Astronomy and the Human Eye

The most important tool you have to see the universe is not the large light buckets that are proliferating, but rather your eyes. We have all learned "tricks" such as averted gaze that we use to see more, but few understand why these aids help. By understanding your eye more, your observations may improve.

Light enters the eye through the cornea, passes across the aqueous humor (the liquid behind the cornea), passes through the pupil and through the lens. The cornea stops high ultraviolet light. The lens stops violet and blue light, especially as we get older, and is damaged by UV light. The human lens grows continuously with age, with new cells added on the outside of the lens, resulting in an onion-like layering of cells with age. The older, central dead cells degenerate and "yellow" the lens with age, A design flaw built into us. Eventually, cataracts (crystallization) of the inner lens may occur, blocking vision. Cataracts are also promoted by exposure to radiation. After passing the lens, the light then crosses the "vitreous Humor" a jelly-like substance that fills the inner eye, and reaches the retina, the light sensitive tissue at the back of the eye. Sometimes clumps of cells detach from the retina and migrate into the vitreous, creating "floaters" when they are in the line of sight, and this gradually progresses with age. (Aging is not kind to astronomers).

The retina is actually an outgrowth of the brain and processes some of the information it receives before passing the signal over the optic nerve to the visual center of the brain (occipital cortex in the back of the head). The actual receptors are the rods and cones of the retina. There are three types of cones(red, yellow-green, and blue), and work best in bright light, and are also used in acuity (resolution). They unfortunately fail us in dim light situations (this is why night vision is black and white). Cones are concentrated in the fovea, the center point of your vision when you stare at an object. As you increase light intensity, a threshold is reached where color vision recurs (as in using a larger telescope to see color in M42).

Night vision is dominated by rods (Black and white vision); but there are no rods in the fovea, increasing rapidly in density in the retina away from the center of vision to peak in an ellipse about 15-20 Degrees out from the fovea. That is why staring 15 degrees away from a dim galaxy improves your view of it. There are about 120 million rods in the human eye (good pixel density). They are identical, but their connections vary significantly. Some are used for normal viewing, but some are only activated by motion. Some are specialized to vertical or horizontal motion, others see only lines for edge sharpening, etc. This is why "flickering" your eye or telescope slightly also allows you to see more, you are recruiting more of your rods.

Your eye is remarkably efficient, able to produce a signal with only one photon, Though the brain routinely suppresses single photons as noise, and generally you need 2-3 photons over a small area in a short time span to produce a signal (pixel) to the brain. Your eye utilizes nearly all of the oxygen that reaches it, and is also dependent on glucose (sugar). You cannot do anything to improve its function, but you can do much to worsen eye sensitivity. Smokers suffer from poor night vision due to a poisoning of their bloodstream with carbon monoxide. For every pack per day you smoke, your blood is 10% diminished in oxygen carrying ability due to the carbon monoxide, and your eye also suffers to this degree as the retina extracts nearly all the oxygen it is presented. High altitudes decrease oxygen pressure, and vision therefore suffers compared to sea level. Vision decreases if blood sugar levels are low, and an occasional snack while observing at night can help. Alcohol diminishes both light sensitivity as well as brain function. Do not drink and observe!

Vitamin A deficiency is simply not seen in this country and extra beyond basic needs does not help. Ignore the false claims of the "megavitamin charlatans" out there of which there are many as they are not FDA controlled as vitamins are a food supplement. There are many very rich makers and sellers of vitamins and "special" nutrients out there who basically lie and make false claims to sell you what you get in your food anyway. Finally, a "bleaching" of rhodopsin (the light sensitive chemical in rods and cones) occurs with prolonged exposure to bright lights or sun. If you plan an observing session after a day of skiing or beach, wear sunglasses.

My recommendations are: Do not smoke or drink (I do not just say this because I am a physician!), take an occasional snack at night, wear sunglasses during the day you plan to observe faint objects, and ignore vitamin charlatans (eat a balanced low cholesterol meal). Finally master the technique of averted gaze and flicking your vision. Happy hunting for those faint and elusive NGC objects.

Last updated: 26-Dec-95