HACKS Cover Up to See in the Dark
By Robert Bruce Thompson and Barbara Fritchman Thompson
Amateur astronomer
Robert Bruce Thompson
protects his night vision.
Our eyes function in two entirely different the site is on private property, it is impossible to install
modes, depending on how much light is available. In permanent screens against the local light pollution.
daylight or bright artificial light, our eyes function in Portable screens are impractical for various reasons.
day-vision mode. After dark, our eyes shift to night- Fortunately, there is a cheap, easy solution to such
vision mode. The physiological changes that occur local light pollution problems, as long as you don’t
in our eyes during the shift from day vision to night mind looking like a complete idiot. All you need is an
vision are called dark adaptation. Dark adapta- old towel and a pirate’s eye patch. The photograph
tion occurs slowly, typically requiring 25 minutes shows Robert working at the chart table, looking like
for 80% adaptation and 60 minutes for 100% an idiot, but with his night vision intact.
adaptation. That’s why astronomers get upset when Dark adaptation occurs individually for each eye.
someone shows a bright light. That means you can keep one eye completely dark-
Night vision is all-important when you observe adapted by covering it with the eye patch whenever
Deep Space Objects (DSOs). For those fortunate you are not using it to look through the eyepiece. The
enough to have access to a truly dark observing other eye is never fully dark-adapted, but that doesn’t
site, it’s not difficult to preserve night vision using matter. You use it for other purposes, such as locating
standard methods — red LED flashlight, covering objects with your computer or charts, or recording
your computer screen with red film, and so on. But observations on your log sheet. Robert sometimes
for many astronomers, the only sites within easy uses his “regular” eye to locate objects in the finder,
driving distance are, at best, semi-dark. The prob- for which full dark adaptation is less important.
lem with these sites is often not so much general As you prepare to observe an object, position the
light pollution as local light pollution — the presence towel to screen your face and the eyepiece from
of streetlights and other nearby bright light sources. local light sources and slide the eye patch out of the
For example, our regular “dark” observing site way. When you finish observing the object, cover
routinely offers mag 5.5+ skies, and on good nights your dark-adapted eye with the eye patch before
mag 6.0 or better. In terms of general light pollution, you remove the towel. At particularly bright sites,
that’s a respectable DSO observing site, at least by you may need to use the same procedure when you
eastern U.S. standards. Unfortunately, there are half use the finder.
a dozen mercury-vapor lights within a few hundred
yards of the site. Their combined light makes it Excerpted from Astronomy Hacks by Robert Bruce Thompson
impossible to become fully dark adapted. Because and Barbara Fritchman Thompson (O’Reilly Media, Inc., 2005)
