How To Conduct A Night Sky Quality Survey

How To Conduct A Night Sky Quality Survey Image

Two hikers taking SQM measurements of the night sky on a trail in Saguaro National Park, Arizona. Photo by Jeremy White, National Park Service.

A thorough night sky quality survey is essential to a successful application to the International Dark Sky Places Program.

There are a variety of ways to approach making a survey. These include:

  1. Sky Quality Meter Survey
  2. Bortle Scale Interpretation
  3. Photographic Evidence



The Unihedron Sky Quality Meter-Lens (SQM-L) is the most widely used device for taking scientific-quality measurements of sky brightness. This small, battery-powered device is available directly from the manufacturer for USD 155.00 plus shipping and handling.

The SQM-L is easy to use: simply hold the device above your head, point the photometer at the zenith, and push the button. The screen will then display the sky brightness at that point in astronomer units (magnitudes per square arcsecond, or mpsas). This unit is somewhat counterintuitive in that the higher the number, the darker the sky is. Take at least six measurements per location per visit, discarding the first measurement. Report all of the measurements.

Never take a measurement directly underneath a light source or anything that might block the clear sky. Always take measurements with a clear, open sky without cloud cover to prevent any skyglow measurements that will deter the accuracy of the device. Also, always take measurements when the Moon is below the horizon. Measurements taken while the Moon is visible will not accurately reflect the natural darkness of your location.

Similarly, keep in mind the impact of twilight on your measurements. Only take readings under conditions of astronomical darkness, meaning that the Sun is at least 18º below the local horizon. Good sources of information for times of the end and beginning of astronomical twilight at your location on any given night can be found here. (Note that for these calculators you may need to know your latitude, longitude, and local timezone offset from GMT.)

It is essential to include areas in the survey that represent the darkest, brightest, and most accessible areas to visitors to achieve a comprehensive survey. This data can then be organized into a map and corresponding table showing approximate locations and readings of measurements. An example map and table are provided below. Advocates are encouraged to use IDA’s standardized spreadsheet when taking measurements out in the field or compiling measurements taken throughout the course of a year. Directions for access and edits are provided at the top of each spreadsheet.

Example of a map depicting SQM locations taken across a site. Numbers correspond with site locations in the legend and are recorded in the accompanying data table. Image from NPS.

Example of SQM-L Data


If you are able, update your data to community science programs like Globe at Night to further help scientists track night sky quality across the world. Data from dark sites and especially communities are essential in monitoring this natural resource and the presence and intensity of light pollution globally. When reporting data to this application, you can indicate that this is a measurement made in support of IDA advocacy by adding the string “IDA” to the ‘Location comments’ section on the Globe At Night form. For measurements made in support of International Dark Sky Places Program nominations, add the string “IDSP” to the same section.



The Bortle Scale works to estimate sky brightness and interpret how light pollution is affecting your view of night sky phenomenon. The lower the number is, the better sky quality. The easiest way to interpret the Bortle Scale is a descriptive flow chart. Follow the questions to reveal what Bortle Class your skies fall under. This may need to be done at different locations across the location if sky brightness varies between Bortle Classes.

Steve Owens, past Dark Sky Places Committee member, is credited with the flow chart below. Learn more about it on his Dark Sky Diary.

Have little to no experience stargazing? No problem. Visit Sky and Telescope’s Beginner’s Guide to learn more about picking up amateur astronomy skills.

We also recommend looking for advice and help from amateur astronomers and local astronomy clubs near you. You may also search for local IDA Chapters or Delegates to assist in night sky monitoring efforts.



Documenting nighttime phenomenon with photography provides verification that the quantitative SQM-L measurements are accurate. Photographs also help document if light domes are visible along the horizon, which the SQM-L device may not detect.

When providing images to IDA, include a breakdown of the camera details used to take it, i.e. exposure time, focal length, ISO, etc. See the example below.

Photographer: Ameé Hennig
Location: Cerro Pachón, near SOAR Telescope in Chile (GPS location if available)
Phenomenon Shown: Milky Way, Large Magellanic Cloud
Camera: NIKON D3000
Exposure: 17.5 sec (1/0)
Aperture: f/3.5
Focal Length: 18 mm
ISO Speed: 1600
Special Editing: None


These methods in unison have proven successful in providing quantitative data and qualitative assessments of night sky quality. If you have additional questions, please contact the Director of Conservation.

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