Assessment of sky brightness in central Portugal: the Dark Sky destination "Aldeias do Xisto" case study

Domingos Barbosa; Dalmiro Maia; Bruno Morgado; Pedro Cruz; Bernardo Relvas; Valério Ribeiro; Allan Kardec de Almeida Jr.; Timothée Vaillant

arXiv · 2610.09028

Assessment of sky brightness in central Portugal: the Dark Sky destination "Aldeias do Xisto" case study

Domingos Barbosa·Dalmiro Maia·Bruno Morgado·Pedro Cruz·Bernardo Relvas·Valério Ribeiro·Allan Kardec de Almeida Jr.·Timothée Vaillant

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Abstract

A combination of remote sensing techniques, open space data and field testing was developed to derive astronomical and meteorological parameters and assess Dark Sky brightness of the hitherto unstudied Schist Villages region in the mountainous Interior Center of Portugal. This study covered the period 2012-2017 and established the methodology for regular light pollution and sky brightness studies in the region using satellite imagery to reduce cost and time and increase accuracy of the final analysis. Long-term Artificial Light At Night (ALAN) data from VIIRS instrument aboard the Suomi-NPP satellite along with cloud cover inferred from MODUS/AQUA satellite data and orographic data, were analyzed in a Geographical Information Systems (GIS) environment. The derived ALAN values were consistent with a preliminary site survey selection, confirming the accuracy of the identified dark sky spots in the region. The analyzed region area spans 1624 Km$^2$ and the darkest site within it showed a sky brightness of 21.4 mag/arcsec$^2$. Ground-based SQM photometer observations of the Night Sky Brightness (NSB) at the zenith validated the satellite derived ALAN data. The devised field testing is itself a low cost solution that can be used by teams in developing countries to easily assess dark sky potential prior to the development of local observatories and astro-tourism activities. This study supported the initial certification of this region as a Dark Sky Starlight Tourist Destination since 2018, an important milestone for dark sky protection in central Portugal.

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Domingos Barbosa, Dalmiro Maia, Bruno Morgado, Pedro Cruz, Bernardo Relvas, Valério Ribeiro, Allan Kardec de Almeida Jr., Timothée Vaillant. 2026-10-06. Assessment of sky brightness in central Portugal: the Dark Sky destination "Aldeias do Xisto" case study. https://arxiv.org/abs/2610.09028

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