1. 1 Night Sky Brightness Monitoring Network in Hong Kong Dr Jason C. S. Pun, Mr Chu-wing SO, Mr Terry C. F. Wong Department of Physics, The University of Hong Kong Osnabrück, Germany October 6, 2011 11th European Symposium for the Protection of the Night Sky 1

2. Nighttime picture of Hong Kong taken from ISS (2007-2008, credit: NASA) Urban: The University of Hong Kong Rural: iObservatory 2 15km

3. 3 Funded by “ Environment and Conservation Fund ” of the Hong Kong SAR government Duration: June 2010 - May 2012 NSN data: –Night Sky Brightness (NSB) by Sky Quality Meters – Lens Ethernet (SQM-LE) at 18 monitoring stations –Cloud amount by IR sensors (Boltwood, AAG) at selected stations –Meteorological data by on-site weather stations and observations by Hong Kong Observatory (HKO) Hong Kong Night Sky Brightness Monitoring Network (NSN) SQM-LE AWS cloud sensors 3

4. NSN stations NSN monitoring station at iObs (rural site) SQM-LE points to zenith (under transparent window) weather station cloud sensor mounting pole mounting frame weatherproof casing: reflective surface 3G modem, router power adapters fans, timer 4

5. NSN stations Special thing about NSN stations: –Measure NSB simultaneously, continuously at multiple locations –10 urban + 7 rural + 1 airport Cooperation with HK Observatory, secondary schools, government agencies –Data collection Each SQM-LE is connected to a 3G router and modem with a fix IP address Server at The University of Hong Kong requests reading from all stations every 5 minutes Store raw data in a database –Custom-made outdoor casing Polycarbonate (after one-year of aging, effect of the case ~ 0.2 mag) Electric fans for ventilation (interior reaches 45 o C in summer!) –Timer (with battery backup) Daily reboot (4pm power up, 8am power off) Hardware and network connection may be self recovered after reboot Avoid overheat during daytime 5

6. Real-time night sky condition map http://nightsky.physics.hku.hk 6

7. Uncertainties of SQM-LE measurements Manufacturer's claimed accuracy: –+/- 10% or +/- 0.1 mag arcsec -2 for all magnitude readings Additional factors in considering uncertainties of SQM-LE readings: –digitization error ranged 0.05 to 0.15 mag arcsec -2 –Measured data spread depends on the incoming luminance level ranged form 0.02 to 0.16 mag arcsec -2 –frequency-period modes switching point uncertainty increased to 0.2 mag arcsec -2 below around ~14.6 mag arcsec -2 Generally, found larger uncertainty for brighter measurements –Could be problematic when making comparisons between urban and rural skies 7

8. Uncertainties of SQM-LE measurements - Digitization error Unit #978 (iObs) Unit #1108 (HKU) <16.5 0.15mag <16.5 0.15mag >17.8 0.05mag >17.8 0.05mag In between 0.1mag In between 0.1mag 8

9. Uncertainties of SQM-LE measurements – measured data spread vs input luminance averaged NSB responses against a constant LED source at distance d inside a light- shield dark platform precision depends on the incoming luminance level 9

10. Attempt to identify the effects of human lightings on night skies Compiled the “darkest” NSB profiles vs time by selecting the dimmest nights at any particular time –Large quantity of data used (over 9 months) –Brightening of night sky due to meteorological or astronomical factors such as moonlight, cloud, air pollution(?) not included –Compare and contrast the profiles from urban and rural sites Effects of human lightings on NSB 10

11. Effects of human lightings on NSB *remark: NSB show by the blue dash curves here are the maximum values sampled over large data sets covering June 2010 to March 2011; The red curves show the their best fits. (1) The urban site is roughly 2 mag arcsec -2 brighter than the rural site (2) NSB stayed constant for a prolonged period until the night sky got dimmer for a few hours (probably due to the reduction of human lightings), after which the night sky stayed roughly at the same level (3) The night sky turned dark earlier for rural site and got to the final level earlier than the urban site (due to different lighting usage pattern?) (4) Greater amount of “jump” (red arrows) in urban site than rural site, due to more human lightings used 1 2 2 3 1 3 4 4 11

12. -urban site -rural site local time good linear correlation good linear correlation clear sky => darker sky cloudy sky => brighter sky clear sky => darker sky cloudy sky => brighter sky clear cloudy clear cloudy 12 Effects of change in cloud amount on NSB

13. 13 Webpage: http://nightsky.physics.hku.hk NSN details Map of night sky condition on Google Maps Research publications Danke! Principal Investigator Dr Jason C. S. Pun Tel: 852-2859-1962 Email: jcspun@hku.hk Project Manager Mr C. W. So Tel: 852-9447-2120 Email: socw@hku.hk Research Assistant Mr Terry C. F. Wong Tel: 852-2859-2372 Email: terryfai@hku.hk 13

14. Backups

15. 15 Overview of Hong Kong High population density (mid-2009): 6,480 /km 2 Mixed land utilization within a small area Complex geological landscape Robust human activities Famous for the spectacular city night lights No regulations on the usage of outdoor lightings at night Case study of the effects of human activities on the quality of night sky Nighttime picture of Hong Kong taken from ISS (2007-2008, credit: NASA) 15km 9.3miles B1

16. NSN stations China Hong Kong B2

17. NSN stations major population centers B3

18. NSN stations Pole mount Wall mount B4

19. B5 Setup of the testing platform

20. 20 Future Plans Compare time series of multiple sites Effects of human lightings at multiple sites Long-term variations of NSB Effects of moonlight on NSB Relation of air pollution vs NSB … B6