A search for the dark side and better astrophotography
One of the Holy Grail quests for astrophotographers is finding the dark sky. Few of us are fortunate enough to live under an ideal dark sky, but most of us are mobile enough to get anywhere better than the center of an urban area.
In 2006, John Bortle published an article in Sky and Telescope describing an informal scale for rating your sky, now aptly known as the Bortle Scale. On its scale, 1 is best, 9 is worst. Bortle 9 is what I live under – I don’t bother with a flashlight when I go to my garden at midnight.
The high-contrast image above was taken after midnight with no moon in the sky. Personally, only a few stars were visible when I took the picture. On the edited image, I circled the three bright stars of the striking summer triangle. The other bright object on the left of the picture is Jupiter.
My local solution is a 100 mile (161 km) drive to my observatory at an altitude of 4,300 feet (1,310 meters), after which on a good night I’ll be under a sky that may be Bortle 4+ sky. The above shot was taken towards the southwest, where the glow of San Diego dominates the central horizon and the glow of the nearby city of Temecula and more distant Los Angeles begins to the right.
The cover photo (repeated above) was taken in the first quarter moonlight at a location in San Pedro de Atacama at an altitude of 2,407 meters in northern Chile, which would likely be classified as a very good dark place (perhaps Bortle 1+). . Despite the first quarter moon, the Milky Way is clearly visible. The volcanic peak in the background is Licancabur, which lies on the border between Chile and Bolivia. The top of the cone is 5,916 meters long.
Above is a mosaic in the (by far) darkest place I have ever visited (Namibia). It is located on a dry high plateau on the edge of the Kalahari Desert with excellent conditions for astrophotography, but is a real journey to get to. The image of the southern Milky Way is a 5-panel mosaic of 40-minute recordings on medium format film.
A more accessible place is Haleakala on the island of Maui. At an altitude of 3,055 meters, the air is very transparent and even, but as you can see in the picture above, the light pollution is clearly visible nearby. Tourism-focused resorts and businesses outline the island’s coastline to the left and right, with central light coming from the cities of Wailuku and Kahului, where the airport is located. On the one hand, the top of the volcano is easy to reach with normal vehicles on a wide, paved road and is a national park. On the minus side, the summit is now so crowded that the National Park Service needs reservations to see the sunrise.
Jump into the search
To help find a suitable dark place, there are now several light pollution maps on the internet, as well as recommended lists of public places with dark skies. The IDA (International Dark Sky Association) is also a source of information if you would like recommendations for reducing the light footprint of your own property.
While reading through the light pollution maps can be helpful, it is wise to remember that, like a map of the average cloudiness of the sky, these are averages. Like the actual weather, the local conditions in a given location are highly dependent on several factors:
- Brightness of the lights in your immediate area
- Lighting technologies
- Air pollution (atmospheric dispersion)
The first point is the obvious one that comes to my mind first. Nearby lights can shine directly into your lens and cause reflection artifacts or affect your night vision. Seasonal effects are wind, fire and fog. Holiday lighting is increasingly becoming a source of light pollution as cheap LED string lights are available.
A more subtle problem is the average glow of distant cities, which is particularly annoying for landscape astrophotographers. Even in deep sky photography, these distant domes of light limit the direction and the minimum height of the image. Wide-angle shots, in which clear color gradients disturb the photo, are particularly affected. But these effects also vary. At my observatory, low coastal fog suffocates the skylights of the surrounding cities at certain times of the year, which significantly improves the Bortle rating.
The type of lighting also plays an important role in the severity of light pollution. Many older types of lighting are in specific spectral bands, which at least allows the use of filters to block some of the interference. Unfortunately, from an astronomical point of view, the ugly low pressure sodium street light spectrum was the easiest to block, but it was so solid orange that it was difficult to find your car in a parking lot!
With the widespread availability of low power LED lighting, many lights have been converted to take advantage of reliability and low cost. Unfortunately, to encourage the move to LED lighting, manufacturers created bluer, more natural lighting, and with that we shot ourselves in the foot. LEDs are inherently very narrow-band light sources, but phosphors have been added to absorb and re-emit the light to cover a wider spectrum. In doing so, we have succeeded in swiveling the light pollution spectrum in the direction of blue, which scatters more strongly in our atmosphere than reddish lighting, as described in a recent study.
Air pollution (atmospheric dispersion)
The problem of blue light scattering also increases the role of particles in the air (whether or not viewed as pollution). The light sources themselves would not be such a problem if the light had no way of scattering and bouncing back on us. Smoke and urban smog are the most obvious factors, but moisture and wind-blown dust can also subtly affect the contrast in our images, even though individual layers of haze may not be visible to the eye.
During the day, you can get an idea of how problematic the scattering is for you by blocking out the sun and seeing how blue the sky looks as you get closer to the sun. Ideally, the sky looks dark blue up to the edge of the sun. At night you can do the same test with the moon. At my observatory location, the sky can often look clear, but as soon as something bright like Venus or Jupiter rises, it immediately becomes clear that there is celestial nebula. With long exposure times, large halos (not in connection with chromatic aberration) become visible.
To get around this problem, one solution is to go up high to get over the low-lying air pollution as well as the clouds. With thin and sufficiently clear air, it is possible to photograph the Milky Way even when the moon (the worst natural source of light pollution) is outside. But even that may not be a good solution when a global event like a major volcanic eruption has brought ash high into the atmosphere. Your personal sensitivity to great heights can also limit this option.
The Simons Observatory (above) in northern Chile is at 5,182 meters and the air is clear enough to see the Milky Way even with a moon in the first quarter of the sky.
Another often ignored source of light pollution is air travel. These cause a double blow – light pollution as well as air pollution. Air traffic exists at any time of the day or night. At night they fly with bright navigation lights. A good strategy is to review not only light pollution maps but also aircraft flight path maps and choose a suitable location.
In addition to the navigation lights of aircraft, engine exhaust gases are emitted high enough to linger for long periods of time, often in the form of visible contrails (altitude ice crystals).
What about satellites? They are not a problem for amateur astronomers. They are much darker than airplanes, have no navigation beacons, and the low-flying ones are mostly visible at sunset or sunrise. They could be a problem for professional astronomers, but amateur astronomers are facing bigger problems.
Other astronomers and astrophotographers
Finally, I have to say that we are our own worst enemies at times. When taking our own astrophotographs, we have to keep in mind that another astrophotographer may be trying to do his own thing 50 meters away. Illuminating the landscape with your flashlight can interfere with the next person’s exposure. The back screen of your cellphone or camera can be just as bad. Even the self-timer countdown flash or memory card write light could be an issue, so have some black tape handy to block out those light sources.
As an astrophotographer, it is also a good idea to avoid groups of amateur astronomers engaged in visual astronomy. You often have groups of people with flashlights pointed everywhere, including straight at your camera. Green laser pointers are often a problem too, and are bright enough to fit in photos (see picture above) even when they are moved. For this reason (as well as for eye safety reasons) I strongly advise against using them as polar finders or pointers to targets.
Do you have a good location to recommend? Please add your comments below!