Night Sky Monitoring Report Metrics & Glossary of Terms

Sample night sky monitoring report from Chaco Culture National Historic Park. NPS Photo
Sample night sky monitoring report from Chaco Culture National Historic Park. NPS Photo
This page provides a quick explanation of the web reports.
A brief description of each table, and each attribute reported within, is included.

Introduction

The report contains three main sections:

  • A table showing general attributes of the data collection event and visual indicators of sky quality
  • Panoramic images of the entire sky shown in false color revealing calibrated sky brightness with links to the high resolution images
  • Photometric indicators of sky quality and the photic environment derived from the all-sky mosaics. Photometric units of measure used include SI units of luminance (candela per square meter) and illuminance (lux), as well as astronomical units of luminance (magnitudes per square arc second) and illuminance (magnitudes) in the V, or visual, band. SI units are linear, astronomical units are inverse logarithmic, that is, smaller values indicate brighter objects, and negative values are possible.
The NPS methods are unique in that a natural sky model is built for each all-sky observation and subtracted pixel-by-pixel revealing the estimated artificial sky glow. The indicator "Sky Quality Index" or SQI is derived from the artificial sky brightness only;therefore it is an index of light pollution from sky glow.

Peer-reviewed journal articles describe the methods of data collection and analysis in detail:

Data mosaic of Chaco Culture National Historical Park shows "all sky" brightness and estimated artificial sky glow
Data mosaic of Chaco Culture National Historical Park shows "all sky" brightness and estimated artificial sky glow

NPS

Observed and Estimated Artificial Sky Brightness Mosaics

First Figure: Full Resolution Mosaic

The full resolution mosaic of the data set's images rendered in false color. Each individual image is placed in the mosaic after correction for pointing errors, and projected into a Hammer-Aitoff equal area projection with the horizon at the center vertically and a fixed azimuth at the center horizontally. The false color scheme reveals a wide dynamic range of sky brightness values in a logarithmic scale from 14 to 23 mag arcsec-2. The all-sky image mosaic (zenith to 6 degrees below the level horizon) contains about 34 million pixels. Land features and individual light trespass sources are often visible in this rendering.

Second Figure: Estimated Artificial Sky Glow

The sky glow mosaic is the sky brightness mosaic subjected to pixel by pixel subtraction of a registered natural sky model mosaic (the natural sky model is not shown as a graphic in the report) rendered in the same false color scale as the full resolution mosaic. The resolution is 0.05 degrees per pixel. Land features and individual light trespass sources are masked out so that only sky luminance from artificial sky glow is shown. This is an at-a-glance representation of the amount of light pollution from sky glow observed at the site. Artificial sky glow will always be brighter near the horizon than at the zenith and its impact on the natural lightscape substantial.

Data Set Attributes and Visual Indicators

  • Park: The name of the park or land manager where the data was collected
  • Site Name: Place name or description of the location of the observing site
  • Longitude: Longitude in decimal degrees (west is negative), Datum WGS 84, taken with GPS receiver, typical horizontal positional accuracy 5 meters
  • Latitude: Latitude in decimal degrees (north is positive), Datum WGS 84, taken with GPS receiver, typical horizontal positional accuracy 5 meters
  • Elevation (m): Elevation above mean sea level in meters, taken with GPS receiver, typical vertical positional accuracy 15 meters
  • Date(LMT): The date at the middle of the imaging sequence in Local Mean Time (LMT);note that LMT is not adjusted for daylight savings time
  • Time (LMT Hours): The time at the middle of the imaging sequence in Local Mean Time (LMT) using a 24 hour clock;23.5 hours is 11:30 PM
  • Camera: Short description of the camera used. The camera name is usually the manufacturer, such as SBIG (Santa Barbara Instruments Group), followed by a number separating multiple cameras used with the same manufacturer. Most of the data reported was imaged with cameras utilizing the KAF 1001E sensor;exceptions include "Apogee" and "MaxCam" which used a smaller sensor and many more images to cover the sky (see Duriscoe, Luginbuhl, and Moore 2007).
  • Observers: The name(s) of the observer(s).
  • Air Temp (C): Air temperature at start of image acquisition as measured by a portable weather meter in degrees Celsius. Typical accuracy is 3 degrees.
  • R.H.(%): Relative humidity at start of image acquisition as measured by a portable weather meter. Typical accuracy is 5%.
  • Wind Speed (mph): Average wind speed at the start of image acquisition as measured by a portable weather meter held at eye level. Typical accuracy is 3 mph.
  • Extinction Coeff: A report of calculated all-sky atmospheric extinction for each data set. Extinction is a measure of the opacity of the air, the units are astronomical magnitudes per airmass. Airmass depends on its zenith angle and the relationship is not linear. A star viewed at the exact zenith is by definition view through one airmass. This value is computed for each data set from measurements of 50-150 standard stars on the images over the entire sky. The number reported is the extinction coefficient in astronomical magnitudes in the V band per airmass.
  • NELM: Naked eye limiting magnitude, or the faintest stars than can be observed visually without optical aid (naked eye) near the zenith, or darkest part of the sky. This observation varies somewhat from observer to observer, but all observers are instructed to practice the same methods. 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, although some observers have confirmed seeing stars as faint as magnitude 8.2 with the naked eye. A number lower (brighter) than 6.3 usually indicates significantly degraded sky quality.
  • Bortle Class: A semi-quantitative measure of the sky quality observed visually, as developed by astronomer John Bortle. Classes are whole numbers 1-9, with 1 the very best and 9 the poorest. See the Sky and Telescope article.
  • Sythetic SQM: A value derived from the all-sky measurements using an algorithm designed to mimic the response of the Unihedron Sky Quality Meter, in magnitudes per square arc-second (mag arcsec-2), aimed at the zenith. It is considered to be more accurate than an actual measure, since it is computed from the sky brightness mosaic based upon accurate alignment to zenith and accurately calibrated CCD camera data. The units are mag arcsec-2. Values of 21.3 and greater (darker) fall within the range of "natural" skies (Bortle Class 1-3), 19.5-21.3 may be considered significantly degraded skies (Bortle Class 4-6), while values less than 19.5 may be considered severely degraded (Bortle Class 7-9). The SQM is only sensitive to areas of the sky 30 degrees above the horizon and higher, so will not measure bright sources of artificial sky glow along the horizon.
  • Sky Quality Index: A synthetic index derived from the distribution of sky luminance values in the artificial sky glow mosaic. Its range is 0-100, where 100 is a sky free of artificial sky glow. Values of 80-100 may be considered to represent skies that retain all of the natural characteristics throughout most of the sky, 60-80 retaining most of the natural sky features, but only in areas within 40 degrees of the zenith, 40-60 represents skies where the Milky Way is not visible or only the brightest parts are visible near the zenith, 20-40 represents skies only stars and planets remaining and the land is illuminated at a level of moonlight, and 0-20 indicates only the brightest stars remain, and the land is in perpetual twilight. Horizon obstructions are masked out before the artificial sky brightness distribution is derived;otherwise this value includes the entire sky.
  • SQI to Z.A. 70°: The same indicator described above but with the sky masked at zenith angle 70° (20° above the level horizon), such that bright areas of skyglow near the horizon may be excluded. This value is included as it may be more comparable from site to site than the all-sky measure. The all-sky measure is considered more representative of a quality of "naturalness."
  • Number of stars visible: An estimate of the total number of stars theoretically visible to the unaided eye. This value is computed from an overlay of the observed sky brightness and a map of the stars visible at the time of the observation. After reduction in apparent brightness of each star by atmospheric extinction, the adjacent sky brightness is compared star-by-star and an algorithm predicts whether or not it will be visible. A count of visible stars is to the observed horizon is reported.
  • Narrative: A descriptive narrative of the conditions observed visually during the night of data collection. This usually includes seeing (a measure of atmospheric steadiness), and transparency (a measure of atmospheric clarity) in semi-quantitative terms. Also may include characteristics of the site, the appearance of certain astronomical features, and the suitability of the site for visual astronomy by casual visitors.
Photometric Indicators

This table is divided into three areas vertically: Observed, Estimated Artificial, and Light Pollution Ratio (Artificial/Natural). Observed refers to the calibrated photometry observed on the images, corresponding to what an observer on the ground would see. Estimated Aritificial is the key to measuring impact from light pollution, the same indicators are reported on but after subtraction of the natural background. Each of these colomns are separated into two different units of measure of luminance and illuminance: astronomical (V magnitudes per square arc second, or mag/ arcsec2, and V magnitudes or mags);and engineering (micro-candela per square meter, or μcd/ m2, and milli-lux). The Light Pollution Ratio expresses each indicator in unitless manner for direct comparison. A natural reference condition for each indicator has been derived, the ratio value demonstrates the amount of artificial or anthropogenic light above the natural condition. 0.10 = 10% above natural, 1.54 = 154%, 31.40 = 3,140%, etc.

Summary indicators are obtained by pixel-by-pixel extraction of values from the all-sky mosaic of images. The full resolution mosaic is simplified by applying a strong median filter to screen out stars, then it is resampled to 0.05 degrees per pixel resolution, for a total of about 8 million pixels covering the entire sky, in an equal-area projection. Bright unshielded lights in the land portion of the mosaic will not be accurately measured for two reasons: they commonly are so bright their recorded luminance exceeds the dynamic range of the detector so they become clipped or saturated at the maximum ADU value, and the median filter will remove most of the light from these sources since they resemble stars or point sources. Therefore the "Brightest" and "Max Vertical" indicators should not be interpreted to include accurate measures of light tresspass from visible individual lights, even if they appear in the full resolution mosaic. Very bright sources such as this will often cause vertical lines or "column bleeds" in the full resolution image;these are removed by the median filter technique before statistics are calculated.
Sky Luminance Measures

  • Zenith: Perhaps the most often reported sky quality indicator in the astronomical literature. This measure is calculated from the median pixel value of an approximately one degree diameter circle centered on the zenith. 22.0 mag arcsec-2or 172 μcd m-2is generally considered to represent the darkest part of pristine skies, any value lower (brighter) than 21.3 mag arcsec-2usually indicates significantly degraded sky quality, unless bright parts of the Milky Way are directly overhead. The level of detection of the artificial component of the NPS method is approximately 17 μcd m-2, values smaller than this are shown in the table as “< 17” or “> 24.5” mag arcsec-2
  • Mean all-sky: This is an important statistic describing the photic environment. It is reported in logarithmic units of mag arcsec-2and linear units μcd m-2. The natural moonless reference condition is set at 21.6 mag arcsec-2or 250 μcd m-2. This is an unbiased measure of the amount of light reaching the observer from sky luminance.
  • Brightest: The brightest part of the sky is an important indicator because the human eye's ability to dark adapt will be impaired by the brightest part of the visual scene, and because bright parts of the sky may cast shadows from 3D objects on the land surface, giving depth to an otherwise uniformly lit natural landscape. The brightest part of the Milky Way is 19.6 mag arcsec-2or 1500 μcd m-2. Brighter values will begin to impair dark adaptation, values brighter than 17.0 mag arcsec-2can cast shadows. The Light Pollution Ratio is scaled to 22.0 mag arcsec-2or 172 μcd m-2.
  • Darkest: The darkest part of the sky is an indicator of the ability to see faint objects anywhere in the night sky. 22.0 mag arcsec-2or 172 μcd m-2is generally considered to represent the darkest part of pristine skies during times of low solar activity. The Light Pollution Ratio is scaled to this value.
  • Median: The middle sky brightness value over the whole sky; a view of the whole sky will reveal most of the areas to be near this value. The Light Pollution Ratio is scaled to the mean all-sky reference condition of 21.6 mag arcsec-2or 250 μcd m-2.
Illuminance Measures

Horizontal and Maximum Vertical Illuminance: Refers to measuring the amount of light striking the ground (horizontal) or a vertical plane (vertical). The units are milli-lux (mlux). The natural reference condition for moonless nights is 0.8 mlux for horizontal and 0.4 mlux for vertical. The maximum vertical illuminance is for a vertical plane facing the brightest part of the sky near the horizon.