 Night Vision & Averted Vision |
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Night Vision
For stargazing, it helps to know a few things about the human eye.
In an eye, light enters through the pupil, is focused by the lens and then forms an image on the retina at the back of the eye. The retina has two different types of receptors, cones and rods, which allow the eye to work in two very different ways.
Cones produce detailed color images when there is enough light. Rods, being more sensitive, produce images when the light is dim, but only in black and white. Another important difference is that cones are found closer to the center of the retina while rods are found away from the center. More on this later.
Stargazing depends largely on rods, but the process of switching from daytime ‘cone’ vision to night ‘rod’ vision is not instantaneous. There’s a process called dark adaptation that can take 20 to 30 minutes or more in which the eye produces a chemical called rhodopsin that switches the rods on.
Go outside from a bright room and look up. Even if the sky is dark, clear and without much light pollution, you still may see only the brightest of stars because your eyes are not yet dark adapted. Look again after 10, 20, and 30 minutes and notice how many more stars become visible including lots of dim ones — that’s dark adaptation.
One thing important to understand: while it takes a while for dark adaptation to occur, it can be lost quickly by seeing a bright light. The rhodopsin breaks down almost instantly and the rods switch off. Surprisingly, a red light does not have this effect, as long as it is not too bright.
What do stargazers do to aid and keep dark adaptation? They….
🌟 Turn off outdoor lights.
🌟 Don’t use a smartphone, tablet or computer unless the brightness is turned way down.
🌟 Use red flashlights instead of white flashlights.
Averted Vision
An important skill for stargazers with telescopes is known as using averted vision. Since the rods tend to be away from the center of the retina (see above), turning the eye to the side a bit when viewing a dim target through an eyepiece can cause the target to appear brighter since it is being viewed with a higher density of rods. Experienced stargazers know that averted vision can make a big difference and so use it a lot while viewing dimmer objects.
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