Note: This page is available for download as a pdf.

Using Sky Quality Meters to Measure Sky Brightness

Contents

Credits: This document on citizen-scientists using Sky Quality Meters to monitor light pollution levels in their community was a collaborative effort between Connie Walker at the National Optical Astronomy Observatory, Chuck Bueter of nightwise.org, Anna Hurst of ASP's Astronomy from the Ground Up program, Vivian White and Marni Berendsen of ASP's Night Sky Network and Kim Patten of the International Dark-Sky Association.

Observations using the Sky Quality Meter (SQM)

The Sky Quality Meters (SQMs) add a new twist to the GLOBE at Night program. They expand the citizen science experience by making it more scientific and more precise. The SQMs allow citizen-scientists to map a city at different locations to identify dark sky oases and even measure changes over time beyond the GLOBE at Night campaign. This document outlines how to make and report SQM observations.

Important parts of the SQM

Using the SQM

There are two models of Sky Quality Meters. Information on the newer model, the SQM-L, can be found along with the instruction sheet at http://unihedron.com/projects/sqm-l/. Information and the instructions sheet on the older SQM model can be found at http://unihedron.com/projects/darksky/.

Both meters are easy to use. The new model (SQM-L) incorporates a lens for collecting more light from a narrower field of view, specifically for city use where surrounding lights or buildings may affect the reading. This narrower field of view is a coned-shaped area in which the detector can "see". The cone is ~20 degrees wide for the SQM-L as opposed to ~84 degrees wide for the SQM. Other differences between the two models include location of the display and the type of push-button. In the SQM-L model, the display is on the same side as a flush-mount push-button, which allows sequential readings to be taken without altering the orientation of the meter. In the SQM model, the display is on the same side as the sensor.

There are five basic steps to participation in GLOBE at Night through SQM measurements.

  1. Find your latitude and longitude.

    There are a few different methods you can choose from:

  2. Go outside an hour after sunset (about 7-10 pm).

    • Observations should be taken on a clear night when the Moon is not up and when the Sun has set for at least an hour. The Sun and Moon would otherwise contribute to the SQM's sky glow reading.
    • Optional — It is best to wait until after astronomical twilight for total natural darkness before taking SQM measurements. To get the times for astronomical twilight for your location as well as Moon phase and Moon rise and set times, see http://www.sunrisesunset.com/custom_srss_calendar.asp.
    • The meter is somewhat temperature dependent. Leave the meter outside for at least 5 minutes to reach ambient temperature before taking any measurements.
    • Avoid using the SQM or SQM-L near lights like streetlights and in areas that are shaded by trees or buildings. A rule of thumb for the SQM is to be as far from the object as it is high. For streetlights, stand at least 7.5 meters (or 25 feet) away, if possible.
  3. Take a measurement.

    • Point the SQM or SQM-L directly overhead (at the zenith). The meter should be held above head level so that shadows or reflections from your body do not interfere with the reading. Keep it steady.
    • Press the start button once to take a reading. The SQM will beep each second while it is accumulating photons (light particles). The beeping will stop once the display reading is ready to be viewed. Under urban skies, a reading will be displayed almost immediately. Under the very darkest skies, the meter may take up to a minute to complete its measurement.
    • You must also take and submit an unaided-eye observation toward Orion for each SQM or SQM-L measurement submitted.
    • (Optional:) Press the start button a second time. The temperature in C° and then F° is displayed. Then the model number of your meter (1.17) and the serial number are displayed.
  4. Report your measurement.

    • Measurements can be submitted on-line at http://www.globe.gov/GaN/report.html during the campaign and up to a week afterward.
    • The observation date, the local time, the latitude and longitude, the SQM reading and the SQM serial number are to be filled out on-line. If you do not have an Orion magnitude chart estimate to submit, select "Cloudy Sky/No Data", no matter what your nighttime sky condition really is.
    • For multiple measurements, see the section entitled "A Data Template for Multiple SQM Measurements".
    • Paper versions of the observation forms as well as additional information are always available in either the downloadable Teacher or Family Packets at http://www.globe.gov/GaN/observe.html.
  5. Compare your observation to thousands around the world.

What do the numbers mean?

The SQM or SQM-L measures how much light strikes the sensor. The meter then converts that amount of light into units of magnitudes per square arc-second. The larger the number read on the meter display, the darker the sky. A meter reading of 21.00 would indicate a very dark site, while a reading of 16.00 would indicate a light polluted sky. For more information on the meaning of the units used by the SQM, see: http://www.nightwise.org/magnitudes.htm.

To compare the SQM measurement with the corresponding magnitude chart chose for the unaided-eye observation, go to http://members.csolutions.net/fisherka/astronote/plan/tlmnelm/html/NELM2BCalc.html. You will see an input box labeled "NELM". That is where you put the number for the magnitude chart you chose (e.g., the limiting magnitude). Click on "Calculate" and a value for the SQM measurement (labeled "MPSAS") will appear in the input box to the right. Similarly, on the next line, you see an input box labeled "MPSAS". That is where you put the value for the SQM measurement. Click on "Calculate" and the limiting magnitude will appear in the input box to the right. How well do the values calculated compare to the values observed or measured?

Typically in a city, limiting magnitudes observed have values of 3 or 4. These correspond to SQM measurements of 16.88 magnitudes per square arcsecond or 18.04 magnitudes per square arcsecond. Most people cannot see beyond a limiting magnitude of 6 which corresponds to almost 21 magnitudes per square arcsecond and a truly dark site.

Demonstrating Light Pollution and Shielding

This demonstration will illustrate the effects of lighting on our view of the night sky. It will also demonstrate how shielding can reduce light pollution while making the lighting more effective. All demo materials are off-the shelf items. They are also provided in the GLOBE at Night Light Pollution Kit. This demonstration is adapted from an activity on the Paper Plate Education website: http://analyzer.depaul.edu/paperplate/lights.htm.

Materials

Background — Light Pollution

There are three main types of light pollution:

These images, as well as others illustrating the effects of different types of lighting, can be found on the Nightwise website: http://www.nightwise.org/examples.htm.

More background on light pollution can be found on the GLOBE at Night website: http://www.globe.gov/GaN/learn_light.html.

Background — Lighting

Quality lighting should:

Keys to quality lighting:

Shielding can maximize the desired effects of lighting by controlling the light output and minimizing the glare, light trespass, and light that travels straight up, as illustrated below.

Doing the Shielding Demonstration

Wrap Up

With your audience/students/family members/other citizen-scientists, discuss your observations and the benefits of shielded lighting. Note that shielded lighting improves our view of the stars (reduces sky glow). It is also safer and more energy efficient, since it directs light down to the ground, where we want lighting, rather than up into the sky. This improves the quality of life for people, animals, and plants, saves money and energy, and preserves our beautiful dark skies.

Capstone activities

For capstone activities, interested citizen-scientists may choose to expand the SQM measurements beyond the campaign period. There are different types of SQM measurements one can take during the rest of the year — from grid surveys of ones town on one night (repeatable every month), to seasonal surveys, to surveys every hour over a night at one or more location, to surveys of 8 cardinal directions along ones horizon (concurrent with regular SQM measurements or measurements at different angles along ones meridian).

Another type of capstone activity encompasses the analysis of the maps. Analysis of the maps could include a comparison with other data sets such as the last couple of years of SQM data and the limiting-magnitude unaided-eye observations, population density, satellite data on nighttime lights (top view looking down versus the SQMs bottom view looking up) and regional environmental concerns (e.g., sea turtles in Florida). GLOBE at Night will endeavor to place the first four of those data sets on-line. The Map Viewer on the GLOBE at Night results webpage enables the citizen-scientist to zoom into an area of interest using these data sets.

Other capstone activities involve citizen-scientists meeting with their local officials to increase their awareness of dark skies preservation issues and to promote the use of compliant fixtures and shielding. For more information on these types of activities, visit the International Dark-Sky Association at http://www.darksky.org. You may also want to encourage others to take part in citizen science projects to measure light pollution (see resources below) by writing letters to the local newspaper, government, and businesses to encourage the installation of shielded lighting in their community.

Online Resources

A Data Sheet for Multiple Measurements

For 500 measurements or more, a data sheet as an Excel spreadsheet is available for either Orion data and/or SQM data. Download the Excel file from: GaN_DataSheet.xls. Once you have entered your measurements and other parameters, send the Excel file to cwalker@noao.edu by March 15, 2011 for the first campaign, and April 15, 2011 for the second campaign.

For less than 500 measurements, you may use the Excel spreadsheet or the pdf file to record data, but you must submit the data yourself by individual entry on-line using the report page at http://www.globe.gov/GaN/report.html. Download the excel file from GaN_DataSheet.xls or the pdf file from GaN_DataSheet.pdf.

Here is an explanation of the columns in the data sheet. Do not fill in the "Tempid" or the "Submitdttm" columns. Columns to fill in include the latitude, the longitude, the observation date and local time, the magnitude chart # for the unaided eye observation toward Orion, and the cloud cover estimate. The optional columns include the comments columns, the SQM measurement, and the SQM serial number. However, we are hoping you will have SQM measurements to submit. (Helpful notes: you must submit an Orion observation for every SQM measurement submitted. Also, citizen-scientists can use the tool on the on-line report form to get the latitude and longitude of their observations. Be careful when using the "-" and "+" signs in the latitude and longitude entries.) An example of a data entry is given below.

Key to the Columns for the Excel or PDF Data Sheet:

Note: This page is available for download as a pdf.

Credits: This document on citizen-scientists using Sky Quality Meters to monitor light pollution levels in their community was a collaborative effort between Connie Walker at the National Optical Astronomy Observatory, Chuck Bueter of nightwise.org, Anna Hurst of ASP's Astronomy from the Ground Up program, Vivian White and Marni Berendsen of ASP's Night Sky Network and Kim Patten of the International Dark-Sky Association.

Questions? Email Connie Walker at cwalker@noao.edu.

Happy star-hunting!