Folks in and around Lyons are accustomed to living on the edge. We’re not quite out on the flats, but not fully into the mountains either. We have one foot in the urban corridor, and the other in the wilderness. We’re right on the edge of the dome of night light that hangs over the Denver metropolitan area.
Late this past summer, I set out to investigate the impact of that glow on both visual stargazing and astrophotography. I ran tests from my rural home in X Bar 7, Sandstone Park in downtown Lyons, and the parking lot at Village at the Peaks (formerly Twin Peaks Mall) in Longmont.
Using this site, there are two different ways to evaluate the impact of light pollution in a given area. One is ground radiance, which indicates the degree of local lighting in the area as seen by satellites. The second is zenith sky brightness, which instead shows how much light one encounters by looking straight up at those satellites. The former method indicates that the town of Lyons has 20 times more photons hitting the ground than at my house, while Longmont has about 12 times more than Lyons. Looking the other way (up), the skies are rated between Bortle Class 3.5 to 5. Given that the Bortle Scale ranges from 1 (the darkest possible skies that astronomers would die for) to 9 (skies under which astronomers would rather be dead), it’s obvious we could do far better, but also far worse. We are indeed on the edge.
At each site I assessed my visual impressions of the sky and then photographed the patch of sky directly overhead (the zenith). I first used a portrait lens to capture the constellation of Lyra with its bright star Vega, and then used a wide-angle lens to include the adjacent Milky Way. I tried test exposures ranging from two to 30 seconds at each locale, and then selected the most acceptable exposures from each.
It was no real surprise that the faintest stars showed up from my home, where 30-second exposures were possible. Oddly, though, I found that I preferred the shot from the town of Lyons over the rural one. The moderate amount of skyglow actually helped the familiar constellation star patterns pop out, and made the image more closely resemble a dark-sky, naked-eye view–as opposed to showing so many stars that I could not see the forest for the trees, so to speak.
The most astonishing finding for me, however, was that even from Longmont, I was able to photograph stars all the way down to 7th magnitude. That’s everything possible to see with the naked eye under pristine conditions! By contrast, there was only one star in the entire sky that I was able to find visually. Fortunately, that lone star was Vega, my target object.
The wide-field views confirmed my expectations. From a rural site, the Milky Way appeared nicely overhead, and could even be made to look spectacular with a 30-second exposure. The galaxy was also apparent overhead from Lyons, provided I used the pine trees to block the glare of the streetlights. Photographically it comes out well, but without a lot of pizzazz.
Longmont, however, was a vastly different story. It was impossible to photographically record, much less visually spot, the Milky Way because of the direct glare from parking lot lights.
It’s my opinion that the zenith sky map is more useful to predict the sky’s darkness for photography, while the ground radiance map more accurately reflects the vagaries of visual observing. The human eye needs to be well adapted to darkness in order to see faint phenomena; at least ten minutes of relative darkness is usually needed to permit one’s pupils to dilate. Even a momentary flash from a streetlight or a car’s headlights is enough to undo one’s night vision, forcing the adaptation process to start all over again. The ground radiance map gives a good idea of the optical clutter one might expect.
Cameras, unlike eyes, are not subject to the need for adapting to darkness: If you tell them to “open wide” while taking a picture, they’ll do just that. Bright ground objects are not distractions if their photons don’t make their way into the lens. The only interference is from the glow overhead, as shown by the zenith sky map.
Perhaps the moral of the story is this: If you live in town, your choices are to find a way to shield your eyeballs from all those lights; head up into the hills; or try photography!