SKYMONITOR: A network to monitor the state of the night sky

Fig. 1 — The NSBM remote unit.

SKYMONITOR is a project to establish a network of Night Sky Brightness Monitors (NSBM) to provide continuous, long-term measurements on the state of the night sky. The heart of this network is a solar-powered, calibrated photometer which operates in a nearly autonomous fashion. Data will be collected at a number of sites, transmitted to a central station where it will be archived and made available to the public via the Web. This project has been supported by the International Dark-Sky Association (IDA), the National Science Foundation and the Vatican Observatory.

Fig. 2 — The base unit, which is connected to the web.

The NSBM unit consists of a remote photometer head with two detectors (Fig. 1) and a base station that receives the data via a wireless link (Fig. 2). The remote head can be up to 100 meters from the base station which is connected to the Web. Each photometer samples an area of the sky about 4 degrees in diameter. The two photometer heads are independently pointed; typically one is toward the zenith while the other is about 20 degrees above the horizontal. The spectral response is much broader than typical astronomical broadband filters, such as Bessel V, thus allowing greater sensitivity for dark skies (Fig. 3).

Fig. 3 — Spectral response of the NSBM photometer.

Calibration of each NSBM unit is crucial to the success of this project. We measure the variation in output of each sensor as a function of temperature and provide a fit to correct for temperature. Also, simultaneous sky brightness measurements have been made with the NSBM photometer and the National Park Service Night Sky Team sky brightness system so that we can correlate our data with a national program.

SKYMONITOR is planned for launch in 2011 with installation of up to 25 NSBM units at astronomical observatories, dark-sky preserves, national parks and other environmentally sensitive sites needing dark sky protection. Data will be sent to a central site at the headquarters of the IDA in Tucson, Arizona where it will be archived and made available to the public via a website. First data is expected be available online by autumn of 2011.