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Measurement of Sky Brightness at the Iranian National Observatory
Davoud Afshari1 and Sadollah Nasiri1,2 1Physics Department, Zanjan University, Zanjan 45195-313, Iran; Institute for Advanced studies in Basic Science (IASBS), Zanjan 45195-1159, Iran;
Measurement of light pollution

2

Introduction

Among 34 candidate sites at the central part of the country, 5 sites namely Garkesh(in Kashan), Kolahbarfi(in Kashan), Deynava (in Qom), Sardar(in Kerman), and Mazarkahi(in Birjand) are selected using meteorological and geophysical data. The seeing parameter is measured for these sites. Therefore among those five candidate sites only deynava(name of mountain in Qom) and Garkesh(name of mountain in Kashan) are remaining, due to their seeing values. We started to study and prepare measuring the sky brightness of these sites by two years ago. Light pollution makes the level of sky brightness higher and results in less signal-to-noise ratio. With low S/N we have to build larger telescopes beside the bad effect of light pollution on them. By studying the effect of light pollution on famous telescopes we can see horrible conclusions. We had studied many numerical and experimental methods. We had gathered sample photometric data with a SBIG ST2000 CCD, a celestron telescope and the UBVRI standard filters to define the sky brightness exactly.
Sky brightness & Light pollution

Various models have been introduced in the literature to estimate the light pollution. The pioneering work in this area was done by Walker, 1970-1973. After some years Walker 's formula corrected. The formula we use to estimate urban sky glow, looking at a zenith of 45 degrees toward an urban source r kilometers away, is

I c Pr

n

"I" is the percent increase of the night sky brightness above the natural background, at 45° down from directly overhead, "P" is the population of the city, "r" is the distance, in kilometers, from the observing site to the center of the city. also "c" = 0.01 and "n" = 2.5 for "r" values between 10 and 50 km, "c" = 0.000632"n" = 1.4281 for values of "r" less than 10 km. Using Walker 's law we estimated the percent increase of the night sky brightness above the natural background for Garkesh and Deynava sites. The results are shown in Tables 1 and 2.
Town or village Qom Kashan Aran & Bidgol Mahallat Delijan Niasar Naragh Population 1030682 248789 55651 48458 31852 4823 2508 2168 1373 963 536 Distance (km) 60 52 52 55 30 25 17 21 8 8 7 Direction N E E SW SW SE SW SE W S E I 0.370 0.128 0.028 0.022 0.065 0.015 0.021 0.011 0.044 0.031 0.021 Ghahrood Meimeh Ghazaan Town or village Kashan Kamo Joshaghan Aran & Bidgol Barzak Population 248789 2000 3477 55651 3211 3566 657 5733 548 Distance (km) 39 7 12 46 16 18 10 28 12 Direction N S S N NW NE E S NE I 0.261 0.078 0.069 0.038 0.031 0.025 0.015 0.013 0.010

Astronomy is suffering from rapidly growing environmental problems. One of these is light pollution. Urban sky glow is taking away the prime view of the stars and the universe. Many things can bring light pollution to us, such airglow, artificial sources as urban lights, zodiacal light, and solar wind and of course moon light.
Sky without light pollution Sky with light pollution

Nashlaj Jasb Khavaran Rahgh

Table 1. The percent increase of the night sky brightness for Deynava

Table 2. The percent increase of the night sky brightness for Garkesh

Sky brightness measurements & Conclusion

The zenith brightness of the moonless night sky at a clear dark observing site, measured at high ecliptic and galactic latitudes, and during solar minimum, for V, B and R band are 21.9, 22.9 and 21.0 magnitude per arcsec squared, respectively. However, the measured sky brightness for observatory sites are normally less than above values. In the first step we estimated the sky brightness by Walker 's law. The results are given in Tables 1 and 2. We also measured the sky brightness of the candidate sites by photometry of the Landolt stars. The data was obtained using SBIG ST2000 CCD, a 11inch celestron telescope with F/10 and the UBVRI standard filters. We used the stars from Landolt catalog with magnitude greater than 10. Taking into account the effects of bias, flat and dark frames on the CCD image of the reference stars, We calculated the sky brightness using the IRAF software for the Deynava and Garkesh sites. The results are shown in Figures 1,2 and 3.
D eynava(Qom)
Garkesh(Kashan)
22.5 22.32 21.77

The problem of light pollution became important mainly since 1960, by growth of urban development and using more artificial lights and lamps at the nighttimes. The natural sky brightness is mainly due to moonlight, zodiacal light and solar wind. One important factor that increases the local sky brightness is the artificial sources of urban lights, or namely light pollution. The earth's atmosphere causes the light coming from sources in an urban area to be scattered, creating the halo of light visible over the city even from great distances. Optical telescopes share mostly the same range of wavelength as are used to provide illumination of roadways, buildings and automobiles. The man made light glow scatters by the atmosphere and enters as an unwished light in the image taken by the observational instruments. Light pollution makes the level of sky brightness higher and results low Signal to Noise Ratio (SNR) which in turn reduces the effective aperture of the telescopes.
TEMPLATE DESIGN © 2007

22.5 22

22.2

Sky Brightness

21.5 21 20.5 20 19.5 19 B

Sky Brightness

21.52 Series 1 20.08

22 21.5 21 20.5 20 19.5 19

20.39

Series 1

V Filte rs

R

B

V Filte rs

R

Figure 1. Sky brightness for Deynava site
22.5 22
Sky Brightness

Figure 2. Sky brightness for Garkesh site

22.222.32 21.77 21.52

21.5 21 20.5 20 19.5 19 18.5 B

20.39 20.08

Deynava(Qom) Garkesh(Kashan)

Figure 3. Sky brightness in B, V and R band for Deynava and Garkesh site

V Filters

R

Sky brightness results in B, V, R filters are acceptable for Iranian National Observatory that can be comparable with other observatories. But there is a worry about light pollution of Qom and Kashan that according to first and second tables.

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