# Towards a Spectro-Photometric Characterization of the Chilean Night Sky
A first quantitative assessment of ALAN across the Coquimbo Region

Rodolfo Angeloni Gemini Observatory, NSF’s NOIRLab, Av. J. Cisternas 1500 N,
1720236, La Serena, Chile  Juan Pablo Uchima-Tamayo Departamento de
Astronomía, Universidad de La Serena, Av. R. Bitrán 1305, 1720256, La Serena,
Chile  Gemini Observatory, NSF’s NOIRLab, Av. J. Cisternas 1500 N, 1720236, La
Serena, Chile  Marcelo Jaque Arancibia  Departamento de Astronomía,
Universidad de La Serena, Av. R. Bitrán 1305, 1720256, La Serena, Chile
Instituto Multidisciplinario de Investigación y Postgrado, Universidad de La
Serena, Av. R. Bitrán 1305, 1720256, La Serena, Chile  Roque Ruiz-Carmona
Gemini Observatory, NSF’s NOIRLab, Av. J. Cisternas 1500 N, 1720236, La
Serena, Chile  Diego Fernández Olivares  Departamento de Astronomía,
Universidad de La Serena, Av. R. Bitrán 1305, 1720256, La Serena, Chile  Pedro
Sanhueza  Dirección de Energía, Ciencia y Tecnología e Innovación, Ministerio
Relaxiones Exteriores, Santiago, Chile  Guillermo Damke  Cerro Tololo
Interamerican Observatory, NSF’s NOIRLab, Av. J. Cisternas 1500 N, 1720236, La
Serena, Chile  Ricardo Moyano  Departamento de Astronomía, Universidad de La
Serena, Av. R. Bitrán 1305, 1720256, La Serena, Chile  Verónica Firpo  Gemini
Observatory, NSF’s NOIRLab, Av. J. Cisternas 1500 N, 1720236, La Serena, Chile
Javier Fuentes  European Southern Observatory, Av. Alonso de Córdova 3107,
763000, Santiago de Chile, Chile  Javier Sayago  OPCC, NSF’s NOIRLab, Av. J.
Cisternas 1500 N, 1720236, La Serena, Chile

###### Abstract

Light pollution is recognized as a global issue that, like other forms of
anthropogenic pollution, has significant impact on ecosystems and adverse
effects on living organisms. Multiple evidence suggests that it has been
increasing at an unprecedented rate at all spatial scales. Chile, which thanks
to its unique environmental conditions has become one of the most prominent
astronomical hubs of the world, seems to be no exception. In this paper we
present the results of the first observing campaign aimed at quantifying the
effects of artificial lights at night (ALAN) on the brightness and colors of
Chilean sky. Through the analysis of photometrically calibrated all-sky images
captured at four representative sites with an increasing degree of
anthropization, and the comparison with state-of-the-art numerical models, we
show that significant levels of light pollution have already altered the
appearance of the natural sky even in remote areas. Our observations reveal
that the light pollution level recorded in a small town of the Coquimbo Region
is comparable with that of Flagstaff, a ten times larger Dark Sky city, and
that a mid-size urban area door to the Atacama Desert displays photometric
indicators of night sky quality that are typical of the most densely populated
regions of Europe. Our results suggest that there is still much to be done in
Chile to keep the light pollution phenomenon under control and thus preserve
the darkness of its night sky - a natural and cultural heritage that is our
responsibility to protect.

Night sky brightness (1112), Light pollution (2318), Observational astronomy
(1145), All-sky cameras (25), Photometers (2030)

##  1 Introduction

Light pollution is today acknowledged as a global problem that, similar to
other forms of anthropogenic pollution, has significant impacts on ecosystems
and wildlife, causing adverse effects on living organisms (Svechkina et al.
2020; Manríquez et al. 2019; Irwin 2018; Bennie et al. 2015; Chepesiuk 2009).
Originally discussed by the professional astronomical community that was
concerned that an inefficient, unnecessary and uncontrolled use of Artificial
Light At Night (hereafter ALAN) could dazzle professional observatories by
impeding them the vision of the starry sky (Walker, 1970; Riegel, 1973),
nowadays ALAN has become a fertile field for interdisciplinary scientific
research (e.g., Cavazzani et al. 2022, 2020; Kolláth et al. 2020; Jechow 2019;
Admiranto et al. 2019; Maggi & Benedetti-Cecchi 2018), socio-economic studies
(e.g., Pérez Vega et al. 2022; Gallaway et al. 2010; Nordhaus 1994), and wide-
ranging cultural debates (e.g., Hamacher et al. 2020; Drake 2019).

Similar to other products of human civilization, the effects of ALAN are
closely tied to the development and subsequent misuse of more efficient (and
often cheaper) energy sources: in this particular case, of the evolving
lighting technology (DiLaura, 2008). While the main concern in the 1970s
revolved around the transition from mercury-vapor to High-Pressure Sodium
(HPS) lamps (Riegel, 1973), we are now facing another, potentially more
complex transition into the era of Light Emitting Diode (LED) lighting. In
fact, the Spectral Power Distribution (SPD) of ALAN is rapidly changing. It
has shifted from displaying only a few emission lines to dozens, if not
hundreds, of lines, accompanied by a significantly increased continuum level,
especially in the blue-green spectral range (see, e.g., Figure 1.14 in Dutta
Gupta & Agarwal 2017).

Intimately linked to this technological transition, there is also multiple
evidence that light pollution is increasing at an unprecedented rate at
virtually all spatial scales (Kyba et al. 2023, 2017; Sánchez de Miguel et al.
2021). It has been estimated that approximately 99% of the U.S. and European
populations experience light-polluted nights, and more than one-third of the
global population has permanently lost the opportunity to observe the Milky
Way (hereafter, MW – Falchi et al. 2016). The cultural and social implications
of this phenomenon (Gullberg et al. 2020) are such that some authors have not
hesitated to address it as a form of cultural genocide (Hamacher et al. 2020),
thus emphasizing the profound impact it has on our cultural identity and
heritage.

Even remote areas with a reputation for unspoiled nature and pristine skies
are not immune to the rapidly evolving effects of light pollution. Chile is
renowned worldwide for its diverse and unique ecosystems, and the outstanding
quality of its northern skies111Along with other historical and geopolitical
reasons whose discussion goes beyond the scope of this work. The interested
reader could refer to Silva Avaria 2022a, b, 2020a, 2020b, 2019. has
transformed it during the last half a century into one of the most important
astronomical hubs of the world (Silva Avaria 2020a). However, even though
there have been longstanding efforts by various public222The Chilean Ministry
of the Environment (MMA) hosts a dedicated section on light pollution in its
official website: https://luminica.mma.gob.cl/ and private333Office for the
Protection of the Night Sky Quality of Northern Chile, a.k.a. OPCC:
https://opcc.cl/ actors to keep the light pollution phenomenon under control,
a coordinated program to scientifically characterize, quantify and monitor the
extent of its environmental impact over the Chilean sky has only recently
begun. Objective and publicly accessible data play in fact a vital role in
supporting the scientific evidence that should inform all regulations aimed at
mitigating light pollution444E.g., https://opcc.cl/revision_ds043.html, while
data-driven indicators of night sky quality are the only reliable way to
measure the effectiveness of any technical legislation and to propose focused
courses of action (Kim et al., 2019).

Our interdisciplinary Research Group on ALAN was born five years ago with the
explicit purpose of filling this need, and aims at providing the broad ALAN
community with the first spectro-photometric characterization of one of the
most pristine skies of planet Earth. Since 2019 we have been regularly
monitoring about twenty distinctive sites (including professional astronomical
observatories, astrotourism centers, natural parks, and metropolitan areas)
across the Coquimbo Region – also known as “Región Estrella” (Region of the
Stars, in Spanish). This semi-desert region lies between the Andes Mountains
and the Pacific Ocean, a few hundred kilometers north of Santiago. Its high-
altitude mountain peaks offer dry and clear skies, making it an ideal host for
numerous international astronomical projects, including the southern
hemisphere AURA-NOIRLab Programs555https://www.noirlab.edu/public/programs/,
the Carnegie Institute Las Campanas Observatory666https://www.lco.cl/, the ESO
La Silla Observatory777https://www.eso.org/public/teles-instr/lasilla/?lang,
and the future Giant Magellan Telescope888https://giantmagellan.org/.
Furthermore,
astrotourism999https://astroturismochile.travel/en/home/,101010https://www.astronomictourism.com/
has emerged as a fundamental asset of the economic, social, and cultural
development of the region. With an abundance of amateur observatories and a
wide range of leisure activities related to astronomy, the Coquimbo Region has
become a destination that attracts astrotourism enthusiasts from around the
world.

In this introductory paper we present the results of a first survey program
aimed at assessing the impact of ALAN on the natural darkness and colors of
four representative sites characterized by an increasing degree of
anthropization, and that therefore effectively sample the wide range of
average night sky conditions encountered in northern Chile. Section 2 outlines
the criteria that inspired the selection of these specific locations,
introduces the instrumental suite, and describes our image acquisition
strategy. We also provide a comprehensive description of the data reduction
process and analysis. In Section 3, we delve into a detailed analysis of the
main results obtained at each location. Moving forward to Section 4, we
contextualize these findings by comparing the quality of the night sky in the
Coquimbo Region with similar sites worldwide, as well as with state-of-the-art
numerical models. Finally, in Section 5 we comment on the overall
significance, and outline the future steps, of our multidisciplinary endeavor
to characterize and preserve the Chilean night sky.

##  2 Measurements and methods

###  2.1 Measurement sites

During the site selection process we chose four representative locations that
could effectively span the wide range of average night sky conditions expected
across northern Chile. In increasing levels of anthropization, they are: i)
Fray Jorge National Park (FJNP), a UNESCO Biosphere Reserve and the first
certified Starlight Reserve in South America; ii) Las Campanas Observatory
(LCO), a professional astronomical observatory situated on the administrative
border between the Atacama and Coquimbo Regions; iii) Collowara Astrotourism
Observatory (CAO), an educational and cultural center located close to the
mining city of Andacollo; iv) ”Greater La Serena” (LS-CQ) that, with a
population of 448,784 inhabitants at the 2017 national census, is the fourth
largest metropolitan area in Chile. Table 1 provides the most relevant
geographical information of these four measurement sites, while Figure 1 shows
their geographical distribution across the Coquimbo Region, overlaid on a
pseudo-color map adapted from the New World Atlas of Artificial Night Sky
Brightness (Falchi et al. 2016).

Table 1: Geographic coordinates obtained from the SQC GPS module of the four measurement sites discussed in this study, listed in order of increasing degree of anthropization. The last four columns indicate the mean distances of each site to the closest urban areas. |  |  |  |  | Distance to
---|---|---|---|---|---
Measurement site | Acronym | Longitude | Latitude | Elevation | LS-CQ | Ovalle | Vicuña | Vallenar
|  |  [$\hbox{${}^{\circ}$}\;\arcmin\;\arcsec$] W |  [$\hbox{${}^{\circ}$}\;\arcmin\;\arcsec$] S | [masl] | [km] | [km] | [km] | [km]
Fray Jorge National Park | FJNP | 71 39 42.71 | 30 37 35.63 | 278 | 85 | 44 | 112 | 244
Las Campanas Observatory | LCO | 70 42 01.57 | 29 00 41.98 | 2283 | 117 | 183 | 114 | 49
Collowara Astroturism Observatory | CAO | 71 03 54.10 | 30 14 56.26 | 1311 | 41 | 41 | 42 | 186
La Serena - Coquimbo metropolitan area | LS-CQ | 71 16 05.85 | 29 56 04.69 | 15 | – | 77 | 53 | 155
Figure 1: Geographical distribution of the four measurement sites analyzed in
this study, represented by red dots with white labels. The dome-shaped icons
indicate professional astronomical observatories. Las Campanas Observatory is
the one included in the current study. CTIO stands for Cerro Tololo
Interamerican Observatory, while Cerro Pachón is home to the Gemini South
telescope, SOAR telescope, and V. Rubin Observatory: they will be discussed in
a future paper. The background color map is adapted from the New World Atlas
of Artificial Night Sky Brightness (Falchi et al. 2016).

###  2.2 Instrumental suite

Today there is a wide range of tools and technologies available for conducting
research on ALAN. Each of these has its own advantages and limitations, as
discussed in a few comprehensive reviews as those by Mander et al. (2023) and
Hänel et al. (2018) that cover instruments, methods, and interdisciplinary
applications. In this subsection, we provide a brief overview of the main
features of the Sky Quality Camera (SQC) equipment that we have been using in
our monitoring campaign since 2019. For more technical details, we refer the
interested reader to the aforementioned reviews and references therein.

Following the classification proposed by Mander et al. (2023), SQC can be
categorized as a ground-based all-sky imaging sensor. Specifically, the SQCs
in our hands are DSLR Canon EOS 6D Mark II cameras, equipped with a 35.8×23.9
mm CMOS sensor of 20.2 (effective) megapixels and coupled with a Sigma EX DG 8
mm f/3.5 fish-eye lens. They provide all-sky multi-spectral information with a
spectral coverage and spatial resolution that are not commonly found in ALAN
studies. The camera comes with its proprietary software111111Euromix,
Ljubljana, Slovenia. that handles vignetting and distortion corrections of the
fish-eye lens and performs astrometric and photometric calibrations. The
latter is conducted against the astronomical Johnson-Cousins V band. SQCs
directly measure night sky brightness (NSB) values in units of V mag/arcsec2
or, equivalently, luminance in mcd/m2. The typical photometric errors of SQC
data is $\pm$0.01 mag/arcsec2 (Angeloni et al., in preparation). In addition
to NSB measurements, the transformation equations from the camera’s RGB
channels to the CIE XYZ color space allow us to obtain color correlated
temperature (CCT) information (Jechow et al., 2019) with a typical error of
$\pm$30 K (Angeloni et al., in preparation). While there are some known
limitations associated with CCT measurements (Esposito & Houser 2022; Durmus
2022), they still prove valuable when comparing the SPD of the different light
sources that contribute to the observed radiance of the night sky (Jechow &
Hölker, 2019).

###  2.3 Data acquisition, reduction and analysis

During the entire surveying campaign, a standardized data acquisition strategy
has been implemented to ensure consistency among all observers. In this
paragraph, we present a concise overview of this strategy. The surveying
expeditions to the measurement sites are deliberately scheduled during
cloudless nights in new Moon periods. In any case, no data is taken when the
Moon is at an elevation $\geq$–10∘. The equipment setup takes place before
sunset when outdoor light levels are high, allowing for an easier and faster
positioning of the SQC121212Mounted on a Manfrotto 055XPro3-3W tripod. along
the north-south, east-west, and zenith axes. This preliminary step is aimed at
aligning the sensor focal plane parallel to the horizontal plane, and the
camera’s optical axis towards zenith. This daytime phase also includes
capturing short-exposure frames of the surrounding landscape, that will then
aid in accurately tracing the local horizon during the data reduction process.
At the nautical twilight, with the sky becoming darker and darker, a series of
test images are taken to optimize ISO and exposure time and thus ensure high
signal-to-noise ratio while avoiding saturation. With the SQC autodark option
enabled, image acquisition can finally start when entering the astronomical
twilight.

The reduction process, which is performed with the SQC software v.1.9.8,
begins with the astrometric calibration of the captured frames. Taking
advantage of both the GPS information stored in the RAW data and the
software’s internal reference star catalog, we align the images following the
standard astronomical convention (N up, E to the left and zenith at the center
of the azimuthal projection). If the on-site camera setting described in the
previous paragraph has been carefully performed, the software corrections are
typically minimal ($\leq$ 1∘) on each of the three main axes. The subsequent
crucial step involves tracing the landscape border to accurately define the
terrain profile and exclude any foreground obstacles (such as distant mountain
tops, trees, or telescope domes) from the active sky area. Before generating
the NSB/CCT maps, the contribution of point-source objects is removed and a
smoothing function is applied across the entire sky field131313 However,
please note that when evaluating SQC vs. GAMBONS NSB maps (see Sec. 4.3) we do
not remove the point source contribution, nor apply any smoothing. This
approach allows for a direct comparison between the observed measurements and
the numerical models.: this step is meant to round off those small, punctual
variations in the measured values that may arise when including very bright
objects (such as planets and first magnitudes stars) in the specific area of
study.

The initial step in the data analysis involves the visual inspection of both
the original RGB and the calibrated NSB/CCT maps to identify any exceeding
light contributions across the landscape. NSB/CCT azimuth profiles are then
calculated for a series of predetermined elevations (usually, the lowest
altitude at which the terrain profile guarantees a 360∘ unobstructed view of
the sky, 15∘, and 30∘) to ascertain directions and magnitudes of the various
brightness peaks recorded in the images. Using the lightpollutionmap web
interface141414https://www.lightpollutionmap.info, we are able to link these
local extremes to specific ALAN sources and then determine the corresponding
azimuth ranges that encompass their angular extent. Additionally, we exploit
the VIIRS/DNB (Liao et al., 2013) radiance maps plotted over Microsoft Bing
base layers to check whether potential ALAN sources visible from space have
been recorded in our topocentric SQC data. In certain cases, when observed
from a particular vantage point, a specific azimuth range can encompass
multiple ALAN bright sources that appear aligned due to the effect of
perspective. For example, from FJNP the cities of Andacollo and Vicuña appear
to lie along the same line of sight despite the latter is almost two times
farther than the former (Figure 2, upper left panel). Similarly, the skyglow
of Andacollo blends with the light halo of LS-CQ metropolitan area from CAO.
In other cases, no ALAN sources are recorded in the VIIRS/DNB radiance maps
over large distances. We term these “dark pathways” ALAN-free directions:
their NSB/CCT profiles serve as a baseline to be compared with other lines of
sight that are visibly polluted, thus providing a starting point of reference
for analysis. For instance, the western horizon of FJNP, facing the Pacific
Ocean, serves as a clear example of an ALAN-free direction. Another example is
shown in the upper right panel of Figure 2: from LCO there is an east-looking
line of sight along which no artificial lights are encountered until reaching
a few remote villages in the Argentinian La Rioja Province, located over 300
km away behind the 6,000 m Andes wall (Figure 2, bottom panel). Their
potential contribution to the brightness of LCO sky is negligible, if
detectable at all.

Having finally obtained fully calibrated NSB/CCT maps and compiled a list of
confirmed and candidate ALAN sources, we can now proceed to measure and
compare different regions of the sky to assess the impact of ALAN on each
individual site.

Figure 2: Left panel: Andacollo and Vicuña are located within the same azimuth range when observed from FJNP. Right panel: from LCO there is a specific line of sight towards the east that is devoid of ALAN sources over a distance of several hundred kilometers. These azimuth ranges define the “ALAN-free” direction. The 2022 VIIRS/DNB radiance maps adapted from http://lightpollutionmap.info are plotted in the background. Bottom panel: elevation profile along the LCO ALAN-free direction (adapted from Google Earth). See Sect. 2 and Table 3 for further methodological details. Table 2: Journal of Observations, showing the local date and time of the expeditions to the sites. Additionally, we report the serial numbers of the SQCs used at each site, the identification numbers of the raw image presented in the study, and the corresponding ISO setting, exposure time, and signal-to-noise ratio. Site | Date | Time | SQC | Frame | ISO | Exp. Time | S/N
---|---|---|---|---|---|---|---
acronym | yyyy-mm-dd | local | serial number | # |  | [s] |
FJNP | 2021-01-09 | 23:25:50 | 183052000778 | 9872 | 1600 | 120 | 18
LCO | 2019-11-24 | 02:39:37 | 183052000778 | 9506 | 3200 | 120 | 22
CAO | 2021-06-10 | 22:41:55 | 423053002003 | 0001 | 1600 | 90 | 30
LS-CQ 1st | 2022-01-02 | 23:25:25 | 423053002003 | 0037 | 1600 | 8 | 39
LS-CQ 2nd | 2022-06-28 | 19:34:42 | 423053002002 | 0065 | 1600 | 8 | 42
Table 3: ALAN individual sources identified at the four observing sites are grouped in areas of interest, and defined by specific azimuth ranges. The first three azimuth ranges are color-coded and can be recognized in the RGB maps of Figures 3, 6, 9, 12. The corresponding NSB/CCT elevation profiles are plotted in Figures 5, 8, 11, 14. Please refer to the main text for further details. Site | Area | Azimuth Range | ALAN | Distance
---|---|---|---|---
acronym | # | [∘] | Main Sources | [km]
| 1 | 16-36 | LS-CQ | 85,8
| 2 | 78-98 | Ovalle | 44,0
| 3 | 225-315 | ALAN-free direction | –
FJNP | 4 | 37-77 | Ruta D-44 | –
| 5 | 143-145 | Alcones Bajo | 19,0
| 6 | 50-57 | Andacollo (Vicuña) | 70,0
| 7 | 150-183 | Gran Valparaíso - RM | 300,0
| 1 | 203-213 | LS-CQ | 117,0
| 2 | 343-2 | Vallenar | 49,0
| 3 | 95-97 | ALAN-free direction | –
| 4 | 260-266 | Ruta 5N @ Cachiyuyo | 19,8
| 5 | 310-331 | Ruta 5N @ Vizcachitas | 20,0
LCO | 6 | 276-292 | Domeyko | 20,3
| 7 | 238-244 | Ruta 5N @ Observatory exit | 26,0
| 8 | 34-39 | Alto del Carmen | 35,0
| 9 | 114-122 | El Veladero open-pit mine | 84,0
| 10 | 175-182 | Vicuña | 114,0
| 11 | 193-197 | Ovalle (Andacollo) | 183,0
| 12 | 7-17 | Copiapó | 186,0
| 1 | 275-5 | Andacollo (275-5) | 2,5
|  |  | LS-CQ (308-345) | 41,0
| 2 | 208-274 | Teck open-pit mine | 2,5
CAO | 3 | 85-93 | ALAN-free direction | -
| 4 | 185-207 | Ovalle | 41,0
| 5 | 50-61 | Vicuña | 42,0
| 6 | 168-181 | RM | 350,0
| 1 | 270-360 | Av. del Mar | 1,3
|  |  | Amusement park | 0,5
| 2 | 0-90 | La Serena | -
|  |  | Cerro Grande | 4,4
LS-CQ | 3 | 90-180 | San Ramón y Huachalalume | 3,8
| 4 | 180-270 | Peñuelas - Industrial Distric | 3,0
|  |  | Coquimbo | 8,0

##  3 Results

In each of the following subsections we provide additional context about each
measurement site by highlighting their distinctive characteristics. We then
present the results of our survey conducted following the methodology
described in Sec. 2.3. The journal of observations is summarized in Table 2,
while Table 3 lists the azimuth ranges of interest and the most notable ALAN
sources recognizable in the data. It is important to notice that the same
color code adopted in Table 3 for the first three azimuth ranges is maintained
in both the topocentric RGB maps (left panels of Figures 3, 6, 9, 12) and in
the corresponding NSB/CCT elevation profiles (Figures 5, 8, 11, 14). These
colors identify the areas that include the two brightest ALAN sources (blue
and red) and the ALAN-free direction (green) at each site151515With the only
exception of LS-CQ, where the very same concept of ALAN-free direction makes
no sense anymore – see Sect. 3.4., thus ensuring consistency and readability
of the graphical representations.

###  3.1 Fray Jorge National Park (FJNP)

FJNP is located in the Limarí Province of the Coquimbo Region, about 100 km
south of LS-CQ. It extends over a total area of 8.863 hectares, of which
$\sim$4% is covered by forests. It is renowned for containing the northernmost
Valdivian temperate rain forests: despite being surrounded by semiarid
scrublands, in fact, the park’s hydrophilic forests have survived for
apparently millions of years thanks to the coastal fog161616Camanchaca in the
Quechua and Aymara language. which hangs on the mountain-slopes and moistens
the subtropical vegetation (Cádiz et al., 2019).

The national park was created in 1941 and is today administered by the Chilean
forest authority CONAF171717https://www.conaf.cl/parques/parque-nacional-
bosque-fray-jorge/.. In 1977, FJNP joined the UNESCO World Network of
Biosphere Reserves. Since 2013, it is also a certified Starlight Reserve (the
first in South America and the fourth in the world), namely “a protected
natural area where a commitment to protecting the quality of the night sky and
access to starlight is established. Its function is to preserve the quality of
the night sky and the different associated values, whether cultural,
scientific, astronomical, or the natural landscape”181818According to the
definition of Starlight Reserve provided on the official website of the
Starlight Foundation:
https://en.fundacionstarlight.org/contenido/43-definition-starlight-
reserves.html..

Figure 3: SQC images of the night sky at FJNP captured on 2021 Jan 09. Left
panel: RAW RGB frame, also showing as spherical triangles the color-coded
azimuth ranges presented in Table 3 and discussed in Sect. 3.1. Central panel:
topocentric NSB map in units of V mag/arcsec2 (upper color bar), highlighting
the most prominent ALAN individual sources and their respective distances.
Right panel: topocentric CCT map in units of Kelvin (bottom color bar). N is
up and E is to the left.

Figure 4: FJNP NSB/CCT profiles averaged within 1∘-wide rings centered at 8∘,
15∘, and 30∘ elevation, as a function of azimuth. Clearly recognizable are the
NSB (CCT) local maxima (minima) that indicate the azimuth directions of the
strongest ALAN sources. Natural sources (in this case the MW) are indicated
with italic gray labels. The typical photometric error of SQC data is
comparable to the line thickness. See also Figure 3 and Table 3.

Figure 5: NSB (left panel) and CCT (right panel) profiles as a function of the
zenith angle ZA for the three color-coded azimuth ranges identified at FJRP.
The typical photometric error of SQC data is also here comparable to the line
thickness. For further details refer to Sect. 3.1, Figure 3, and Table 3.

Our expedition at FJNP took place during the 2021 southern summer. We chose an
observing spot located at an elevation of 278 meters above sea level,
approximately 4.5 kilometers away from the coastal line. The natural slope of
the terrain causes a small obstruction ($\sim$10%) of the celestial
hemisphere, particularly in the western direction where the lowest $\sim
10\hbox{${}^{\circ}$}$ of the sky are hidden by hills.

The SQC RGB RAW frame of FJNP (Figure 3, left panel) captures a stunning view
of the MW arching high across the sky, accompanied by the presence of the
Large Magellanic Cloud (LMC) that has just crossed the southern meridian. The
eastern hemisphere, which overlooks the continent, appears already slightly
brighter than the western one facing the Pacific Ocean. By examining the
corresponding NSB/CCT maps (Figure 3, middle and right panel), we can clearly
identify on the horizon three distinct bright spots of similar intensity. The
southern one overlaps with the MW region containing the constellations of
Carina, Crux, and Centaurus that have just risen. Figure 4 allows us to
pinpoint these three regions to specific azimuth directions, and in turn to
specifically ALAN sources. In order of increasing azimuth and NSB, we
recognize: i) the metropolitan area of LS-CQ, with a NSB peak at an azimuth
angle AZ$\approx$26∘; ii) the city of Ovalle191919Capital of the Limarí
Province. According to the 2017 national census, Ovalle has a population of
121,269 inhabitants. It is the third largest city in the Coquimbo Region,
following Coquimbo and La Serena., with a NSB peak at AZ$\approx$86∘; iii) the
distant metropolitan areas of Gran Valparaíso202020The third metropolitan area
of Chile, with 951,150 inhabitants censored in 2017. and Gran Santiago,
located at least 300 kilometers away, with a NSB peak AZ$\approx$166∘. The
coincidence of azimuth values between the local NSB maxima and CCT minima
provides direct evidence of the artificial nature of these light sources. It
also exemplifies the valuable role of CCT information in helping differentiate
between natural and artificial contributions to the observed radiance.

Figure 5 illustrates the variation of NSB/CCT as a function of ZA for the two
brightest ALAN sources and the ALAN-free direction (westward quadrant facing
the ocean) at FJNP. At ZA$\approx$26∘ (ZA$\approx$33∘), towards the direction
of Ovalle (LS-CQ), the NSB profiles exhibit local peaks, which can be
attributed to the natural modulation of the MK transiting at zenith. As the ZA
increases towards the horizon, the NSB virtually parallel profiles of LS-CQ
and Ovalle diverge from the ALAN-free profile at ZA$\approx$75∘, suggesting
that the western horizon is dominated by ALAN sources up to an elevation of
approximately 15∘. Interestingly, the ALAN-free profile never intersects the
other two profiles. The presence of the galactic plane in that direction
complicates the interpretation (as the galactic starlight represents one of
the most significant contributors to the natural NSB - Duriscoe 2013; Masana
et al. 2021) but this evidence could be the hint that even at higher elevation
(i.e., for ZA$\geq 60\hbox{${}^{\circ}$}$) ALAN may contribute up to
$\sim$0.15 mag/arcsec2 to the NSB of the western horizon at FJNP. When
observing under very dark conditions like the ones encountered at this site,
the only technique able to confirm this kind of insights seems to be
spectroscopy (Kolláth & Jechow, 2023).

Based on various measurements and quantitative indicators that include the NSB
ratio between the brightest and darkest points in the sky ($\sim$6), the
absolute value of the darkest point in the sky (V=22.25 mag/arcsec2) and at
zenith (V=22.05 mag/arcsec2), and the dominance of CCT$\geq$4000 K over the
celestial hemisphere, it can be confidently stated that the sky at FJNP is
still very close to pristine conditions. Its minimal level of light pollution
could make it a world-reference location for monitoring the interplay between
the various natural components of the night sky radiance without significant
interference from artificial sources.

###  3.2 Las Campanas Observatory (LCO)

LCO is a branch of the Astronomy & Astrophysics division of the Carnegie
Institution for Science. It was established in 1969 on a mountain top at an
elevation of 2,300 meters in the southern extreme of the Atacama Desert, in
visual contact with the ESO La Silla Observatory which is only 26 km away, and
approximately 120 km from LS-CQ. Beyond the original 1-m Swope and 2.5-m Du
Pont reflectors, LCO now hosts the twin 6.5-m Magellan telescopes and will
also be home to the 25-m Giant Magellan Telescope (GMT) currently under
construction, one of the first extremely giant telescopes (ELTs) expected to
see light in the XXI century.

Figure 6: Same as Figure 3, but for LCO on 2019 Nov 24. For further details
refer to Sect. 3.2 and Table 3.

The observing run at LCO took place during the 2019 southern springtime,
shortly before the pandemic lockdown that suspended its scientific operations
for several months. The reference measurement spot was chosen in the vicinity
of the Swope telescope, whose dome is the only relevant obstruction visible
towards SE in the RGB frame of Figure 6, left panel212121The careful observer
could locate the Magellan telescope domes on the background hill at
AZ$\approx$110∘. The extremely careful observer could even spot the Du Pont
telescope dome at AZ$\approx$324∘.. The observed sky configuration at LCO was
similar to that described for FJNP, with the Galactic plane positioned high
over the eastern hemisphere and LMC at the meridian transit.

Figure 7: LCO NSB/CCT profiles averaged within 1∘-wide rings centered at 5∘,
15∘, and 30∘ elevation, as a function of azimuth.

Figure 8: Same as Figure 5, but for LCO. For further details refer to Sect.
3.2, Figure 6, and Table 3.

Upon initial examination of the SQC maps in Figure 6 (middle and right panels)
and the corresponding azimuth profiles in Figure 7, we can identify two
prominent NSB maxima (CCT minima) at approximately AZ$\approx$207∘ and
AZ$\approx$352∘, i.e., the skyglow of LS-CQ and Vallenar222222Capital of the
Huasco Province in the Atacama Region, with a population of 51,917 inhabitants
at the 2017 census., respectively. The NSB peak resulting from the rising
portion of our Galaxy is here partially obscured by the Swope dome. Figure 8
illustrates the variations of NSB (left panel) and CCT (right panel) with
respect to ZA. The skyglow from LS-CQ dominates the sky up to an elevation of
$\sim$15∘ , while the skyglow from Vallenar gradually merges with the MW,
resulting in a nearly linear trend in the NSB profile (red line). It becomes
more challenging to determine at which elevation the artificial light from the
city overwhelms the natural contribution, but a distinctive slope change in
both NSB and CCT profiles at ZA$\approx$84∘ suggests that this transition may
occur at $\sim$6∘ above the horizon. Along the ALAN-free direction (already
discussed in Sec. 2.3 for this specific site), the most notable feature is
again the modulation visible between
$30\hbox{${}^{\circ}$}<ZA<40\hbox{${}^{\circ}$}$ due to the galactic plane.
Further examination of Figure 6 reveals additional fainter ALAN sources along
the horizon, more easily noticeable in the CCT map. These include the skyglow
caps originating from the distant urban areas of Andacollo and Ovalle (aligned
along this line of sight, with the latter located approximately 180 km south
of LCO) and Copiapó232323Capital of the Atacama Region, with 174,039
inhabitants. (around 190 km north of LCO).

The CCT map, combined with the georeferenced VIIRS data, also reveals a series
of apparently more diluted sources with CCT$\sim$3000 K at the western
horizon. They can be ascribed to the (highly debated – Blanc 2019) renewed
lighting system of Ruta 5N, the panamerican highway which connects La Serena
with Vallenar and passes about 20 km away from LCO. Four specific directions
of interest can be identified: the crossroad between the same highway and the
observatory route (AZ$\approx$241∘), and the localities of Cachiyuyo
(AZ$\approx$263∘), Domeyko (AZ$\approx$288∘), and Vizcachita
(AZ$\approx$320∘). Their contribution to the observed NSB appears minimal even
at very low elevation (Figure 7): when comparing these directions with their
corresponding antipodals, there is no significant increase in NSB levels above
$\sim$3∘242424For example, considering a 5∘ reference elevation: at AZ=263∘
(i.e., Cachiyuyo) V=21.32 mag/arcsec2, while at the antipodal AZ=83∘ V=21.38
mag/arcsec2, for a $\Delta$V=0.06 mag/arcsec2.. However, our SQC data do not
allow us to firmly weight the relative contribution of artificial and natural
sources: a diffuse light, already perceivable in the RGB frame and clearly
visible in the NSB map, runs in fact across the entire horizon and suggests
that during that observing night there was a fairly high airglow activity.
Airglow is known to contribute significantly to the intrinsic variability of
the natural sky (Kolláth & Jechow 2023; Duriscoe 2016), being its composite
and complex multiperiodicity sensitive to both atmospheric and space weather
factors (Grauer & Grauer, 2021). As it was the case for FJNP, under near-
pristine conditions like these ones at LCO it becomes challenging to separate
the artificial (observed - natural) from the observed (natural + artificial)
sky radiance towards specific directions without any spatially resolved
spectral information.

###  3.3 Collowara Astroturism Observatory (CAO)

CAO is an astrotourism and educational complex situated at an altitude of
1,300 masl on Cerro El Churqui, approximately 2 km away from both the city of
Andacollo252525Population of 11,044 inhabitants at the 2017 census. and the
open-pit mine TECK “Carmen de Andacollo”. It is positioned at the center of a
hypothetical triangle, equidistant (around 40 km) from LS-CQ, Ovalle, and
Vicuña. Managed by the local lodging network, CAO was officially inaugurated
on June 25th, 2004, and has been welcoming tourists of all ages and
nationalities year-round ever since (with the notable exception of the
pandemic period, during which our study visit was nevertheless authorized).
During our visit to CAO in the winter of 2021, we installed the SQC on a
plateau slightly higher than the main observatory building. This vantage point
allowed us to gain an unobstructed 360∘ view of the local horizon and a direct
line of sight over the entire city and mine areas.

In the winter and at these southern latitudes, the bulge and inner disk of our
Galaxy reach their highest point above the horizon. They constitute by far the
strongest natural contribution of the zenith sky radiance during Moon-less
nights (see, e.g., Figure 4 in Duriscoe 2016). But the RGB image of CAO shown
in Figure 9 immediately tells us that the impact of light pollution at this
site can no longer be considered negligible. The celestial dome appears
divided into two almost equal sections. Because of a higher sky background, an
apparently discolored MW is perceptible towards the east, still showcasing its
dark dust lanes that have long fascinated the local ancestors in pre-ALAN eras
(Gullberg et al. 2020). But, on the other side of the celestial vault, three
equally spaced ALAN caps dazzle the entire family of setting constellations.
The NSB/CCT maps in Figure 9, along with their corresponding azimuth profiles
in Figure 10, confirm that the largest and brightest skyglow, with an NSB peak
at AZ$\approx$317∘, is the merged ALAN of LS-CQ and Andacollo. The other two
skyglows can be linked to the nearby open-pit mine and to the city of Ovalle,
this latter located sixteen times farther away (with NSB peaks at
AZ$\approx$244∘ and AZ$\approx$198∘, respectively). The skyglow from Ovalle
also exhibits pretty symmetric wings, with the southern one being the
contribution of the even more distant Gran Valparaíso (approximately 310 km
away) within the azimuth range 183∘-190∘, and the southwestern one arising
from the D-43 street connecting Ovalle and LS-CQ. Additionally, our data
detects the skyglow from the Gran Santiago metropolitan area (approximately
350 km away!) towards AZ$\approx$174∘.

The CCT map provides a clearer identification of Vicuña (AZ$\approx$53∘) and
seems to have also recorded the ALAN effects of the Argentinian cities of San
Juan ($\sim$280 km away) and Mendoza ($\sim$360 km away), as indicated by a
CCT minimum of $\sim$3000 K spread over the azimuth range 115∘-152∘. While the
proposed interpretation may sound questionable for the presence of the
physical barrier embodied by the Andes, it is worth noticing that satellite
meteorological data262626Available from, e.g., https://zoom.earth/ reported
the presence of high clouds passing over west Argentina during the night of
our observation. These scattered cloud coverage may have amplified the effects
of urban light pollution (Kyba et al. 2011; Jechow et al. 2017), making them
detectable from the Chilean side of the Cordillera up to the moderate altitude
of CAO. However, this hypothesis remains highly speculative, and additional
measurements under all possible weather conditions, preferably in the form of
continuous all-sky monitoring campaigns, are highly encouraged.

The NSB/CCT profiles graphed in Figure 11 confirm that light pollution at CAO
makes the western horizon approximately seven times brighter (at a reference
elevation of 5∘) compared to the chosen ALAN-free direction. Light pollution
at CAO has not only canceled out the majority of $\geq$5 mag stars, but it has
already lightened zenith (the entire sky) by 54% (130%) over the natural
value, invariably altering the night sky quality indicators shown in Table 4.

Figure 9: Same as Figure 3, but for CAO on 2021 Jun 10. For further details
refer to Sect. 3.3 and Table 3.

Figure 10: CAO NSB/CCT profiles averaged within 1 ∘-wide rings centered at 5∘
elevation, 15∘, and 30∘, as a function of azimuth (see also Table 3).

Figure 11: Same as Figure 5, but for CAO. For further details refer to Sect.
3.3, Figure 9, and Table 3.

###  3.4 La Serena - Coquimbo metropolitan area (LS-CQ)

La Serena and Coquimbo are neighboring cities that together shape the fourth
largest metropolitan area in Chile. Located approximately 470 km north of
Santiago, it serves as a gateway to the Atacama Desert. Situated along the
Pacific Ocean, these two cities are renowned for their historical charm and
cultural significance. La Serena, the regional capital, is also the second
oldest city in Chile after the capital Santiago. For the past half a century,
it has been home to the Chilean headquarters of AURA-NOIRLab and LCO. In
recent years, the entire metropolitan area has witnessed rapid and somewhat
uncontrolled urban and demographic expansion (Orellana Mc Bride, 2020).

Our reference spot to study the night sky quality in the urban fabric of LS-CQ
became the football field of the local “A. De Gasperi” Italian School. Despite
its relative proximity to both the Ruta 5N panamerican highway and the
bustling tourist promenade of “Avenida del Mar”, this didactic compound still
provided a relatively large surface with minimally intrusive lighting
installations. In 2021 we visited the Italian School twice, first in January
during the summer tourist season and then in June at the beginning of the
local winter. Because of the recent opening of an overly illuminated sports
ground in close proximity, today this monitoring site is no longer suitable
for conducting NSB measurements - already an indirect hint of the rapidly
increasing level of light pollution in the city.

Figure 12: As in Figure 3, but for LS-CQ on 2022 Jan 2 (upper row) and Jun 28
(bottom row). In the second epoch RGB image (bottom left), the 5∘ zenith
circle and the 30∘ elevation ring which are mentioned in the text and
described in Table 4 are explicitly marked. It is important to note the
different color scale used in the NSB maps, that needs to accommodate the
shifted range of brightness levels in this heavily light-polluted urban sky.
Additionally, there is a significant presence of areas with CCT well over 4000
K. Unlike the case discussed for FJNP in Section 3.1, here we are witnessing
the invasion of white LED sources, particularly from private buildings and
commercial establishments. For further details refer to Sects. 3.4 and 4.

The SQC RGB images and corresponding NSB/CCT maps from our urban survey are
shown in Figure 12. The RGB images (left panels) reveal that the MK is
completely lost in the artificial glare, and even second magnitude stars are
barely discernible. The sky is so bright that a different color scale has to
be adopted in order to maintain readability in the SQC maps (middle and right
panels). Analysis of the azimuth profiles (Figure 13) confirms a uniformly
bright sky due to artificial lighting, with one direction even exhibiting an
unnatural whitish hue.

Figure 13: LS-CQ NSB/CCT profiles averaged within 1∘-wide rings centered at
7∘, 15∘, and 30∘ elevation, as a function of azimuth (see also Table 3).

Under this heavily contaminated sky, the very same concept of ALAN-free
direction loses its significance. For our analysis we therefore divided the
sky into four cardinal quadrants (Figure 14). Except for a few degrees above
the horizon, where the different lighting installations can still be
individually discerned, the NSB profiles of the four quadrants overlap each
other almost perfectly. This is a direct consequence of being virtually
immersed in a homogeneously lit ALAN bubble. During our first visit in
January, the primarily residential area towards the eastern horizon was
dominated by low-CCT HPS sources still commonly used for public street
lighting; while the western horizon already exhibited high CCT values
predominantly from white-blue LEDs installed at private facilities such as
amusement parks and commercial malls.

However, it is the comparison between the first and second visit (which are
only one semester apart) to be truly dramatic. In June, we encountered
slightly higher brightness level but, more significantly, the sky color had
changed so much as to become discomforting to the naked eye in certain
directions. The higher CCT values that in January were confined to a large but
still delimited azimuth range, in June had expanded from multiple directions,
covering almost the entire sky. Under these conditions, where even the darkest
point in the sky is brighter than the brightest points recorded at all
previous sites (cfr. Table 4), and the brightest point approaches the boundary
between mesopic and photopic vision (Green et al. 2022; Stockman & Sharpe
2006), only the Moon, the brightest planets and less than 1% of stars (Kyba et
al. 2023; Cinzano & Falchi 2020) remain visible from the capital of the Región
Estrella.

Figure 14: Same as Figure 5, but for LS-CQ on 2022 Jan 2 (solid lines) and
2022 Jun 28 (dotted lines). For further details refer to Sect. 3.4, Figure 12,
and Table 3.

##  4 Discussion

In the preceding section, we have outlined the most significant findings
achieved by this first survey at each monitoring location. In this concluding
section, we discuss them in a more general context by comparing the quality of
the night sky in the Coquimbo Region with similar sites worldwide, as well as
with state-of-the-art numerical models.

###  4.1 Photometric indicators for cross-site comparisons

For comparative studies across different locations, a wealth of photometric
indicators of visual night sky quality have been suggested in literature
(Deverchère et al. 2022; Falchi & Bará 2021; Duriscoe 2016). Each indicator
has its advantages and disadvantages. For instance, the brightest section of
the sky is considered significant for biological studies, including human
health, while the darkest one serves as a reliable proxy for sky quality since
it represents the celestial field least affected by light pollution, and
therefore closest to natural conditions. Although zenith values are widely
reported in the literature, their suitability as a general indicator of sky
quality has been questioned, particularly for areas with low-level light
pollution (Kolláth & Jechow 2023; Falchi et al. 2023). At the same time, the
average NSB in the 30∘ altitude ring is considered a reliable index for
astronomical research, given that most professional astronomical observations
are conducted above airmass 2. The most objective indicator to describe
overall sky quality is however agreed to be the all-sky averaged NSB value
(Falchi et al. 2023; Duriscoe 2016). In Table 4, we provide the alt-az
coordinates and corresponding NSB/CCT values for the darkest and brightest
points in the sky, as well as at zenith (i.e., averaged over a circle of 5∘
radius), within a 1∘ wide ring centered at 30∘ elevation, and across the
entire celestial hemisphere (i.e., all-sky) for each site during our
observations. Figure 15 displays histograms of the all-sky NSB/CCT
distributions, suitable for quickly identifying at any given location the
portion of celestial hemisphere above or below a specific reference threshold.

Table 4: Alt-az coordinates and corresponding NSB (in V mag/arcsec2) and CCT
(in K) values of the darkest and brightest points in the sky at the time of
our observations. Also shown are the averaged values at zenith, at 30∘ fixed
elevation, and over the entire celestial hemisphere.

Site | Brightest Pointa | Darkest Pointa | Zenithb | 30$\hbox{${}^{\circ}$}^{c}$ | All-Sky
---|---|---|---|---|---
acronym | AZ | ZA | NSB | CCT | AZ | ZA | NSB | CCT | NSB | CCT | NSB | CCT | NSB | CCT
FJNP | 86 | 86 | 20.33 | 3325 | 216 | 25 | 22.25 | 4416 | 22.05 | 4926 | 21.57 | 4452 | 21.55 | 4311
LCO | 208 | 90 | 19.96 | 2456 | 223 | 24 | 22.08 | 4004 | 22.02 | 4233 | 21.44 | 3978 | 21.45 | 3836
CAO | 317 | 90 | 18.95 | 3042 | 145 | 57 | 21.19 | 3476 | 21.09 | 3485 | 20.77 | 3255 | 20.64 | 3250
LS-CQ 1st | 304 | 87 | 14.29 | 4978 | 264 | 5 | 18.36 | 3533 | 18.34 | 3484 | 17.35 | 3466 | 16.90 | 3351
LS-CQ 2nd | 309 | 86 | 13.99 | 5881 | 241 | 8 | 18.27 | 4398 | 18.26 | 4352 | 17.37 | 4594 | 16.81 | 4234

Note. — a Average value within 1 deg2; b Average value for zenith angle
ZA$<$5∘; c Average value at $30\hbox{${}^{\circ}$}$ elevation, in a
$1\hbox{${}^{\circ}$}$ wide ring (i.e., for
$59.5\hbox{${}^{\circ}$}<$ZA$<60.5\hbox{${}^{\circ}$}$).

Figure 15: The histograms represent the distribution of all-sky NSB (left
column) and CCT (right columns) values for each of the four measurement sites.
The bin width is set at 0.2 mag/arcsec2 and 100 K, respectively.

###  4.2 Cross-site comparisons

FJNP. Despite the asymmetry that sees at FJNP the western horizon (when
considering only ZA$\geq$60°) 28% brighter than the eastern one, the reference
photometric indicators presented in Table 4 indicate that FJNP is one of the
darkest measured locations on Earth272727Along with few other natural parks
located in continental US, such as Banner Creek Summit and Galena Lodge in
Idaho’s Sawtooth National Recreation Area –
https://www.nps.gov/subjects/nightskies/skymap.htm (Alarcon et al., 2021). The
further comparison with state-of-the-art numerical models (Figure 17 and Table
5) confirms this conclusion by estimating that ALAN contributes only $\sim$4%
to FJNP overall NSB. This evidence, coupled with the exceptional ecological
importance of FJNP as a one-of-a-kind ecosystem, highlights the urgent need
for coordinated conservation efforts that go beyond the existing measures in
place. While the importance of FJNP as a UNESCO Biosphere Reserve is widely
acknowledged and the access to the site is regulated accordingly, the same
level of active protection has not been extended to its status as a Starlight
Reserve. To the best of our knowledge, in fact, there are currently no
concrete actions focused on preserving the pristine quality of its night sky,
nor ongoing monitoring programs able to provide timely alerts in the event of
any significant deviations of the relevant variables.

LCO. The increasing evidence in the VIIRS yearly data releases of ALAN spots
associated with the renewed lighting system of the panamerican highway has
raised concerns regarding its potential impact on the night sky quality of
nearby professional observatories, particularly LCO and ESO La Silla (Blanc
2019; Falchi et al. 2023). Falchi et al. (2023) estimated that “the lights
from Ruta 5 alone, within a 40 km distance from the observatory, contribute to
over 50% of both the artificial zenith radiance and the average radiance at
30∘” at LCO. However, the recorded SQC data towards Ruta 5 approximately
indicate the same NSB levels as those in the antipodal direction. The lack of
spectral information on a night characterized by high airglow activity
prevents us from accurately determining the specific contributions of each
ALAN source, nor to weight it against a measured (i.e., not assumed a priori)
natural reference level. We are therefore not in the position to confirm or
dismiss Falchi’s statement. As we have emphasized throughout this paper,
determining a reliable ratio of artificial to natural NSB without dedicated
spectroscopy is extremely challenging. On the other side, it is also worth
mentioning that the convolution of a point spread function with the
geographical distribution of light sources, although useful for mapping ALAN
radiance over large areas, may still suffer from considerable uncertainties
and could overestimate the artificial contribution by up to 50% (Simoneau et
al., 2021). Further data collection and collaborative studies are thus
necessary to improve our current understanding of this situation. Nonetheless,
the analysis of various metrics that includes the NSB value of the darkest
point in the sky (V=22.08 mag/arcsec2) and at zenith (V=22.02 mag/arcsec2) as
shown in Table 4, and the comparison with numerical simulations (for which
ALAN contributes only $\sim$11% to the overall NSB - Figure 17 and Table 5)
reaffirm that LCO is one of the best locations worldwide for conducting
cutting-edge astronomical research. If it will remain so also in the upcoming
ELT era, it only depends on our present ability to defend it against the
threat of the surrounding ALAN sources, first and foremost of the rapidly
expanding urban areas.

CAO. Due to the geographical distribution of ALAN sources surrounding CAO, the
western celestial hemisphere is heavily affected by skyglow up to at least 50∘
elevation. In contrast, the eastern hemisphere appears noticeably darker, with
the darkest region in the sky located only 30∘ above the horizon, pretty close
to the brightest natural feature represented in this case by the MK bulge. Our
SQC data allows us to quantify this asymmetry: the western hemisphere is
approximately 70% brighter than the eastern one, and this percentage increases
to about 130% when considering only the first 30∘ above the horizon. The
artificial contribution to NSB overwhelms the natural one even at the zenith.
It is thought-provoking to realize that the night sky quality indicators at
CAO are comparable with the ones of Flagstaff, Arizona282828SQC images
recorded in Flagstaff on August 16, 2017 are publicly available at
https://lightpollutionmap.info., highlighting the fact that a city with
$\sim$11,000 inhabitants (Andacollo) is plagued by light pollution levels that
are similar to those of a much larger urban area with nearly 140,000 residents
like Flagstaff. On one hand, this comparison praises the long-term measures
taken by Flagstaff local administration to regulate ALAN, which was awarded in
2001 the title of world’s first Dark Sky
Community292929https://www.flagstaff.az.gov/3799/Dark-Sky-Community; it also
proves that well-coordinated efforts to protect the night sky do not
necessarily hinder any economic or demographic growth, nor represent a
specific threat to public safety. On the other hand, it highlights that there
is still much work that can (and should) be done in Chile to keep the light
pollution phenomenon under control.

LS-CQ. Probably one of the most significant findings of this study is that LS-
CQ consistently emerges as the most invasive source of artificial light at the
horizon of all of our observation sites, regardless of whether we sit high in
the Andes near the northern border of the Coquimbo region, or deep in a ocean-
side forest near its southern extreme. It is therefore imperative to
prioritize efforts in controlling light pollution over this urban area.
As we have previously discussed (Section 3.4), the NSB/CCT values in LS-CQ
indicate a rapid degradation of its night sky, which can be primarily
attributed to the wide-spreading use of white-blue LED lighting. Interestingly
enough, we observationally confirm the trends suggested by Luginbuhl et al.
(2014) models, namely that blue-rich LED sources “produce a dramatically
greater sky brightness than yellow-rich HPS sources […] when matched lumen-
for-lumen and observed nearby”, but at the same time their contribution
decreases faster with distance because of a higher scattering efficiency. In
other words, the copious LED contribution from LS-CQ causing the high CCT
values of Figure 12 is largely scattered out by the time it is recorded at the
other sites, explaining the CCT dips to $\sim$ 3000 K in the direction of LS-
CQ of Figures 4, 7,10.

To truly grasp the severity of the situation, it is enlightening to compare
the sky quality indicators of LS-CQ with those of Padova, a city in northern
Italy of similar size and at approximately the same elevation above sea level,
but located in one of the most densely populated (and light-polluted – Falchi
et al. 2016) areas of Europe. Publicly available SQC images from downtown
Padova captured on November 11, 2019 reveal virtually identical NSB within a
few tenths’ magnitude303030Padova’s darkest spot in the sky: 18.28
mag/arcsec2; NSB within the 30∘ ring: 17.46 mag/arcsec2; all-sky radiance:
17.49 mag/arcsec2. Compare these values with the last two rows of Table 4. and
even lower CCT values compared to those measured in LS-CQ during our two
visits in 2022. It is therefore extremely concerning to realize that if one
can still refer to a general darkness of the Chilean skies, and La Serena be
praised as the capital of the Región Estrella, it is not for the results of an
active and effective control of light pollution, but it is fortuitously due to
the urban sparse distribution of a low-populated country with a unique
geography.

To monitor with the deserved pace the rapid evolution of ALAN in LS-CQ, we
have recently installed a Telescope Encoder and Sky Sensor (TESS-W, Zamorano
et al. 2016) in the AURA-NOIRLab headquarters in La Serena. This TESS-W
(stars1036) has been continuously monitoring the night sky at zenith since
March 2023, and its NSB measurements are summarized in Figure 16. They reveal
a clear bimodal distribution and are consistent with the SQC-derived zenith
values recorded under clear-cloud conditions during our two 2022 visits (Table
4). It is well-known that in urban environments variable cloud coverage can
amplify ALAN (Kyba et al., 2011): in LS-CQ our TESS data suggest that
completely overcast nights appear roughly 25 times brighter than moonless,
cloud-clear nights. Thanks to publicly funded programs, we are currently
assembling a network composed of $\sim$40 TESS stations that will be
monitoring professional and astrotourism observatories, natural reserves, and
rural schools across the entire Región Estrella (Jaque Arancibia et al., in
preparation).

Figure 16: TESS-W distribution of zenith NSB in LS-CQ. The histogram reflects
over 115,000 TESS-W measurements taken in the $\sim$160 days leading up to
2023.08.26. The top panel collects measurements obtained under any weather
conditions, while the bottom panel represents only measurements obtained under
clear sky conditions. Clear conditions were observed approximately 37% of the
time, partially cloudy conditions were observed 23% of the time, and
completely overcast conditions were observed 40% of the time.

###  4.3 Comparison with GAMBONS numerical models

Figures 17 and 18, along with Table 5, provide a direct comparison between our
SQC data and the GAMBONS numerical models (Masana et al. 2021, 2019). GAMBONS
offers a user-friendly web
interface313131https://gambons.fqa.ub.edu/index.html that allows users to
generate realistic models of the sky’s natural brightness in cloudless and
moonless nights for any given time and location on Earth. These models take
separately into account the brightness contributions of stars from Gaia ED3
and Hipparcos catalogs, diffuse galactic and extragalactic light, zodiacal
light, airglow, and atmospheric attenuation and scattering effects. The models
have been run using default airglow and aerosol parameters and can be
considered a good first approximation to the typical conditions of our
monitoring sites. However, it is important to note that the models available
on the public web interface adopt a simplified approach to evaluate the impact
of atmospheric scattering in order to expedite computation time. These
simplified simulations are known to overestimate the natural NSB near the
horizon and underestimate it at zenith by up to 0.1 mag/arcsec2 (Masana et
al., 2019). Taking into account both the modeling accuracy and the precision
of our data, the agreement between the natural (as provided by GAMBONS) and
observed (i.e., natural + artificial, as provided by the SQC) zenith NSB for
the most pristine site is remarkable. Within this level of confidence, it
becomes also possible to quantify the artificial contribution to the overall
radiance of the night sky (last column of Table 5). It is worth noticing that
if FJNP is confirmed to currently have a negligible level of light pollution,
and LCO is still in the list of excellent sites for conducting cutting-edge
astronomical observations, the skyglow at CAO has already erased nearly half
of the stars that were visible by the naked eye until a few human generations
ago. ALAN in LS-CQ has already killed the natural starlight, resulting in
nights virtually devoid of stars.

Figure 17: NSB maps according to the GAMBONS numerical models (left panels)
vs. SQC-based observations (right panels) for FJNP (upper row) and LCO (bottom
row). Please refer to Sect. 4 and Table 5 for further details.

Figure 18: As for Figure 17 but for CAO and LS-CQ. Refer to Sect. 4 and Table
5 for further details. Table 5: Comparison between GAMBONS numerical models
(GMB) and observed (SQC) NSB at zenith and all-sky, in V mag/arcsec2. The last
column indicates the artificial contribution to the overall (i.e., observed
$=$ natural $+$ artificial) radiance of the night sky. See Sect. 4 for further
details.

Site | Zenitha | All-Sky
---|---|---
acronym | GMB | SQC |  $\Delta$m | GMB | SQC | ALAN %
FJNP | 21.83 | 21.90 | -0.07 | 21.52 | 21.48 | 4.0
LCO | 21.78 | 21.95 | -0.17 | 21.52 | 21.39 | 11.3
CAO | 21.52 | 21.05 | 0.47 | 21.51 | 20.61 | 56.6
LS-CQ 1st | 21.80 | 18.35 | 3.45 | 21.53 | 16.89 | 98.6
LS-CQ 2nd | 21.66 | 18.26 | 3.40 | 21.47 | 16.81 | 98.6

Note. — a Average NSB for ZA$<$5∘.

##  5 Concluding Remarks

We want to close this work by further calling the attention on the potentially
confusing similarity between the high CCT values recorded at both FJNP and LS-
CQ (Figure 15). In Section 1 we had briefly discussed the ALAN SPD reshaping
that it is taking place because of the so-called LED revolution: among the
various detrimental effects it is at the origin of, there is in fact also the
increasing evidence that most of the photometric technologies and techniques
that have so far been used to monitor ALAN worldwide may not able to keep pace
with the massive and fast spread of these cheap, and therefore widely
accessible, lighting sources. This is mainly due to the diverging spectral
emissivity of lighting sources on one side, and the spectral sensitivity range
of the instruments devoted to the measurements of night sky quality on the
other side (Kyba et al., 2023).

Falchi et al. (2016) already noted that the VIIRS DNB sensor spectral response
“leaves out the blue and violet parts of the visible spectrum […] thus
preventing a good control of the evolution of light pollution in this
important spectral band, where the white LEDs now being installed have strong
emissions.” Even the more advanced DSLR all-sky cameras which are increasingly
used in professional ALAN investigations (e.g., Vandersteen et al. 2020;
Jechow & Hölker 2019; Kolláth & Dömény 2017; this same study) may face
challenges in distinguishing between natural and artificial sources of NSB as
soon as the blue-shifting of ALAN SPD becomes even more pronounced
(Vandersteen et al., 2020; Jechow & Hölker, 2019; Kolláth & Dömény, 2017).
While incandescent and HPS lamps can be differentiated from the natural sky
emission because of their different CCT, the contamination from typical LED
sources that emit 70% of light in the blue-green (400-480 nm) range becomes
basically indistinguishable to the eyes of SQC and similar devices, as we have
also reported in our study. This highlights the need for improved measurement
techniques and analysis methods to effectively separate natural and artificial
sources of NSB in the face of increasing LED lighting.

Significant progress in the quantitative monitoring of ALAN could be achieved
through spectroscopy, which so far has primarily been conducted at
professional astronomical observatories (Patat, 2008). This approach allows
for a much direct and trusty identification of natural and artificial
contributions to the sky brightness level, also enabling the precise tracking
of its complex and multi-periodic variations over time. In collaboration with
the University of Padova, we are currently working on the development of a
portable spectrograph capable of capturing low-resolution (R$\sim$600),
optical (350-750 nm) flux-calibrated spectra of large portion ($\sim$30∘) of
the night sky at once. The first prototype is already collecting continuous
data at the Agency for Environmental Protection of Veneto Region (ARPAV)
headquarters in Padova, Italy. An improved version is planned to be installed
near the 8.1 m Gemini South telescope on Cerro Pachón during semester 2024A,
while a second, portable one, will be joining our SQC monitoring campaign
across the Coquimbo Region and beyond.

All these scientific and instrumental projects, along with free-of-charge
lighting consultancy initiatives and outreach campaigns323232See, e.g., the
educational videos of the Universidad de
La Serena web series #IluminAconCiencia on YouTube., are currently playing a
crucial role in raising public awareness about the harmful impacts of light
pollution in Chile. They help dispel misconceptions and promote a better
understanding of appropriate lighting practices. The scientific evidence
presented in this paper has also already offered valuable support to public
committees at the municipal, regional, and national levels and has empowered
them to formulate data-driven strategies to protect the darkness of the
Chilean sky: a natural and cultural333333https://www3.astronomicalheritage.net
heritage that is our scientific, social, and moral responsibility to defend
and preserve for all future generations.

The authors would like to thank the referee for helpful comments and
suggestions that improved the manuscript. The authors also express their
gratitude to all those who provided logistical support during the visits to
the observing sites: special thanks are extended to Solange Mondaca of COA and
Carlos Slomp Burger of the Italian School of La Serena for granting and
coordinating access to their respective facilities during the challenging
period of the pandemic. JPUT and RA acknowledge financial support of
DIDULS/ULS through project PTE1953851. JPUT also acknowledges financial
support of the National Agency for Research and Development (ANID) Scholarship
Program/Doctorado Nacional/2021-21210732. MJA acknowledges the financial
support of DIDULS/ULS through project PR2353855. DFO, MJA and RA acknowledge
financial support of DIDULS/ULS through project PTE2053852.

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