Night Skies Data Report: Photometric Assessment of Night Sky Quality—Chaco Culture National Historical Park

Hung, Li-Wei; Duriscoe, Dan; White, Jeremy; Meadows, Bob; Anderson, Sharolyn

This report characterizes night sky conditions in Chaco Culture National Historic Park using measurements made in the park unit and models of regional conditions based on satellite data. Calibrated night sky imagery was obtained to characterize the night sky at four sites. These ground observations were collected on nine nights from 2001 to 2016. Satellite data collected in 2016 was used to create a map of predicted night sky conditions in and around the park. Overall, the photometric measurements demonstrate the night sky quality at Chaco Culture NHP is excellent. The average horizontal illuminance indicates the park preserves to a large extent of the natural illumination on the plain. Although the artificial lights have a measurable effect in brightening the night sky along the horizon, the maximum vertical illuminance from all sources is less than one mlx (about one tenth of a quarter moon). The low illuminance level provides a refuge for crepuscular and nocturnal species in the park. The sky overhead remains pristine with the average zenith brightness of 21.9 mag/arcsec2. We estimate more than 90% of stars were still visible for the most of the time, providing an outstanding opportunity to observe the natural night sky from the park. Excluding the observation taken in 2014 under hazy conditions, the whole sky over Chaco Culture NHP is only 12-19% brighter than average natural levels, indicating excellent dark sky conditions. The visual observations also suggest the darkest part of the sky remains pristine, and the whole sky is only slightly brighter than the natural conditions. In Chaco Culture NHP, we classified the sky as Bortle Class 3: rural sky, based on the visibility of astronomical objects. The average naked eye limiting magnitude (NELM) is 7.0, approaching the sensitivity limit of human eyes under good atmospheric conditions. Our SQM measurements average to 21.6 mag/arcsec2, indicating the zenith is darker than what we can accurately measure with a SQM. From most locations within the park, visitors can find places free of direct glare and allow their eyes to be fully dark-adapted. During clear and dark nights, visitors have an opportunity to see the Milky Way from nearly horizon to horizon, complete constellations, faint astronomical objects, and natural sources of light such as the Andromeda Galaxy, zodiacal light, and airglow. The main impacts to Chaco’s night sky quality were the light domes from Albuquerque, Farmington, Rio Rancho, Gallup, Crownpoint, Santa Fe, Bloomfield, and Grants. These light domes were observed along the horizon, with a few exceeding the natural brightness of the Milky Way. Additionally, glare sources associated with oil and gas development sites are visible along the north and east horizons. In a dark environment such as Chaco Culture NHP, small changes of lighting will have a large impact on the night sky quality. While effects from light domes and glare sources are moderate to low during clear nights, their brightness can be increased significantly by clouds and increases in atmospheric aerosols (e.g. dust, soot).

Night Skies Data Report: Photometric Assessment of Night Sky Quality—Chaco Culture National Historical Park

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