Modelling the night sky brightness and light pollution sources of Montsec protected area

https://doi.org/10.1016/j.jqsrt.2018.05.037Get rights and content

Highlights

  • Light propagation modelling applied to an astronomical protected area.

  • Effects on light pollution of different light inventories scenarios.

  • Light propagation modelling vs. real measurements.

Abstract

We proceeded to the modelling of the night sky brightness of Montsec area (north-east of Spain), an astronomical protected area certified as a Starlight Reserve. We have used the hyperspectral version of ILLUMINA, an artificial sky brightness model. Ground based measurements for Montsec and other areas of Catalonia [15], [16], including both photometric and spectroscopic data, has been used to fit and evaluate the input parameters of the model. In this first modelling attempt, Lleida, the biggest city in the area, has been considered as the unique source of light pollution. In 2014 there was an update of the lighting infrastructure in Lleida. A detailed comparison of the sky brightness before and after the change is shown in order to measure the effects that different kind of lamps can produce. This information could be used to plan for future updates and improvements of the lighting systems in the area.

Introduction

Light pollution was already a noticeable problem for astronomy in the second half of the twentieth century when trying to find new locations for astronomical observatories [17], [19], [20].

The first attempts to derive a propagation law for artificial night-sky illumination came soon after. Bertiau et al. [6] and Treanor [18] presented simplified models that estimates the contribution of aerosol scattering to the zenith brightness due to light coming from nearby cities. The artificial zenith brightness was modeled by the population-brightness function F(D) and the brightness-distance function Q(D) of the surrounding cities. Berry [5] modified the brightness-population function in agreement with the work of Normandin [14] in the province of Québec, to better fit the data obtained from Southern Ontario. Walker [21] described new F(P) and Q(D) using measurements from California.

A major step towards a more complete model was proposed by Garstang (1986). In Garstang’s model, cities were not considered as point sources but as circular uniform surfaces; the amount of aerosols was an adjustable parameters and different scale heights for molecules and aerosols were established. It also accounted for ground reflectance and the percentage of light emitted above the horizontal with a prescribed angular emission function.

Nowadays, there are two prominent light pollution models: MSNsRAu [12], and ILLUMINA [3]. Both models accept the heterogeneity of ground-based light sources (spectra and light output angle distribution). MSNsRAu takes into account first order scattering only, which is suited for optically thin atmospheres and observers relatively close to cities and for low elevation angles. ILLUMINA on the other hand includes first and second order scattering that allows experiments in any direction and distance. Taking into account second order scattering makes ILLUMINA very time consuming with respect MSNsRAu. For that reason MSNsRAu is better suited for experiments involving a large number of grid point (large geographical domain with high spacial resolution). However, Aubé [2] showed that the second-order relative contribution rises with distance from city limits and may contribute up to 66 percent of the total zenith radiance for remote sites, therefore experiments with observer’s location far from cities should take into account second order scattering.

The present work is based on ILLUMINA because of the low number of lighted grid points and the large distance between the observer and the sources. Our aim is to better understand what kind of sources produces light pollution and how they affect astronomical observations. The output of ILLUMINA is converted to astronomical magnitudes to build all-sky maps comparable with the ones obtained with ground based measurements. We focus on the effect of light pollution in the Johnson–Cousins photometric system visual filters B, V and R [7], [10], [11] in any line of sight from the observer.

The main goal is to evaluate the light pollution that Parc Astronòmic Montsec (PAM) is receiving from Lleida, the biggest city in the area, located approximately 50 km south from PAM. Montsec is an astronomical protected area in the Northeast of Spain, labeled as reference point accordint to law 6/2001 of government of Catalonia due to its pristine conditions and also certified as Starlight Reserve. This case of study is interesting not only for the relevance of the PAM but to compare the effects of two different city lighting systems. In 2014 Lleida updated its light source inventory mostly by replacing mercury vapor and some metal halide lights by LEDs.

The paper is organized as follow. In Section 2 we introduce the methodology, describing briefly ILLUMINA and how its output is converted to astronomical magnitudes. In Section 3 we characterize the parameters of the study: location, atmosphere, lines of sight, inventory of lights, etc. In Section 4 the results are presented in the form of sky brightness all-sky maps. Finally the results are discussed in Section 5.

Section snippets

Methodology

The methodology used to compute sky brightness above the observer is divided in two main blocks.

The first one is based on the ILLUMINA project [3], [4], an open source code that produces several outputs. We will exclusively use the sky spectral radiance output. The second block is the post-processing, which consists in the conversion of ILLUMINA spectral radiance to proper astronomic magnitudes and displaying them in all-sky maps. Such conversion will allow to proceed to comparison with images

Case of study: Parc Astronòmic Montsec

Parc Astronòmic Montsec (PAM) is located in the North–East of Spain (Fig. 3). Scientific studies certified this location as ideal for astronomic purposes due to its low level of rainfall and humidity, the high ratio of clear nights and its elevation (1600 m above sea level).

The sky above the PAM is considered as free from light pollution and hence it is protected by the local legal framework. Only some bright sky areas close to the horizon are detected. However, there are some cities and towns

Results

The first set of figures in this section (Fig. 7) show all-sky ASTMON images in the Johnson–Cousins filters B, V and R taken in November 2012 and May 2015. They are used to check the ILLUMINA model. All the images represent the sky from the PAM, with the azimuth’s origin pointing to the North. The elevation starts at 5° in the outer radius and increases as the radius shortens, with the zenith (h=90) at the center.

Fig. 8, Fig. 9, Fig. 10, Fig. 11, Fig. 12, Fig. 13 show all-sky results from

Discussion

When comparing ILLUMINA results with ASTMON data, we need to take into account that, as seen in Fig. 4, Lleida is not the only source of light pollution. There are other directions that are affected such as 120°, which coincides with the direction to Barcelona, and 160° that points to Balaguer, a town smaller than Lleida but much closer. Furthermore, other elements can affect the measurement, such as the presence of the Milky Way. This makes difficult to perform an accurate comparison between

Conclusions

The methodology presented in this work shows to be suitable to study the contribution of artificial light sources to the sky brightness. In our case of study, Lleida has been detected in the expected azimuth and elevation. Lleida enlightens the sky of Montsec, but the light pollution is confined in a set of lines of sight around 190° in azimuth and up to 25° in elevation. In spite of simplifications as not taking into account natural sources of light, such as the Milky Way, and other cities

Acknowledgment

M.A. thanks the Fonds de recherche du Québec – Nature et technologies (FRQNT) (Grant no. 210478). for financial support through the Research program for college researchers. S.R. and H.L. thank European Cooperation Project POTEFA Pyrenees La Nuit EFA 233/16/PLN.

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