Your Most Sensitive Instrument - Your Eye

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You need to know a little bit about how your eyes work. Ultimately, all this hype is about getting the image of something into your eye so it can tell your brain what you are looking at.

Anatomy (not a lot). On the back of your eye is the retina. This is where the image is produced. It is made up of two kinds of cells called rods and cones. Cones are sensitive to the amount of light entering the eye. They transmit to the brain the brightness of the image. Rods are sensitive to color. They transmit the colors to the brain. Between the two, you brain will process these impulses and produce a colored image of the light entering your eye.

In the front of your eye is a lens with a pupil. The lens focuses the incoming light on the retina. The pupil opens and closes to let more or less light in to the eye. In bright light, the pupil is smaller and in dim light, it opens up to allow more light to enter the eye.

Averted Vision.The rods are much larger cells than the cones, and rods are much fewer in number. On the retina, almost directly behind the lens, is an area called the fovea. The fovea is an area made up almost exclusively of rods. So, when you turn your eye slightly away from the image, the light hits more cones, and may appear brighter. We call this Averted Vision. Using averted vision, you may actually see more detail than looking directly at a dim object like a nebula or a galaxy.

Light-catching abiity. Now lets discuss how much your eyes see. The light-catching abiity of any optical system depends on the F number, or focal ratio, (See Optics for a discussion of focal ratios). The typical human eye, when the pupil is opened to its widest aperture, is about 7 - 8 millimeters. The distance to the retina, is approximately 17 mm. Therefore the F number for the human eye is about 2.1 - 2.4. The lower the F number, the brighter objects seem. For instance a cat's eye has an opening of about 13 mm and an F number of 0.89. No wonder a cat can "see in the dark"! But the winner in this deal is the Net Casting Spider. With a pupil opening of only 1.325 mm, it has an F number of 0.08! This is the winner in the animal kingdom of light-catching ability.

Night Vision. When you first leave a bright area and enter a dark one, your pupils open up, and your cones are used to bright light. After about 15-20 minutes, you can see better in the dark. Your cones have slightly changed shape, you brain is used to getting less information, so it processes the information differently, and your irises have opened up a little bit more. You have now entered "NIGHT VISION MODE". You can see pretty well, and things in the eyepiece of your telescope seem brighter. It only takes an instant of bright light and your night vision is gone for another 15-20 minutes. This includes looking at a bright moon or planet in the telescope.

Red light and night vision. If you have been to a star party, you probably have heard that red light is all we use. Sometimes astronomers get down right nasty about white light. Red light is very low in energy. There is not enough energy in red light to activate the muscles around the iris or cause the cones to revert to bright light mode. It is also why red light can be used in a photo dark room. There is not enough energy in red light to spoil the photo paper or film.

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