Measuring light pollution in Beijing and effects on Xinglong Station of National Astronomical Observatory

Baozhou Zhang, Ligen Lu Department of Astronomy, Beijing Normal University

Focus Meeting 21 Mitigating Threats of Light Pollution & Radio Frequency Interference

Contents

Light pollution & night-sky brightness

Measuring night-sky brightness in Beijing Method and instrument

Observations

Data and results

Summary

Light pollution & night-sky brightness

Light pollution refers to any “adverse effect of artificial light including sky glow, glare, light trespass, light clutter, decreased visibility at night, and energy waste.” — International Dark-Sky Association

Urban sky glow

Light trespass

Clutter

Glare

Night-sky brightness Natural sky brightness airglow, zodiacal light, scattered starlight, the Milky Way

Artificial sky brightness poorly designed and excessive outdoor lightings

Measuring night-sky brightness in Beijing

Method Night-sky brightness / luminance

V(mag/arcsec2) = 12.59 – 2.5lgB(cd/m2)*

astronomical unit mag/arcsec2

photometric unit

cd/m2

* Garstang, R. H. 1986, Pub. A. S. P., 98, 364-375.

*candela(cd), one of seven base SI units

Instrument – Night-sky luminance meter

Objective aperture 50 mm Focal length 70 mm

Field of view 2 degrees Sensitivity resolution 10-5 cd/m2

Measurement range 0 ~ 19.99 cd/m2 Measurement error ± 4.6% (± 0.05 mag/arcsec2)

Table 1 Specifications of the Night Sky Luminance Meter. Objective lens

Eyepiece Reflector

Reflector with a hole

PMT A ADC θ

The view seen from the eyepiece

Observation conditions

The necessary conditions to perform all measurements in the survey:

Moonless night Clear night, no clouds or fog The Sun is at least 18 degrees below the horizon (astronomical

twilight) Avoid a site with road lightings or other considerable artificial

lightings nearby

Site 1 – Beijing Normal University (BNU)

Site 1–BNU Site 2

Site 3

Site 4

Site 5

Site 6–Xinglong Station

Site 7–Lingshan

• BNU, Beijing Normal University, 7 km from the city center • Xinglong Station of National Astronomical Observatory, is located about 110 km

northeast to Beijing at an altitude of 950 m. • Lingshan site, at an altitude of nearly 1 km, is about 80 km from the city center.

Night-sky brightness in one night (at Site 1 - BNU) Continuous measurement from earlier night (22:30 p.m. local time) to later

time (04:00 a.m. the next day) Including the zenith night sky and skies at 45 degrees zenith distance in four

directions of north, south, east and west

Figure 2. Variation of night-sky brightness with time.

Observation results

Site 1–BNU

Zenith brightness

Unit 10-3 cd/m2 mag/arcsec2

Max 7.68 17.88 Min 4.65 18.42

Zenith night-sky brightness at various distances from the city center

These brightness data were all measured in late nights, representing the darkest night sky at each observing site.

Observation location

Distance from the city center (km)

Measurement time Weather Night-sky brightness

10-3 cd/m2 mag/arcsec2

Site 1 - BNU 7 6m 16d 01h 20m Clear 4.65 18.42 Site 2 18 8 22 02 30 Clear 2.82 18.96 Site 3 40 7 30 00 00 Clear 1.30 19.81 Site 4 61 8 22 01 30 Clear 0.67 20.52 Site 5 85 8 22 00 00 Clear 0.33 21.29

Site 6 - Xinglong 114 4 3 00 40 Clear 0.26 21.55

Site 7 - Lingshan 80 9 11 02 00 Clear 0.23 21.68

Table 2 Zenith night-sky brightness at seven observation sites.

Beijing

Site 6–Xinglong Station

Site 7–Lingshan

Chengde City

Xinglong Town

Zunhua City

Summary A portable night-sky luminance meter

• Independent of the telescope • With narrow field of view (2 degrees) and high sensitivity

(10-5 cd/m2)

Measuring night-sky brightness in Beijing • Seven observation sites with different distances from the

city center, including Xinglong Station, a major optical site for astronomical observations in China

• Potential risks to night sky of the Xinglong Station: outdoor lightings in nearby cities

Future work Modifications of the night-sky luminance meter

• Remote data transmission and operation

Beijing night-sky brightness monitoring network • Detailed geographical and temporal variations of the night-

sky brightness in Beijing • Assess whether new rules and regulations are necessary to

control light pollution and protect astronomical sites

Thank you for your attention!