Notes from Under Sky

Protecting Your Night Vision

Things you can do to see deeper

Observing is universal. It's something we all take for granted in normal, ordinary situations, and we naturally expect that same facility when it comes to astronomical observing. But instead, we discover that it takes some experience and some skill to observe well under the night sky. Why is that?

Well, we aren't built to observe at night. We see well enough by dim light to avoid obstacles, and to detect when something might be moving out there in the darkness, but as far as observing detail is concerned, we are far behind many of our animal counterparts.

As a result, we have to stretch our faculties to the limit to see as much as we can. Developing night vision is really a number of skills that are designed to help us see deeper at the telescope. Broadly speaking, those skills can be broken up into two categories: physiological techniques and observing tricks.

Physiology. Most people are familiar with the ability of the eye's iris to shrink and expand in response to bright or dim environments. When it's bright outside, our irises contract to keep our eyes from getting saturated by all the light. When we go into a dark room, our irises expand to allow more light to enter our eyes, to improve their sensitivity.

Many amateur astronomers think that the iris is all there is to night vision. Since irises respond to external light stimuli in a matter of a few seconds or so, they think that night vision is gained in a correspondingly short period of time.

Not so! It turns out that along with the physical change in our irises, there is also a chemical change in our retinas. We detect light when certain precursor chemicals are transformed by light to other chemicals. These chemicals in turn cause signals to be sent to the brain representing the image seen by the eyes. The sensitivity of our eyes is directly related to the concentration of precursor chemicals.

Our bodies respond to a dark environment not only by expanding our irises, but also producing more of the precursor chemicals. This chemical change takes as much as an hour to acclimate to the extremely dim light levels involved at the telescope. This dark adaptation must be guarded jealously, as even the subdued glow of a suburban observing site can retard chemical adaptation, and a sudden burst of direct light will break down practically all of the precursor chemicals and ruin the dark adaptation for another hour.

My first recommendation, therefore, is to be patient. Just because you don't see very much on your first target during an observing run, doesn't mean that you won't see much more later. Give your eyes a chance to get used to the dark. Aside from that, avoid bright lights both before and during your observing period. Some observers even wear a patch over their dominant eye, and use their off eye to check their charts, change eyepieces, and perform other routine activities when they aren't peering at dim fuzzies. This extends their dark adaptation to the limit.

What else? Well, it also happens that your eyes are more efficient when they're well oxygenated. So hyperventilating a little before turning to the eyepiece may also help you see dimmer objects. (But see Tricks, below, for an apparently contradictory tip.)

Finally, some foods are purported to help night vision. Everyone knows, or at least suspects, that carrots help you see in the dark. Among amateur astronomers, however, the "sacred nectar" is bilberry jam. Supposedly, some pilots reported better seeing during night flights when they fed up on bilberries, and it has become something of an in-joke to mention bilberry jam on a list of things to pack for an observing trip. Of course, no one actually eats this stuff (do they?)

Observing Tricks. The second set of tips has to do with little tricks that may help you see whether or not your eyes are completely dark adapted. The most well-known trick of the trade is called averted vision. Your eye is most sensitive to detail in the center of its field of view, which is where your attention is directed most of the time. But this area, the fovea centralis, is not the area most sensitive to low levels of light. Therefore, one trick to see dimmer objects is to focus your eye toward the edge of the field of view of the eyepiece, but to direct your attention to the object in question. Averted vision can often help you to see fully half a magnitude deeper than direct vision.

Another trick that might help you has to do with the fact that the brain is more adept at picking up dim objects in motion than those that are still. One thing to try jiggling the scope, just a little, to see if you can pick up any dim fuzzy jiggling about in the same way. (On the other hand, it's been suggested that the eye can in some instances act like a film emulsion. That is, if the eye is held absolutely still, the brain can learn to accumulate little bits of light to detect something that would be invisible after even 10 seconds of observing. My advice would be to try both techniques if you have trouble discerning an object.)

Along those lines, you might try holding your breath while you look for an object. This might appear to run counter to our hyperventilation suggestion, above, but in fact, they can work together. Hyperventilate before you get to the eyepiece; once there, hold your breath. The idea is that the very act of breathing and all the motion involved can distract you just enough for the object to elude observation.

I hope that if you try all of these tips—well, maybe all of them besides the bilberry jam!—you'll learn to detect dimmer and dimmer objects. No, it won't be like seeing the pictures you see in the astronomy magazines, but you'll have found them yourself, and with your own eyes!

Copyright (c) 1999 Brian Tung