September 1998

"Seeing Science"

In the course of the nightly public presentation, guests are given the opportunity to observe and see many wonderful objects in the night sky. These things can be seen by using one's unaided eyes, binoculars and, of course, the telescope. One of the important ideas that is presented is that professional astronomers do not look through telescopes with their eyes to make scientific measurements; instead they use sensitive instruments that are much more appropriate for doing the job. In fact, I generally do not have to work very hard to show people why that is. Human eyes are not designed for observing very dim objects in the night sky or a telescope. Ironically, unless you can get your hands on a larger telescope and plop it down at a darker site, the views given at the Visitor Center Observatory are as good as it gets! This program upholds the romanticism of astronomy: you get to see things like planets and galaxies with your own eyes through the telescope.

So, although our eyes are not suited for observing very faint sources, they are still remarkable light detectors. In order to use them to their best capablities a little optical biology is necessary. When we enter low-light situations, our eyes begin to adjust and become more sensitive to light. Please refer to the diagram for an illustation of structures of the eye. After light enters through the cornea and passes through the pupil, it is focused by the lens behind the pupil on to the cells of the back of the eye (retina). The brain senses the intensity of the light and increases or decreases the size of the pupil accordingly. In bright sun light the pupil may open as little as 2 mm to restrict the amount of the light that enters the eye. In the dark the pupil may open to as much as 8 mm!

There are two kinds of light-detecting cells on the retina: cones and rods. (See zebra rod cells as viewed through a microscope to the right). The cones are responsible for high light-level observing (photopic vision), while the rods are for low light vision (scotopic). In the human eye there are approximately 5 million cones and a hundred million rods! The cones occupy a small spot centrally located on the retina (fovea) whereas the rods surround the cones and cover a much greater area(macula ). Cones specialize in color perception, while rods are the cells used to detect things that you see in your peripheral vision, especially under low light levels.

So, when the public begins observing the dark night sky, everyone's pupils dilate and let in as much light as possible. In addition (and more importantly) the eye begins to produce a chemical called rhodopsin (visual purple) and increases the sensitivity of the rod cells in the eye. This process may take from 10-20 minutes to reach a good level of dark adaption. This extra sensitivity is very necessary in order to observe the dim and diffuse objects in the night sky (and through the telescope). Looking at bright sources of light will cause the eye to re-adjust and lose its night vision. You can feel this adjustment when bright lights are suddenly turned on! You may notice that most of the lights used at observatories or by amateur astronomers are colored red. This is because at low light levels the eye is not very sensitive to this wavelength (or color) of light and one's dark adaption can be maintained while still being able to see what one is doing.

Having said all of this, the main challenge with observing dim objects is knowing how to see them. When I point something out in the sky, it is very natural for the guests to begin by looking directly at it (especially in the telescope, when the object is put in the center of the field of view). However, under low light levels the most sensitive cells (the rods), which are not concentrated in the "center" of vision (fovea), do not come into play when looking directly at something. Furthermore, this is why you do not see color well (if at all) in dim conditions. Those cones need alot of light to get them to go!

For example, when looking in the telescope at an object like M13 (see picture), if you look directly at the cluster, it, will appear dim and possibly disappear! Instead, if you look off to the side (up, down, left, or right just a little) suddenly the cluster will appear to brighten and look like the picture shown! This is an effect that most people are unfamiliar with. The method is called using "averted vision". Amateur astronomers use this to their advantage whenever they look through telescopes. It will help you out tremendously!

There are a few other unfamiliar effects that can be noticed when observing in the dark. Although the above may seem to highlight inadequacies of the eye, there are many remarkable strengths. For example, when observing objects that have both very bright and very dim aspects, the eye, unlike most linear detectors, can "see" both portions at the same time. This is due to the logarithmic response to brightness levels. The Great Orion Nebula is one of the few objects that I believe is better seen in an eyepiece than photographed. This is because the eye can clearly see both the bright Trapezium stars without saturation and, at the same time, the eye can detect the faint wisps of gas that enshroud those stars.

The other effect is even more subtle. Guests will often describe how the "blackness" of space in the telescope field of view seems to somehow shimmer or look spotted. This is actually noise in the signal from our eyes to our brain! Under very low light levels it can look as if the background (not just in the telescope) is a dim version of static on television, rather than appearing completely black. The neurons that are connected to the cells on the retina can "fire," indicating a detection of light without any light actually entering the eye. This level of noise is very low, but easily seen, and perhaps startlingly so, when you are in the dark.

- Adam Block
Kitt Peak Visitor Center

Updated: 9/23/98