How To Conduct A Sky Quality Survey

How To Conduct A 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 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


Sky Quality Meter (Photo by Unihedron)

The Unihedron Sky Quality Meter (SQM) 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 US$119.99 plus shipping and handling.

The SQM is easy to use: simply hold the device above your head, pointing the photometer at zenith, and click the button. The screen will then display the sky brightness at that point in ‘astronomer’ units (magnitudes per square arcsecond). 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 clear open sky, cloudless, to prevent any sky glow measurements that will deter the accuracy of the device. Also, always take measurements on clear nights 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 and here. (Note that for these calculators you may need to know your latitude, longitude, and local timezone offset from GMT.)

Using this device a “grid” of the locations may be made. It is essential to include areas that will be the darkest and the brightest to achieve a comprehensive survey of the location. The chart below is useful for visualizing how to plan the number and location of a set of measurements. Include measurements of the borders, active areas, and presumed darkest areas of the park. This data can then be organized into a map showing approximate location of measurements and a table. An example table is also shown below.

SQM Average Location Reserve Section (for Reserves only) Notes
21.0 Border Buffer Zone
20.5 Active Area Critical Buffer Brightest Area
21.5 Darkest Core Zone Darkest Area

View a Chinese translation of these instructions: 點擊此處下載指南:  夜空調查說明 (Night Sky Survey Instructions)
Translation courtesy of IDA Beijing / Starry Sky Project of China


The Bortle Scale works to estimate sky brightness and interpret how light pollution is affecting your view of night sky phenomenon. The easiest way to interpret the Bortle Scale is an easy flow chart. Follow the questions to reveal what Bortle Class your skies fall under. This may need to be done at different locations in the location if sky brightness varies, for instance, you may have a Bortle Class between 3 and 4. The lower the number is, the better sky quality.

This method is less quantitative than the Sky Quality Measurement. An application using only this method to document Sky Quality is not as strong as an application using the SQM method or using two or more methods.

Steve Owens, International 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 article “How to Start Right in Astronomy” to learn more about picking up amateur astronomy skills.

Also see Sky & Telescope’s easy guides “Getting Started in Astronomy” for the northern and southern hemispheres for star maps and other information on how to proceed.

We also recommend looking for advice and help from local amateur astronomers. Find local astronomy clubs near you hosted by NASA’s Night Sky Network (USA only).

View a Chinese translation of these instructions: 點擊此處下載指南:  Bortle 暗天空的等级 (Bortle Dark Sky Level)
Translation courtesy of IDA Beijing / Starry Sky Project of China


Like the Bortle Scale method this method is less quantitative than the Sky Quality Measurement. An application using only this method to document Sky Quality is not as strong as an application using the SQM method or using two or more methods.

However, it can be very useful to support the Bortle Scale method and document specific nighttime phenomenon. For example, imaging the Milky Way, the Zodiacal light, and the Gegenschein make for a much stronger use of the Bortle Scale.

When including images also 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

Also take context images of the horizon showing light domes from cities or other sources of light that impact the proposed Dark Sky Place.

This method, along with others, have proven successful. If you have additional questions, please contact the Dark Sky Places Program Manager.

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