Dear Fellow Astronomer,Spring is (finally) upon us and although the cold weather didn’t necessarily stop us from going outside, the nicer weather is sure to entice more of us out under clear skies. A must-have resource for any of us (beginners, experts, and fair-weather astronomers alike), is a red light.
Texas Star Party upper field, 2009..
Ron Ronhaar and Todd HargisIdeally, visual astronomers would work without any light source – it takes time for the human eye to adapt to darkness. Fully dark-adapted eyes are much more likely to make your observing session a success, particularly if you’re searching for dimmer objects with averted vision. But even on the most organized observing nights, it can be useful to have a little bit of light assistance when changing eyepieces and filters, or thumbing through a sky atlas in search of an unexpected target. In these situations-which should be infrequent to best preserve your dark adaption – only a dim, red light should be used.
As you might recall from your high school biology, there are two types of photoreceptors in our retinas: rods and cones. Our night vision relies mostly on the rods, as cones work only in bright light. The rods are very sensitive to changes in light and dark, but only minimally useful for color vision. They’re most sensitive in the blue-green range, and less sensitive to red, which is why amateur astronomers use dim, red lights while observing. (As an aside, rods are more numerous than cones at the periphery of our retinas, which is why we use averted vision to see dim galaxies at the eyepiece. Peripheral, rod vision is often the best tool for deep-sky objects.)
Experienced amateur astronomers will already be well versed in this bit of observing etiquette, but it bears repeating: use the dimmest light possible, and make sure it’s pure red. I admit, in my younger, less organized days, I was prone to taping a piece of red cellophane over a white flashlight when I needed light in a hurry. But this made me my own worst enemy, since red-filtered white light still radiates in wavelengths that affect night vision. Besides, regular flashlights are bright, even when filtered. Do yourself (and others) a favor and use the dimmest light possible. The brighter the light – even when red – the greater the loss of dark adaptation.
This is why I recommend an adjustable true red light like our Skylight Mini. The light from its two red LED bulbs can be dialed down to ultradim or up to ultrabright (but only use the brightest setting when it’s really necessary, such as when you’re trekking through a root-laden forest to your observing destination). Perhaps best of all, the Skylight Mini comes with an attached lanyard, which means you’ll never have to search for your light in the dark again – it’s already hanging around your neck.
JR
Observing Editor, Sky & Telescope
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