In 1991, Flagstaff Arizona was designated the worlds first International Dark Skies Community. It's a great distinction that I'm proud our city has achieved. As a Astronomy hobbyist, I've often gazed in wonder at the night skies visible in our area, and appreciate the efforts that have gone into the preservation of one of our most precious vistas.
When I read in the Az Daily Sun that the "Year of the Night Sky" in Flagstaff was to kick off on March 28th, it inspired a question: How dark are our skies, actually? How much sky glow is visible in photographs from different parts of the City? What do the night skies of a 'Dark Skies' community actually look like? My geekish tendencies got hold of me, and I put together a plan. I gathered up my equipment and a Thermos of hot coffee, and set out for an all-night photographic endeavor to find some answers.
My goal was to accurately document the brightness of the Flagstaff sky glow from a number of locations in and around the Flagstaff area. Since I wanted the darkest skies possible, a night was chosen near the 'new' moon (no moon interference), no clouds, and around the midnight hour. The night of March 27-28 served to be ideal.
Since this was to be a meaningful comparison, I made sure that I used identical exposures from each location: 30 sec @ f / 2.8, iso 6400, color balanced to 5150K, with a wide-angle 14mm lens on my tripod-mounted Canon 6D. No post-processing (change in brightness, contrast, color, etc) was performed on any of the shots. They were directly converted from RAW, cropped, then re-sized for web display. With the in-camera GPS each location was logged.
Over the course of the night I shot from 14 different locations with a wide variety of sky glow conditions. I went as far north as the Wupatki Loop Intersection at Hwy 89 (25 mi N of town), as far south as Kachina Village, to several locations in Flagstaff, and up at Snowbowl. The long exposures brought out the sky glow very well, and gave a good representation to the limitations of photographing our night sky, and the visual impact of the Flagstaff sky glow.
By the time I was done at 3:30 AM - I had some answers. Frankly, I was quite surprised by what I saw. Here's a map of where the photographs were taken.
The Good, The Bad, and the Ugly.
First, the good news. Areas just outside the periphery of Flagstaff have very little overhead sky glow. At the zenith, the skies can be wonderfully and magically dark!
That bad is, you only have to turn the camera toward Flagstaff to see the obvious influence of the lights of our city. I don't know how the glow from our town compares to others. Given it's designation as an International Dark Sky Community, I assume the glow we see from Flagstaff is significantly LESS than we'd see from another town of equivalent size. I was quite surprised to see how much sky glow we actually had, and what spectacular limitations it can put on astrophotography in the region.
For the darkest skies of this survey, check out this image from 25 miles north of Flagstaff. This (and all of the other shots cropped to a square) were shot looking straight up at the stars overhead:
Now compare that to a shot taken on the east side of Flagstaff, near the Country Club neighborhood. Same exposure, same settings.
I found that most locations in Flagstaff had a similar amount of light pollution. The worst was downtown. As might be expected, Lowell Observatory exhibited levels of light pollution similar to other parts of town:
Just getting out of town a couple miles makes a considerable difference. Look at these two zenith photos from Kachina Village (left) and Doney Park (right):
But looking toward town from these two locations reveals the nasty sky glow dominating the horizon:
From the few images above you can see the basic results of my observations. There's enough sky glow from within Flagstaff to obscure many details of the night sky, and significantly deteriorate attempts for astrophotography. Traveling 5-10 miles outside the city makes a big difference - truly dark skies aren't too far away.
However, I'm lacking in perspective here. Do these observations challenge Flagstaff's 'Dark Sky' designation, or support it? I don't know. Should we expect Flagstaff's skies to be darker, or should we be happy with the level of sky glow we have?
I've put all of the overhead star photographs together below - simply click on the image to scroll through the images from the 14 locations in and around Flagstaff.
That's all for now...if you'd like to see the complete set of images from this analysis (including many more horizon images), you can find them on my Pbase site here.