Sky Brightness Research Articles

Alteration of twilight sky brightness profile by light pollution

The phenomenon of twilight has been stipulated as naturally dependent on the variables of solar depression, aerosol concentration and ozone composition. The unique color spectrum of twilight is attributed by ozone layer and aerosol, whereas its brightness is heavily relying on the altitude of sun below horizon. While all the natural dependence is being researched extensively, the impact of light pollution on the brightness of the twilight is not being explored thoroughly. The objective of this study is to find out the alteration of light pollution towards the natural behavior of twilight sky brightness. Total of 84 data of twilight brightness was collected from 2014 until 2022 using Sky Quality Meter, and the specific location with profile classified as urban, suburban and pristine is investigated to study the impact of light pollution on twilight brightness. The result demonstrates that the stability of twilight brightness, which naturally factored by solar depression within the range of 18°, discovered to be heavy influenced by the zenith light pollution of the location sky, as evidenced by location that has zenith pollution of 17.11, 19.77, and 21.39, has horizontal twilight brightness of 14.84, 19.55 and 21.2 and solar depression during brightness stability at 11.5°, 15.67° and 17.49°.

The cloud cover and meteorological parameters at the Lenghu site on the Tibetan Plateau

Abstract The cloud cover and meteorological parameters serve as fundamental criteria for an astronomical observatory working in optical and infrared wavelengths. In this paper, we present a systematic assessment of key meteorological parameters at the Lenghu astronomical observing site on the Tibetan Plateau. The datasets adopted includes the meteorological parameters collected at the local weather stations at the site and in the Lenghu Town, the sky brightness acquired by the Sky Quality Meters and all-sky images from a digital camera, the ERA5 reanalysis database and global climate monitoring data. From 2019 to 2023, the fractional observable time of photometric condition is 69.70%, 74.97%, 70.26%, 74.27% and 65.12%, respectively, which is influenced by a variety of meteorological parameters. Large-scale air-sea interactions affect the climate at Lenghu site, which in fact delivers a clue to understand the irregularity of 2023. Specifically, precipitable water vapor at Lenghu site is correlated to both the westerly wind index and the summer North Atlantic Oscillation index, the yearly average temperature of Lenghu site is observed to increase significantly during the occurrence of a strong El Niño event, and the relative humidity anomaly at Lenghu site is correlated to the Pacific Decadal Oscillation index. The decrease of fractional observing time in 2023 was due to the ongoing strong El Niño event and relevant global climate change. We underscore the substantial role of global climate change in regulating astronomical observing conditions and the necessity for long-term continuous monitoring of the astronomical meteorological parameters at Lenghu site.

Quantifying Night Sky Brightness as a Stressor for Coastal Ecosystems in Moreton Bay, Queensland

Growing light pollution is increasingly studied in terrestrial environments. However, research on night lights in coastal ecosystems is limited. We aimed to complement spaceborne remote sensing with ground-based hemispheric photos to quantify the exposure of coastal habitats to light pollution. We used a calibrated DSLR Canon camera with a fisheye lens to photograph the night sky in 24 sites in the rapidly developing area of Moreton Bay, Queensland, Australia, extracting multiple brightness metrics. We then examined the use of the LANcubeV2 photometer and night-time satellite data from SDGSAT-1 for coastal areas. We found that the skies were darker in less urbanized areas and on islands compared with the mainland. Sky brightness near the zenith was correlated with satellite observations only at a coarse spatial scale. When examining light pollution horizontally above the horizon (60–80° degrees below the zenith), we found that the seaward direction was brighter than the landward direction in most sites due to urban glow on the seaward side. These findings emphasize the importance of ground measurements of light pollution alongside satellite imagery. In order to reduce the exposure of coastal ecosystems to light pollution, actions need to go beyond sites with conservation importance and extend to adjacent urban areas.

Daytime Sky Brightness at Dome C, Antarctica: Results from All ESCAPE Campaigns

The study of the solar corona is a prominent focus in the field of solar physics. However, conducting ground-based observations of the corona is a challenging task due to the interference caused by the diffused sky brightness, which obscures the faint coronal signal. As a result, such observations are primarily carried out during total solar eclipses. The requirement of a sky-brightness level as low as 10−6\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$10^{-6}$\\end{document} times the solar disk brightness (B⊙\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$B_{\\odot }$\\end{document}) is met by few places on Earth, and currently there are only two sites hosting solar observatories that satisfy this criterion, Mauna Loa and Haleakala, both located in Hawaii. Nevertheless, another candidate coronagraphic site was discovered in the Concordia Station at Dome C plateau, Antarctica (≃3300\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$\\simeq 3300$\\end{document} m a.s.l.). In this article, we show the last results of the Extreme Solar Coronagraphy Antarctic Program Experiment (ESCAPE) during the 38th summer campaign of the Italian Piano Nazionale di Ricerche in Antartide (PNRA). Here, we report a model for estimating the air column, which allows for the first time to account for variations in the Sun’s altitude above the horizon during different observation periods, and we use it to compare the obtained results with previous campaigns. Our results confirm that Dome C is an ideal coronagraphic site with the required sky-brightness level, reaching 1.0−0.7×10−6B⊙\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$1.0-0.7\ imes 10^{-6}B_{\\odot }$\\end{document} in optimal conditions.

Intensity-sensitive Quality Assessment of Extended Sources in Astronomical Images

Radio astronomy studies the Universe by observing the radio emissions of celestial bodies. Different methods can be used to recover the sky brightness distribution (SBD), which describes the distribution of celestial sources from recorded data, with the output dependent on the method used. Image quality assessment (IQA) indexes can be used to compare the differences between restored SBDs produced by different image reconstruction techniques to evaluate their effectiveness. However, reconstructed images (for the same SBD) can appear to be very similar, especially when observed by the human visual system (HVS). Hence, current structural similarity methods, inspired by the HVS, are not effective. In the past, we have proposed two methods to assess point-source images, where low amounts of concentrated information are present in larger regions of noise-like data. But for images that include extended source(s), the increase in complexity of the structure makes the IQA methods for point sources oversensitive because the important objects cannot be described by isolated point sources. Therefore, in this article we propose the augmented low-information similarity index (augLISI), an improved version of LISI, to assess images including extended source(s). Experiments have been carried out to illustrate how this new IQA method can help with the development and study of astronomical imaging techniques. Note that although we focus on radio astronomical images herein, these IQA methods are also applicable to other astronomical images and imaging techniques.

Detecting changes in anthropogenic light emissions: Limits due to atmospheric variability

Monitoring the evolution of the anthropogenic light emissions is a priority task in light pollution research. Among the complementary approaches that can be adopted to achieve this goal stand out those based on measuring the direct radiance of the sources at ground level or from low Earth orbit satellites, and on measuring the scattered radiance (known as artificial night sky brightness or skyglow) using networks of ground-based sensors. The terrestrial atmosphere is a variable medium interposed between the sources and the measuring instruments, and the fluctuation of its optical parameters sets a lower limit for the actual source emission changes that can be confidently detected. In this paper we analyze the effect of the fluctuations of the molecular and aerosol optical depths. It is shown that for reliably detecting changes in the anthropogenic light emissions of order ∼1 % per year, the inter-annual variability of the annual means of these atmospheric parameters in the measurement datasets must be carefully controlled or efficiently corrected for.

An Overview of Sky Brightness Surrounding Timau National Observatory (TNO), Indonesia

This study aims to analyze skylight conditions surrounding the newly constructed Timau National Observatory (TNO) in Kupang, Indonesia. The analysis was carried out with the primary reason of determining the initial conditions and surroundings of the area, thereby providing valuable information for future observers who will be using the facility. To achieve the stated aim, the sky brightness in the study area was measured in units of magnitudes per square arcsecond (mpsas) at a 20-kilometer radius. Furthermore, numerous observations were conducted at 32 distinct observation points using Sky Quality Meter (SQM) LU-DL (SQM-LU-DL), which was directed towards the zenith. The results obtained from these observations were subsequently analyzed using Surfer software with the primary aim of determining sky brightness in the study area. The analysis showed variations in sky quality in a 20 km radius, ranging from 21.64 to 20.37 magnitudes per square arcsecond (mpsas). The measurements showed that sky quality fell between the categories of great dark and semi-suburban transitional skies. However, it was important to comprehend that recent investigations had identified bright areas surrounding the observatory area, capable of diminishing the visual quality of sky if left unaddressed.

Fisheye Night Sky Imager: A Calibrated Tool to Measure Night Sky Brightness

The Night Skies Team of the U.S. National Park Service (NPS) has developed a new camera system equipped with a fisheye lens to measure night sky brightness. This NPS Fisheye Night Sky Imager comprises a back-illuminated Sony IMX455 CMOS sensor housed in the ZWO ASI6200MM camera, a Johnson V filter, and a Sigma 8 mm F3.5 fisheye lens. All of the components are commercially available. The fisheye lens allows us to capture the entire sky in a single frame. The wide-angle view also introduces calibration challenges. To address this, we have customized tools and devised innovative methods for calibrating the system, including flat-field correction, pixel scale determination, and field of view characterization. Additionally, we use standard stars for precise positional and photometric calibration. Furthermore, we create an open-source Python pipeline to process these fisheye images and leverage Git and GitHub for source code version control and distribution. The resulting images provide a positionally and brightness-calibrated fisheye view of the night sky, with a photometric calibration uncertainty of 0.12 mag. When comparing our Fisheye Night Sky Imager measurements to images taken with other calibrated systems, we reliably achieve consistent results under both dark and bright skies. Our Fisheye Night Sky Imager not only enables rapid assessment of sky quality but also supports the feasibility of establishing permanent monitoring locations. Recognizing that the natural dark sky is a critical element for natural, cultural, educational, and visitor experiences, this new camera system enhances our ability to assess resource conditions and fulfill the NPS mission of conserving resources unimpaired for the enjoyment of this and future generations.

Experimental Scattering Matrices of Martian Dust Aerosols with Narrow Particle-size Distributions

We present experimental scattering matrices of the JSC Mars-1, MMS-2, and MGS-1 simulants at 488 and 640 nm. The analogs were processed so that narrow size distributions representative of Martian dust aerosols during different dust cycles were obtained. We find that the forward peak of the phase function depends on particle size as it becomes narrower with increasing size, whereas the side- and backscattering directions depend on both composition and size so that increasing size and decreasing absorption produce a flatter curve. The position and maximum of the degree of linear polarization varies based on particle size and composition, and the negative polarization branch is more prominent for wavelength-scale particles diminishing with increasing size. The linear depolarization is strongly affected by size and composition. Finally, we compare sky-brightness curves measured by the Navcam and Hazcam engineering cameras on board the Mars Science Laboratory rover to the measured phase functions. The observations show a narrower peak at the forward direction and a flatter curve toward the side- and backscattering directions with an increasing dust load in the atmosphere, similar to what can be seen for the measured phase functions of the analogs with increasing particle size. In the case of the analogs, the flattening of the curve can be caused by an increase in multiple scattering within a particle by wavelength-scale surface roughness and/or internal inclusions. For the observed sky brightnesses, particle aggregation and multiple scattering among particles in denser dust conditions play a major role.

Illusory light drives pupil responses in primates.

In humans, the eye pupils respond to both physical light sensed by the retina and mental representations of light produced by the brain. Notably, our pupils constrict when a visual stimulus is illusorily perceived brighter, even if retinal illumination is constant. However, it remains unclear whether such perceptual penetrability of pupil responses is an epiphenomenon unique to humans or whether it represents an adaptive mechanism shared with other animals to anticipate variations in retinal illumination between successive eye fixations. To address this issue, we measured the pupil responses of both humans and macaque monkeys exposed to three chromatic versions (cyan, magenta, and yellow) of the Asahi brightness illusion. We found that the stimuli illusorily perceived brighter or darker trigger differential pupil responses that are very similar in macaques and human participants. Additionally, we show that this phenomenon exhibits an analogous cyan bias in both primate species. Beyond evincing the macaque monkey as a relevant model to study the perceptual penetrability of pupil responses, our results suggest that this phenomenon is tuned to ecological conditions because the exposure to a "bright cyan-bluish sky" may be associated with increased risks of dazzle and retinal damages.

Energy Efficiency in Public Lighting Systems Friendly to the Environment and Protected Areas

Solid-state lighting technology, such as LED devices, is critical to improving energy efficiency in street lighting systems. In Ecuador, government policies have established the obligation to exclusively use LED systems starting in 2023, except in special projects. Ecuador, known for its vast biodiversity, protects its national parks, which are rich in flora, fauna and natural resources, through international institutions and agreements such as UNESCO, CBD and CITES. Although reducing electrical consumption usually measures energy efficiency, this article goes further. It considers aspects such as the correlated color temperature in the lighting design of protected areas, light pollution and the decrease in energy quality due to harmonic distortion. Measurements of the electromagnetic spectrum of the light sources were made in an area in the Galápagos National Park of Ecuador, revealing highly correlated color temperatures that can affect ecosystem cycles. In addition, the investigation detected levels of light pollution increasing the night sky brightness and a notable presence of harmonic distortion in the electrical grid. Using simulations to predict the behavior of these variables offers an efficient option to help preserve protected environments and the quality of energy supply while promoting energy savings.

An Image Quality Evaluation and Masking Algorithm Based On Pretrained Deep Neural Networks

With the growing amount of astronomical data, there is an increasing need for automated data processing pipelines, which can extract scientific information from observation data without human interventions. A critical aspect of these pipelines is the image quality evaluation and masking algorithm, which evaluate image qualities based on various factors such as cloud coverage, sky brightness, scattering light from the optical system, point-spread-function size and shape, and read-out noise. Occasionally, the algorithm requires masking of areas severely affected by noise. However, the algorithm often necessitates significant human interventions, reducing data processing efficiency. In this study, we present a deep-learning-based image quality evaluation algorithm that uses an autoencoder to learn features of high quality astronomical images. The trained autoencoder enables automatic evaluation of image quality and masking of noise affected areas. We have evaluated the performance of our algorithm using two test cases: images with point spread functions of varying full width half magnitude, and images with complex backgrounds. In the first scenario, our algorithm could effectively identify variations of the point spread functions, which can provide valuable reference information for photometry. In the second scenario, our method could successfully mask regions affected by complex regions, which could significantly increase the photometry accuracy. Our algorithm can be employed to automatically evaluate image quality obtained by different sky surveying projects, further increasing the speed and robustness of data processing pipelines.

Atmospheric extinction coefficients and night sky brightness at ITB-Undana remote telescope site

Abstract A remote telescope system has been established at Universitas Nusa Cendana, Kupang. Bosscha Observatory, Institut Teknologi Bandung (ITB), and the Physics Study Program at Universitas Nusa Cendana (Undana) collaborated to develop the system. The ITB-Undana Telescope system has a QSI 616 CCD camera and BV RI Bessel filter attached to a 20-cm (f/10) Schmidt-Cassegrain reflector. Several research and public outreach activities have been conducted using the ITB-Undana remote telescope throughout 2023. When establishing a new astronomical observation site, it is imperative to conduct sky characterization and quality assessment. This process involves measuring night sky brightness and atmospheric extinction coefficients. In May 2023, we observed photometric standard stars using the absolute photometry technique with BV RI Bessel filters. We also utilized the Sky Quality Meter (SQM) to measure night sky brightness over three nights from May 24th to May 26th, 2023, to compare the sky brightness values obtained through the absolute photometry method. Our initial measurement shows a first-order extinction coefficient value ranging from k b ′ = 0.269 to 0.520, k v ′ = 0.216 to 0.297, k r ′ = 0.019 to 0.206, and k i ′ = 0.127 to 0.163, for B, V, R, and I filter, respectively. Application of extinction coefficient value in the sky field gives a mean sky brightness value in V bandpass, V sky = 19.431 ± 0.022, with color index (B − V )sky = 0.492 ± 0.039. These night sky brightness values are suitable for measurements using the SQM.

DIY Photometer Performance Test in Determining Morning Twilight

Abstract Do-It-Yourself (DIY) Sky Quality Meter (SQM) known as D.I.Y – CJ’01 Photometer has been successfully built and tested. Using SQM as a calibrated portable photometer is a hot topic of astronomical research regarding night sky brightness measurement. The output data obtained by the photometer is in unit of magnitude per square arc second (mag/sq arcsecond ∼ mpsas) value with a maximum value of 21.7 mpsas as a quantization of the representation of a very dark sky with the presence of natural airglow. The sky brightness then gradually increases with values up to 16.7 mpsas, which is when the sky is brighter due to the presence of artificial airglow from outdoor artificial lighting such as in big cities. This research aims to determine the D.I.Y – CJ’01 Photometer performance in measuring the sky brightness of morning twilight. The research method used in this work is an experiment with data collection from D.I.Y - CJ’01 Photometer and SQM Unihedron gathered from two locations in Cimahi and Bandung. Data processing shows that the D.I.Y - CJ’01 Photometer mimics the performance of SQM Unihedron, with the the smallest difference for the value of the solar depression angle in morning twilight measurements is 0.06%.

Analysis of sky brightness variations in full days based on sky quality meter measurement in Pekalongan

Analysis of sky brightness variations in full days based on sky quality meter measurement in Pekalongan

A new analytical model of the cosmic-ray energy flux for Galactic diffuse radio emission

Low-frequency radio observations of diffuse synchrotron radiation offer a unique vantage point from which to investigate the intricate relationship between gas and magnetic fields in the formation of structures within the Galaxy, spanning from the diffuse interstellar medium (ISM) to star-forming regions. Achieving this pivotal objective hinges on a comprehensive understanding of cosmic-ray properties; these dictate the effective energy distribution of relativistic electrons, which are primarily responsible for the observable synchrotron radiation. Notably, cosmic-ray electrons (CRe) with energies of between 100 MeV and 10 GeV play a crucial role in determining the majority of the sky brightness below the GHz range. However, their energy flux (je) remains elusive because of solar modulation. We propose a way to derive observational constraints on this energy gap of interstellar CRe through the brightness temperature spectral index of low-frequency radio emission, here denoted βobs. We introduce a new parametric analytical model that fits available data for je in accordance with the βobs values measured in the literature between 50 MHz and 1 GHz for diffuse emission in the Milky Way. Our model accounts for multiple observations considering magnetic-field strengths consistent with existing measurements below 10 μG. We present a first all-sky map of the average component of the magnetic field perpendicular to the line of sight and validate our methodology against state-of-the art numerical simulations of the diffuse ISM. This research makes headway in modeling Galactic diffuse emission with a practical, parametric form. It provides essential insights that will help preparations for the imminent arrival of the Square Kilometre Array.

Let there be Skyglow—light pollution from a large outdoor music festival (Lollapalooza Berlin 2016)

Live music is often linked to elaborate light shows, particularly at large outdoor music festivals. However, artificial light at night is one form of environmental pollution, light pollution, and because outdoor festivals emit a substantial amount of artificial light into the environment, they are a potential source of light pollution. So far, no studies that quantified the impact of such festivals on urban light pollution and skyglow exist. Here, the light pollution produced by a major rock festival (Lollapalooza Berlin 2016 with 70,000 visitors per day in an urban park) was investigated with ground-based radiometry and night-time light data. A small night-sky radiometer installed near the main stages and a calibrated digital camera from a nearby observation spot inside of the park were used to quantify changes in night sky brightness and direct light emissions within the park. The impact of the music festival on the urban skyglow was indeed measurable. Zenith luminance increased locally by up to a factor of 8 and illuminance increased by about 50% at the observation spot within the park. The radiance detected by night-time satellite was also increased during the festival. This is the first time, that light pollution from such a major rock music event was quantified.

An Optical Daytime Astronomy Pathfinder for the Huntsman Telescope

Abstract Observing stars and satellites in optical wavelengths during the day (optical daytime astronomy) has begun a resurgence of interest. The recent dramatic dimming event of Betelgeuse has spurred interest in continuous monitoring of the brightest variable stars, even when an object is only visible during the day due to their proximity to the Sun. In addition, an exponential increase in the number of satellites being launched into low Earth orbit in recent years has driven an interest in optical daytime astronomy for the detection and monitoring of satellites in space situational awareness (SSA) networks. In this paper we explore the use of the Huntsman Telescope as an optical daytime astronomy facility, by conducting an exploratory survey using a pathfinder instrument. We find that an absolute photometric accuracy between 1 - 10% can be achieved during the day, with a detection limit of V band 4.6 mag at midday in sloan g‚ and r‚ wavelengths. In addition we characterise the daytime sky brightness, colour and observing conditions in order to achieve the most reliable and highest signal-to-noise observations within the limitations of the bright sky background. We undertake a 7 month survey of the brightness of Betelgeuse during the day and demonstrate that our results are in agreement with measurements from other observatories. Finally we present our preliminary results that demonstrate obtaining absolute photometric measurements of the International Space Station during the day.

Study of Light Pollution Risk Evaluation Model

The invention and widespread use of artificial light are important progress in human history, but the light pollution formed by its excessive or poor use is a serious danger to biodiversity and ecosystems, etc. Behind the bright lights of the busy city, it is also a metaphor for the bright starry sky far away from us. Though using the theory and methods of risk identification and evaluation hierarchical model, and unconfirmed measurement model, the index system and the measuring method on the risk level of light pollution are established, the Light pollution risk evaluation model is formed, and they are applied to interpret their results for the four diverse types of locations. The order of the ratings is urban communities, suburban communities, rural communities, and protected land according to high to low. They meet the results from the literature, which confirms the reasonableness of the risk level measures developed. Our work provides a certain reference for the research and treatment of light pollution.

Evaluation of the light pollution in the nature reserves of China based on NPP/VIIRS nighttime light data

ABSTRACT Nature reserves play a key role in the conservation of habitats, species and ecosystems in China. However, the issue of light pollution within these reserves has not received well recognition. Whereas previous studies have investigated light pollution in protected areas by remote sensing, the influence of the atmospheric scattered light from neighborhoods was ignored by using only direct light emissions. This study mapped the nighttime all-sky brightness from NPP/VIIRS data to quantify light pollution over the national nature reserves in China from 2013 to 2022. Based on the remotely sensed sky brightness, the spatio-temporal variations of sky brightness were analyzed. The mean sky brightness across all nature reserves was 0.63 mcd/m2 and the mean trend was 0.0189 mcd/m2/a, indicating obvious and accelerating light pollution. Furthermore, the sky brightness around the nature reserves was estimated and compared with the reserves. In general, the surrounding regions showed higher sky brightness and brightening trends, suggesting a certain but insufficient protective effect on the sky quality of nature reserves. This study provides a comprehensive understanding of the light pollution conditions in the national nature reserves in China and serves as a valuable reference for future assessments of light pollution using remote sensing data.