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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

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Nighttime picture of Hong Kong taken from ISS (2007-2008, credit: NASA)

Urban: The University of Hong KongRural: iObservatory 2

15km

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• 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

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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 surface3G modem, routerpower adaptersfans, timer

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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 45oC 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

Real-time night sky condition maphttp://nightsky.physics.hku.hk

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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

Uncertainties of SQM-LE measurements - Digitization error

Unit #978 (iObs)

Unit #1108 (HKU)

<16.50.15mag

<16.50.15mag

>17.80.05mag

>17.80.05mag

In between0.1mag

In between0.1mag

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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

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• 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

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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

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2

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1

3

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-urban site

-rural site

local time

good linear correlation

good linear correlation

clear sky => darker skycloudy sky => brighter sky

clear sky => darker skycloudy sky => brighter sky

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clo

ud

ycl

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rcl

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Effects of change in cloud amount on NSB

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Webpage:

http://nightsky.physics.hku.hk• NSN details

• Map of night sky condition on Google Maps

• Research publications

Danke!

Principal InvestigatorDr Jason C. S. PunTel: 852-2859-1962

Email: jcspun@hku.hk

Project ManagerMr C. W. So

Tel: 852-9447-2120Email: socw@hku.hk

Research AssistantMr Terry C. F. WongTel: 852-2859-2372

Email: terryfai@hku.hk

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Backups

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Overview of Hong Kong• High population density (mid-2009):

6,480 /km2

• 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

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NSN stations

China

Hong Kong

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NSN stations major population centers

B3

NSN stations

Pole mount Wall mountB4

B5

Setup of the testing platform

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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