Getting used to the Dark - Eye Adaption

By Tim Trott, 2nd May 2013 in Astrophotography

In order to gaze successfully around the starry skies you need to take into account a very important factor to do with your eyes. This is called dark adaptation.
 

You may have noticed that your pupils are larger in dark conditions and smaller on a bright sunny day. This is the way the eye controls how much light to let in - exactly the same way as the aperture works on a camera lens. In dark conditions the iris opens and the pupil becomes larger to let in more light, while in bright conditions the iris closes and the pupil becomes smaller to limit the light coming in and preventing you from becoming dazzled. This is actually only a small part of what your eyes are up to, and the process of getting used to the dark is called Dark Eye Adaptation.

Dark Eye Adaption
Pupils are small in bright Conditions so that the light does not dazzle, and in the dark, your pupils grow bigger to let in more light.

Your eyes adapt to whatever the prevailing lighting conditions are. Let's take an example - a room at night with the lights on. It all looks fine because your eyes have set themselves to work in whatever light there is around. Now turn the lights off and the first thing you'll notice is that the room appears almost black for a short time. Your eyes, sensing the lack of light, have gone into dark-adapting mode - your pupils grow to let in more light and then the all-important chemical changes begin to switch on the low-light-intensity 'rods' which fill the backs of the eyes. This process is Dark Eye Adaptation and it actually takes around an hour, but a good proportion is complete within 10 minutes or so.

In order to see the best of the faint night sky, shield your eyes from all bright lights for a good few minutes before you start stargazing. You should also make sure you cannot see any bright lights, such as street lighting, while you are observing. If you need to use a torch whilst observing you can get special red light torches that will help preserve your night vision.

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