Your Eyes: Key to Observing
As you observe a galaxy, each photon has traveled millions of light years to reach your retina. It takes only a few of those photons to trigger a nerve cell in your eye. With really faint objects some people report seeing a bit of scintillation as the individual cells fire. So what goes on in your eye to create this effect and others?
There are two different kinds of receptors inside our eyes - rods and cones. Each operates a bit differently and has a specific function, which becomes especially significant to us amateur astronomers. The rods are more sensitive to faint light than the cones. The cones lie near the center of our inner eye and are better able to discriminate fine detail. When we use averted vision, we are looking at a field out of the "corner of our eye", and in so doing are using primarily the rods. This allows us to glimpse very faint objects. Sometimes, if you then try to look directly at the faint object you saw with averted vision, the object disappears from view. That's because at the center of view we are now using the less-sensitive cones.
When it comes to dark adaptation, our visual sensitivity improves markedly during about the first eight minutes we are in the dark. This time is when the cones in our eyes are adapting and becoming more sensitive. Afterwards, we start to use the rods. Our night vision then continues to improve for at least twenty more minutes as our rods adapt to the dark. Once dark-adapted, it takes our vision about thirty minutes to fully recover from white light. Any light adversely affects our dark adaptation to some extent, but the effect is least serious with red light, and according to some observers, green light as well. (But the jury is still out on the green light issue.) Visual sensitivity is also circadian in nature; that is, it varies over the space of twenty-four hours. As the night wears on, your eyes become more sensitive to light. Some research suggests that this process comes to an abrupt halt at 4 to 5 AM, just before sunrise. For more on dark adaptation, check the web site:
Getting back to the red-light-versus-green-light issue, I saw an application of this while on vacation in Costa Rica a few years ago. We were staying at a resort intended to be in harmony with the ecosystem. It was designed so that throughout the day and night it would not disrupt the movement patterns of local wildlife such as bats, monkeys, birds and the occasional jaguar. The nighttime illumination was minimal, with fixtures placed low to the ground. There were a few downward-aimed white lights, but most of the ambient lighting was green-colored. I found my dark adaptation was pretty well preserved when searching the night sky for far-southern deep sky objects.
There is also an effect called temporal summation, which allows our eyes to respond very much like a long exposure causes film or a CCD chip to accumulate more light. This is a collective aspect of vision where adjacent rods, each triggered by a photon during some short time interval, may send a nerve impulse that the individual rods would not have launched.
If you're curious about how light is converted to a signal recognized by our brain, the following is a technical explanation from Paul Jones, a professor of photochemistry at Wake Forest University: "When we detect a photon with our vision, the photon is absorbed by retinal in the retina. 11-cis-Retinal is a molecule that is bound to a protein, opsin, in the resting state. When it absorbs the photon, the energy of the photon is added to the ground state of retinal, resulting in an excited state. Literally, a bonding electron in retinal is 'promoted' to an antibonding molecular orbital. Excited states, in general, are very reactive and don't last very long. In this case, the excited state leads to the isomerization of a carbon-carbon double bond in retinal. This changes the shape of retinal and it detaches from opsin. This initiates a cascade of enzyme reactions that, ultimately, leads to a signal down the optic nerve. When this happens, the energy of the photon is used to promote an electron to a higher energy orbital. Essentially, the photon disappears into the molecule. Some molecules (not retinal) may fluoresce or phosphoresce, a process that results in the emission of a photon. Similar chemistry occurs in light detectors (film, CCDs, etc.). The energy of the photon is converted into some type of signal that can be detected, disappearing in the process."
Another issue is whether looking at a bright object, such as the full moon, with one eye could also affect the other eye. According to Ralph Chou, a professor of optometry at the University of Waterloo (Ontario), "Light in one eye would not affect the dark adaptation of the other eye. The photopigment in the receptors of the unexposed eye would not be bleached. It should be noted, however, that each eye supplies information to both halves of the visual cortex (there is a cross-over of optic nerve fibres at the optic chiasm) so that at the time of exposure, there could be a minor perceptual effect related to brightness of the visual world. This would cease with the cessation of the light stimulus." This reminds us that the dark adaptation in your observing eye might be better preserved if it is covered with an eye patch while looking at charts, even using a red light.
Many observers who use binocular viewers with their telescopes report dramatic increases in the detail visible. I've noted this myself, one example being the craterlets on the floor of the large lunar crater Plato, which are visible using a single ocular, but seem to jump out when using a bino-viewer. A binocular viewer gives the impression of a three-dimensional picture. It splits the image and each side is fainter than the image would be in a single ocular. However, this is more than compensated by the fact that we are using both eyes with a bino-viewer - double the number of rods and cones. Sharpness is usually enhanced because defects in one eye are filtered out by our brain in processing the two channels back into one. Of course, this is also one of the benefits of using regular binoculars for wide-field observing where each eye has it's own separate small telescope for gathering light.
Bino-viewers also eliminate the effect of floaters in most peoples' eyes. Floaters are cellular debris floating in the aqueous humor inside our eyes: mostly dead cells. At high magnifications this is seen as silhouettes on our retinas which appear to move around. They tend to get worse with age, and often block vision in annoying ways. Since they're random in each eye, they are less visible in a bino-viewer, as our brain filters out the things that differ between out two eyes.
With regard to common vision defects, it has been said that almost everyone suffers some level of astigmatism, but it's only noticeable in everyday situations for the more serious cases. If stars seem to be elongated as we look at them in a telescope, it would be the result of our own astigmatism if the "tails" on the stars change direction as we move our head. But according to optometrist and amateur astronomer Roland Schwarz, "It's somewhat common for some forms of astigmatism to decrease with age. Increase in lenticular (internal) astigmatism can often help to reduce corneal (external) astigmatism. Smaller pupil size can also help reduce the eyes' optical defects." I suppose that's some small consolation as we grow older.
However, age takes another toll on our pupils, which become less elastic and don't dilate as much in the dark. Wide field oculars typically have large exit pupils - in simplest terms, the diameter of the light cone that strikes your eye at the point of best focus. If your dark-adapted pupils expand to only 5mm while the eyepiece in your telescope presents an exit pupil of 7mm, not only is light being wasted, but you may see distortion at the edge of the field. To determine the exit pupil diameter of an eyepiece, divide the focal length of the eyepiece by the focal ratio of your telescope. For example, a 25mm eyepiece in a scope with a focal ratio of f/5 gives an exit pupil of 5mm.
There's a simple way to approximate how much your pupils dilate at night. According to Geoff Gaherty of the Toronto RASC, you can use a set of metric Allen key wrenches (hex wrenches). Give your eyes a half-hour to accommodate to the darkness so they're fully dilated, then look at a fairly bright star while holding an Allen wrench in front of your eye. Because the star is at infinity, you don't have to hold the key particularly close to your eye; a few inches away is fine. Hold it up vertically so the tip is well above your eyebrow. Slowly move the key across in front of the star. If the key is larger than your pupil, the star will wink in and out. If the key is smaller than your pupil, the star may appear double for a second, but will never completely disappear. The first key size to wink out the star is your pupil diameter in millimeters.