Site Name: Place name or local name of the monitoring site.
Longitude: Longitude in decimal degrees (west is negative), Datum WGS 84, taken with GPS receiver, typical horizontal position accuracy 5 meters.
Latitude: Latitude in decimal degrees (north is positive), Datum WGS 84, taken with GPS receiver, typical horizontal position accuracy 5 meters.
Elevation (m): Elevation above mean sea level in meters, taken with GPS receiver, typical vertical positional accuracy 15 meters.
Date (UT): The date at the beginning of data acquisition in Universal Time (GMT)
Time Start (UT): The time in hours, minutes and seconds (24 hour clock) at the beginning of data acquisition in Universal Time (GMT).
Data Quality: A semi-quantitative description of the quality or reliability of the data collected. Classes include: Excellent, Very Good, Good, Fair, and Poor. Data is classed according to many factors, including equipment malfunctions, battery voltage, unexpected image contamination by stray light, occurrence of frost or dew, observed layered haze or clouds, deviation of standard star measures from predicted values, and obvious visible defects in the images.
Equipment: Short description of the camera, lens, and filter used. The filters used are all Bessell V astronomical filters transmitting green light.
Observers: Name(s) of observers.
Air Temp (°F): Air temperature at start of image acquisition as measured by a portable weather meter. Typical accuracy is 3 degrees.
Rel Humid (%): Relative humidity at start of image acquisition as measured by a portable weather meter. Typical accuracy is 5%.
Wind Sp (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.
CCD Temp (°C): Temperature at which the CCD camera's thermoelectric cooler is set at for data acquisition.
Exp (seconds): Exposure (or integration) time of each image in seconds.
Bortle Class: A qualitative measure of the sky quality observed visually, as developed by professional astronomer John Bortle. Classes are whole numbers 1-9, with 1 the very best and 9 the poorest. Further explanation of the Bortle Class can be found on Sky and Telescope's Dark Sky webpage.
ZLM: Zenith 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. There are 14,000 stars visible at 7.0 conditions, 5000 stars visible at magnitude 6.0 conditions, and only a few dozen stars visible at 1.0. This estimate varies somewhat from observer to observer. Faintest star visible is affected not only by artificial light pollution, but humidity, atmospheric clarity, and air turbulence also. Generally, pristine skies range from 6.6-7.5 while urban skies range from 0-4. More information can be found on the More information can be found on the Sonomo State University Observatory limiting magnitude webpage.
Narrative: A descriptive narrative of the conditions observed visually during the night of data collection. 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 and the visibility of certain astronomical features.
Image: A compressed image of the all sky map of sky brightness in hemispheric projection. On this image, north is up and west is to the right- as if you were lying on your back looking at the sky with your head pointed north. Higher resolution images are available by clicking on HEMI in the Sky Brightness Data box.
Data set number: Multiple data sets are usually taken on a given night. A data set is comprised of all the images necessary to cover the entire sky plus calibration images. Data sets are numbered sequentially.
Time (UT): The time in hours minutes and seconds (24 hour clock) of the start and end of the data set in Universal Time (GMT).
Extinction coefficient (magnitude/airmass): A measure of the opacity of the air, in astronomical magnitudes per airmass (atmospheric thickness). A star will be diminished in brightness or extincted by this amount multiplied by the airmass through which it is observed. A star viewed at the exact zenith (straight overhead) is by definition viewed through 1 airmass. A star at 30 degrees above the horizon is viewed through 2 airmasses, and at 10 degrees above the horizon 5.6 airmasses. As an example, if the extinction coefficient is 0.20, a star with V magnitude of 4.0 will appear as magnitude 4.2 at the zenith, 4.4 when it is 30 degrees above the horizon, and 5.12 when it is 10 degrees above the horizon. The extinction coefficient is computed for each data set from measurements of 100-150 standard stars on the images through a range of airmasses. The extinction coefficient is corrected for a fixed color term for each instrument due to a slight bias in the V-band filter and camera combination.
Std Err Y Extinction stars (magnitudes): Statistical standard error of Y of the extinction regression equation, expressed in astronomical magnitudes in V-band. Smaller numbers indicate a better fit to the linear model, which implies better data quality. A standard error of 0.04 magnitude is typical and considered to represent very good data quality.
Zenith (magnitudes/square arc-sec): Sky brightness measured straight overhead. This and subsequent sky brightness measures are calculated from the median pixel value of a one degree diameter circle centered on the area of interest. This circle includes more than 1000 pixels. This value is converted to astronomical magnitude per square arc second (1/3600th of an angular degree or 1/12,960,000th of a square degree), a standard unit in the astronomical literature, in V-band. 22.0 is generally considered to represent pristine skies, lower values indicate a brighter sky due to variations in natural conditions or increased artificial light.
Whole Sky (magnitudes): The brightness of the entire sky (excluding stars and glare from unshielded lights but including the Milky Way, Airglow, Zodiacal Light, and Light Pollution) in astronomical magnitudes in V-band. A value between -7.5 and -7.0 is typical for near pristine locations. Value more negative than -8.0 usually indicates degraded sky quality.
Sky Above 20 degree Altitude (magnitudes): Like the previous measure but with the area between the horizon and 20 degrees altitude excluded. This would exclude small light domes from cities and most of the bright parts of the natural airglow. This is a more sensitive indicator of sky quality for dark locations. The presence of the Milky Way will significantly affect this value, however.
Brightest (magnitudes/square arc sec): The brightness of the brightest part of the sky in magnitudes per square arc second in V-band. This is a median of a one degree circle, and excludes stars. It may be in a light dome from a city or in the Milky Way or other natural phenomena. The brightest value typically expected from a natural source (the Milky Way or Zodiacal Light) is about 19.5. A value brighter (lower number) than this usually indicates degraded sky quality. A number lower than 17.0 represents a very bright area of the night sky and usually would significantly impair human night vision and cast obvious shadows on the land.
Darkest (magnitudes/square arc second): The brightness of the darkest part of the sky above a point 20 degrees above the true horizon in astronomical magnitudes per square arc second in V-band. This is possibly the most directly comparable statistic between data sets and nights. However, it can vary significantly under natural (unpolluted) conditions from as bright as 21.3 to as dark as 22.3.
Links to Sky Maps: Display maps of sky brightness derived from this data set in either panoramic (PAN) or hemispheric (HEMI) projection.
Light Dome Data
City: The name of the cities, towns, or light sources producing the observed dome of light near the horizon.
Distance (km): The distance of the light source from the monitoring site in kilometers.
Azimuth: The azimuth or bearing in degrees (clockwise from true north) from the site to the light source.
Data set Brightness (magnitudes): The total integrated brightness of the light dome in astronomical magnitudes in V-band calculated from the data indicated. Any number brighter (smaller) than -2.0 usually indicates a degradation in sky quality. For comparison, the planet Venus at its brightest is magnitude -4, a two day old crescent moon is magnitude -7.