observer, especially of solar system objects, and has been writing about amateur astronomy sporadically for 14 years. He operates the private Rockland Observatory of Hereford, Arizona, which is dedicated to astronomy education and journalism. In 1997, Medkeff was appointed an assistant coordinator within the Association of Lunar and Planetary Observers' Solar Section.

More of Jeff's articles, which he freely provides to the astronomical community, may be found on the web at his home page