Metrics Guide to Night Skies Report

This document explains the measurements and images reported in the NPS Night Sky Quality Reports on the NPMaps website.

Metadata

• Latitude: site latitude in decimal degrees taken with a GPS.
• Longitude: site longitude in decimal degrees taken with a GPS.
• Elevation: elevation above mean sea level in meters taken with a GPS.
• Date: local date at the middle of data collection.
• Time: local mean time (LMT) at the middle of data collection.
• Air Temp: air temperature in °C measured at the start of image acquisition.
• R.H.: relative humidity in % measured at the start of image acquisition.
• Wind Speed: average wind speed in mph measured at the start of image acquisition.
• Extinction Coeff: extinction coefficient measured in magnitude per airmass. It characterizes the amount of light attenuated by the atmosphere due to absorption and/or scattering. Extinction values indicate the quality of atmosphere transparency. Increasing the aerosol content in the atmosphere will increase the extinction value. Extinction greater than 0.3 mag/airmass is considered high, indicating poor atmospheric conditions. Aerosols from the haze, dust, and air pollutants can enhance the skyglow.
• NELM: naked eye limiting magnitude. NELM is the magnitude of the faintest star we can see in the sky with naked eyes. As the night sky brightness increases, the limiting magnitude will degrade to a lower value. The NELM will also depend on the observer and will increase with the eye’s dark adaptation. 6.6 is considered near pristine under average conditions. 7.0 is achievable under good seeing conditions and with proper dark adaptation of the eye. 7.4 is excellent, just about the faintest attainable. A number lower than 6.3 usually indicates degraded sky quality. The limiting magnitude is also a common metric used by citizen scientists to assess the sky brightness globally.
• Bortle Class: a nine-level numeric scale that classifies the night sky’s brightness of a particular location based on visible sky objects (Bortle, 2001)[1]. The rating of one indicates pristine night sky that is completely free of skyglow, and nine indicates heavily light polluted sky often found in the inner cities.
• Synthetic SQM: the hypothetical SQM value approximated from the image containing all light sources.
• SQM: Unihedron Sky Quality Meter measurement in magnitudes per square arcsecond aimed at the zenith. The SQM has a wide field of view; its full width half maximum of the angular sensitivity is 42°. Although getting a reading is fast and easy, the instrument does not reliably measure the sky brightness when the sky is darker than 21.5 mag/arcsec².
• Stars Visible: the percentage of stars that remains visible under the influence of skyglow. On average, about 4000 stars are visible to the naked eye under the natural dark sky. This metric estimates the direct impact of skyglow to human visual observation of stars and night sky features.

Observed and Estimated Artificial Sky Brightness Mosaics

Two panoramic images are associated with each data set. The date and time are in local time. The images are taken with a visual band or V-band filter, allowing the detected signal to closely represent what human eyes can see based on our spectral sensitivity. This V-band filter is most transparent to green light, blocking purple through ultraviolet and orange through infrared. Sky brightness is measured in magnitude per square arcsecond (mag/arcsec²). Magnitude (mag) is a common unit for measuring the brightness of astronomical objects, and it is in inverted logarithmic scale. Lower values are brighter. A difference of 5 magnitudes corresponds to a 100x difference in brightness. The images are shown in false color, with warmer colors representing brighter skies.

The top image shows the observed night sky, which contains light from both natural and artificial sources. A sky brightness of 22 mag/arcsec² would be considered pristine, and a sky brightness < 20 mag/arcsec² would be considered greatly deviated from the natural condition. Purple and dark blue colors indicate unpolluted sky, and the Milky Way under the natural condition appears green in this color scheme. The bottom image shows only the skyglow from artificial sources. Light domes along the horizon from the nearby area are more apparent in this bottom image.

Field Narrative

Field observation notes taken by the observer.

Photometric Indicators

The following metrics are calculated based on the observed and estimated artificial images.

Sky Luminance Measures

Luminance describes the brightness of a surface. It specifies the amount of light that passes through or is emitted from an area, and falling within a solid angle. The sky luminance refers to the sky surface brightness, or sky brightness in short. We report the sky luminance in both the log unit magnitudes per square arcsecond (mag/arcsec²) and linear unit microcandela per square meter (μcd/m²).

• Zenith Luminance: zenith sky brightness. This measure is calculated from the median pixel value of an approximately one-degree diameter circle centered on the zenith. 22.0 mag/arcsec² or 172 μcd/m² is generally considered to represent the darkest part of pristine skies. Any value lower (brighter) than 21.3 mag/arcsec² usually indicates significantly degraded sky quality, unless the Milky Way is overhead or the natural airglow or the zodiacal light is bright.
• Brightest Luminance: brightest sky brightness. The brightest feature of the natural moonless sky is Milky Way, which is 19.6 mag/arcsec² or 1500 μcd/m². Values brighter than Milky Way will begin to impair dark adaptation, and values brighter than 17.0 mag/arcsec² can cast shadows.
• Mean All-sky Luminance: averaged sky brightness over the whole sky. The natural reference is 21.6 mag/arcsec² or 250 μcd/m².
• Median Luminance: median sky brightness, representing the 50$^{th}$ brightness percentile over the whole sky.
• Darkest Luminance: darkest sky brightness, representing the darkest 1% of sky. The natural reference is 22 mag/arcsec² or 172 μcd/m².

Illuminance Measures

Illuminance describes the amount of visible light incident on a unit surface area. The impact from a light source is weighted by the cosine of its angle of incidence. We report the illuminance in both the log unit (mags) and linear unit (mlux).

• Horizontal illuminance: The amount of light landing on a horizontal surface. It provides the illuminance measurement of the entire sky at a glance but is not sensitive to skyglow near the horizon. The natural reference condition for moonless nights is 0.8 mlux for horizontal illuminance.
• Vertical illuminance: The amount of light striking a vertical surface. The vertical illuminance better reflects the brightness of sources near the horizon. Since a vertical surface can be held facing many different directions, we report the vertical illuminance measured at the direction that gives the largest value. Note that the maximum vertical illuminance can be greatly influenced by natural sources such as the Milky Way, zodiacal light, and airglow. The natural reference condition for moonless nights is 0.4 mlux for vertical illuminance.

Light Pollution Ratio

Light Pollution Ratio (LPR) is calculated by dividing the artificial light by the natural reference light level. The ratio has no unit. In the case with no light pollution, all LPR measurements will be zero. LPR of one indicates the artificial light has reached the same brightness as the natural light. The first five metrics (zenith, brightest, mean all-sky, median, and darkest) are measures for sky luminance. The last two (horizontal and max vertical) are measures for illuminance.

• Mean All-sky Light Pollution Ratio (ALR): the total skyglow brightness divided by the natural dark sky set at 250 μcd/m². Generally, ALR values less than 0.3 indicate excellent conditions, 0.3 to 2.0 indicate impaired sky quality (though areas of the sky may reveal important natural features), and greater than 2.0 indicate the natural night sky is not readily visible.

References