Have you ever seen the Milky Way? Only 20 percent of people in the U.S. live in a location where they can. Away from light pollution, the wide, ghostly band almost looks like clouds, and the stars overhead look so big and real that you feel you might fall into the sky. Through the eyepiece of a telescope, galaxies and nebulae normally obscured by the haze of city lights stand out in stark contrast.
The story is a little different for the astrophotographer — light pollution matters less these days with the ability to stack hundreds of subframes and use light pollution or narrowband filters to block out city lights. It’s difficult to lug your equipment out to a dark-sky site, so many are often content to stay home and image from the backyard — myself included. You may even live hours away from rural skies and ask: Is it worth the trip?
For city-based imagers, narrowband filters are especially effective at cutting down on the worst of light pollution; they block most light except for tiny slices of the visible spectrum that correspond to wavelengths emitted by deep-sky objects. There are also traditional light-pollution filters, which reduce light at wavelengths produced by mercury- and sodium-vapor lamps. Although many cities have switched to broad-spectrum LED lights, light-pollution filters still make a difference. And in post-processing, background extraction and gradient removal tools can do wonders for reducing background haze and allowing dim features to shine through.
With these tools, going to a dark-sky site may feel superfluous. However, a brighter background isn’t the only problem with light pollution — all those extra photons introduce additional “shot noise,” which is a quantum mechanical effect related to the discrete nature of the electrons and the photons that created them in the camera sensor. To beat down the light pollution and added noise, one must take many more hours of subframes to increase the signal-to-noise ratio. For astrophotographers who live in places with less-than-ideal weather, every hour of clear sky is precious. Typically, I will spend 20 to 40 hours on a target from my backyard, but from a dark-sky site, I can usually get a better image in three to six hours — sometimes less.
We quantify how dark the sky is in units of stellar magnitudes per square arcsecond (mag/arcsec2). For example, in the downtown area of a large city, the sky might have a brightness of 17.5 mag/arcsec2, which is essentially the same as filling the whole sky with dim stars; one can practically read by the reflected city light. This is the bright extreme of the Bortle dark-sky scale, at Bortle 9. Most constellations are not visible, with only the brightest stars and planets shining through.
On the other hand, when I was at the Okie-Tex Star Party in 2022, it got as dark as 21.7 mag/arcsec2, on the border of Bortle 1 and 2. In such a sky, the Milky Way is vivid and shows structure, several galaxies and star clusters are visible naked-eye, and bright stars and planets can cast shadows. The strangest thing about being at a dark-sky site is that the clouds are black, since there is no light from the ground for them to reflect!
Sometimes we must put up with the light pollution we have and try to observe and image what we can. But I highly recommend getting out to a dark-sky site as often as possible; a weekend camping trip to your local astronomy club’s dark-sky site can be fun and rewarding.
In the longer term, there are ways to reduce light cast to the sky; primarily pointing lights at the ground and cutting off sideways- and upward-looking light fixtures. DarkSky International (IDA) has ideas for responsible lighting solutions and resources for advocating for your local city or municipal government to implement responsible lighting ordinances.
It can sometimes feel impossible to escape light pollution, but it is well worth it to get out under a dark sky, even if it’s only a few Bortle steps lower. You will be amazed at how much more you can see with your camera and your eye. Make some time to bask in the light of the Milky Way and fall into the thousands of stars that lie overhead.
