Measuring Night Sky Brightness

Figure 1: Mosaic camera system. Photo credit: NPS / Li-Wei Hung

We collect high resolution images of the night skies from horizon to horizon using two types of highly specialized camera systems: our mosaic camera system (Duriscoe, 2007) (figure 1) and our fisheye camera system (figure 2). The mosaic camera system has been used since the early 2000s for collecting high resolution images, and we use the new fisheye camera system when a park needs a quick snapshot of the entire sky or long-term monitoring of sky brightness. Images from both cameras are positionally and photometrically calibrated.

Figure 2: Mosaic camera system deployed in the field.

NPS / Li-Wei Hung

Figure 3: Fisheye camera system.

NPS / Li-Wei Hung

Figure 4: Fisheye camera system deployed in the field.

NPS/ Jeremy White

Both camera systems are equipped with a V-band filter, so the captured images closely measure the night sky brightness as perceived by the human eye. These images are photometrically and positionally calibrated.

Coming soon: time lapse video that shows how we set up our mosaic camera system in the field.

Figure 5: Example of the night sky image collected at Chaco Culture National Historical Park. (a) The observed sky showing light from all sources, both natural and artificial. (b) The image of estimated skyglow from artificial sources. (Hung et al., 2019)

Next, we separate out natural vs artificial light in the captured images. Components of natural sky brightness include zodiacal light, Milky Way, airglow, and atmospheric diffused light (see “Natural Light in Night Sky” page). Based on the time and the location of the observation, we can construct a natural sky model accounting for all these natural brightness components (Duriscoe, 2013). The modeled natural brightness can be subtracted out to obtain panoramic images showing anthropogenic light only. These assessments provide baseline conditions of the quality of the night sky and help park managers identify light pollution sources, brightness, and their impacts on the surrounding environment. This data is used to inform management actions and implement sustainable lighting practices to improve night sky resources.

Explore night sky resources across the National Park Service using this interactive map of NPS all sky data.

References

• Duriscoe, D. M., Luginbuhl, C. B., & Moore, C. A. 2007. Measuring Night-Sky Brightness with a Wide-Field CCD Camera. Publications of the Astronomical Society of the Pacific. 119, 192
• Duriscoe, D. M. 2013. Measuring Anthropogenic Sky Glow Using a Natural Sky Brightness Model. Publications of the Astronomical Society of the Pacific. 125, 1370
• Hung, L.-W., D. M. Duriscoe, J. M. White, B. Meadows, and S. J. Anderson. 2019. Night skies data report: Photometric assessment of night sky quality—Chaco Culture National Historical Park. Natural Resource Report NPS/NRSS/NSNSD/NRR—2019/1914. National Park Service, Fort Collins, Colorado.