Dark Adaptation: “Looking” Your BestMarch 14, 2008
To best see faint deep-sky objects like galaxies and nebulae, you have to make sure your eyes are “dark adapted”. Here’s what you need to know.
• The human eye evolved to operate in two modes, photopic for seeing in well-lit conditions and scotopic for seeing faint objects in the dark.
• As you learned earlier, your retina has two types of cells, rods and cones. In photopic mode, the cones detect bright light and colors. But in scotopic mode, the rods detect faint light. Both types of cells contain dyes that undergo a chemical change called “bleaching” when hit by light.
• In light-adapted or photopic mode, the dyes in your rods are fully bleached, so they can’t detect faint light… they’re out of action. Turn the lights off and the rods to return to dark-adapted mode, but it takes a long time, about 20-60 minutes. That’s why astronomers get so angry when someone carelessly shines a white light in their eyes… they have to wait a long time to recover their dark-adapted vision.
• Going from a dark to light adapted state happens much faster, in only a few seconds.
A Deeper Look
• Each eye reacts separately to light, so you can keep one eye dark adapted while using your other eye to read star charts and slew your telescope. An eye patch is ideal.
• You can keep unwanted streetlights out of your eyes by throwing a towel over your head when looking through the eyepiece of your scope with your dark adapted eye.
• You often see astronomers using bright red LED flashlights when looking at star maps and gear around the telescope. That’s because red light cannot bleach the dye in the rods if the wavelength is > 620 nanometers. So the chemical structure of the dye in the rods is completely unaffected, while the dye in the cones still enables scotopic vision.
Good To Know
Your body cannot by itself make the dyes for the rods and cones in your retina. It needs an external chemical-beta carotene-to synthesize the dyes. A good source of beta-cartone? Carrots. So carrots really can be good for your eyesight.
You can imagine the reaction I get from the police officers who occasionally find me in the local park wearing an eye patch with a towel over my head to block out the nearby street lights. “Really officer, I’m just trying to see the faint structure in M97. Want a carrot?”