Night sky photometry and spectroscopy performed at the Vienna University Observatory

https://doi.org/10.1016/j.jqsrt.2013.08.019Get rights and content

Highlights

  • We performed night sky brightness photometry and spectroscopy in and around Vienna.

  • The level of light pollution in the city ranges from 15 to 19.25 mag arcsec−2.

  • In Vienna, the curfew at 11 p.m. and 12 p.m. was detected.

  • Fluorescent lamps are responsible for the strongest lines of Vienna's night sky.

  • Circalunar rhythm of night sky brightness is extinguished due to light pollution.

Abstract

We present night sky brightness measurements performed at the Vienna University Observatory and at the Leopold-Figl-Observatorium für Astrophysik, which is located about 35 km to the southwest of Vienna. The measurements have been performed with Sky Quality Meters made by Unihedron. They cover a time span of roughly one year and have been carried out every night, yielding a luminance value every 7 s and thus delivering a large amount of data. In this paper, the level of skyglow in Vienna, which ranges from 15 to 19.25 magSQM arcsec−2 is presented for the very first time in a systematic way. We discuss the influence of different environmental conditions on the night sky brightness and implications for human vision. We show that the circalunar rhythm of night sky brightness is almost extinguished at our observatory due to light pollution.

Additionally, we present spectra of the night sky in Vienna, taken with a 0.8 m telescope. The goal of these spectroscopic measurements was to identify the main types of light sources and the spectral lines which cause the skyglow in Vienna. It turned out that fluorescent lamps are responsible for the strongest lines of the night sky above Vienna (e.g. lines at 546 nm and at 611 nm).

Introduction

Until recently, astronomers have mainly been measuring the brightness of the night sky (in magnitudes arcsec−2) at dark sites, especially at modern mountain observatories, or at potential observatory sites as a part of “site-testing” and “site-monitoring”. Within the past few years, it has become evident that increasing night sky brightness and light pollution have far-reaching consequences for many branches of human life as well as wildlife (e.g. [1]).

Therefore, it is desirable to measure and monitor the night sky brightness not only at remote mountaintop observatory locations (as done, e.g. by Patat [2]), but also close to the centers of modern civilization, and to do so every night, in a reproducible way, with the aim of performing long-term studies (such as in climate research). Only in this way can the impact of skyglow on biological rhythms on animal behavior, and human health be assessed (see [3], [4], [5]).

This is the aim of our ongoing night sky brightness measurements in Vienna and at the “Leopold-Figl-Observatorium für Astrophysik” (LFOA) on Mount Mitterschöpfl. We complemented our measurements with spectroscopic studies of the night sky at the Vienna University Observatory, which hosts the “Institut für Astrophysik” (IfA).

It should be noted that all our measurements refer to scattered light. We measured only brightness values and spectra of the sky background, while we explicitly avoided to measure any direct radiation from streetlamps. This is what we call “night sky brightness” (NSB). In other words: the brightness values and the spectra presented in this paper refer to the total backscattered light of the night sky. Its origin is the whole ensemble of streetlamps, facade illuminations, illuminated billboards, etc. at the respective observing site and in its near and far surroundings. The natural nocturnal radiation from Earth's atmosphere, which is produced by different processes such as recombination of atoms that have been ionized by Sun's radiation during daytime, contributes very little to the NSB that we measured at our observing sites, since the latter is dominated by the influence of artificial light.

Section snippets

Measurement sites

First measurements began in November 2011 at the IfA. Since April 2nd, 2012 the measuring device (label “IFA”) has been mounted at its ultimate place and points exactly to the zenith. About the same time (March 21st, 2012) we started measuring the NSB at our remote mountaintop site too. Currently we use three devices in total, two of which are located at LFOA (label “FOA” and “FOA2”). The geographical positions of our sites are given below (Table 1).

Units used for our photometric measurements

The devices that we use for our NSB

Data analysis

In the following all given daily mean values in magSQMarcsec2 and other statistical parameters for the NSB are calculated for time intervals between the astronomical twilights of the given date. As described in Section 2 the device “FOA2” mounted at our remote site was calibrated against our older SQM “IFA” located at our institute in Vienna.

Conclusions

After one year of NSB measurements at an urban site (in Vienna) and at a rural mountaintop, we may draw the following conclusions:

  • (i)

    Due to its high sensitivity covering the bulk of the visible spectrum, Sky Quality Meters are appropriate not only for long-term studies of the NSB, but also for short-term events. Nevertheless, an adequate site, weather proof housing and shade are crucial, such that the sensor is protected against scattered (moon) light. Once tested, the instrument will monitor the

Acknowledgments

We thank Otto Beck, Erich Schäfer and Werner Zeilinger for their help with the installation of the SQMs in Vienna and especially at the Mitterschöpfl Observatory. We gratefully acknowledge Stefan Meingast's help with the spectroscopic measurements. S. Uttenthaler acknowledges support from the Austrian Science Fund (FWF) under project P 22911-N16. JP acknowledges financial support from the Provincial Government of Upper Austria. The authors wish to thank Anthony Tekatch (Unihedron) for fruitful

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