Sky Quality Meters (SQM)

SQM

So what exactly is an SQM? SQM stands for sky quality meter. It is an instrument about the size of a cellphone that is pointed straight up to measure how dark the sky is at night. Based on internationally accepted standards, the SQM keeps a record of the measurement of darkness every 15 minutes. About every 60 days, data is offloaded from the SQM to a computer, where it is saved and sent off for analysis.

For you techies, the SQM quantifies the skyglow aspect of light pollution and uses units of “magnitudes per square arcsecond.” It measures the Night Sky Brightness (NSB) at the zenith, in the wavelength range of 390 nm to 600 nm. The view straight up provides 20 degrees of coverage. One SQM covers an area with a diameter of 4.34 miles at an altitude of 65,000 feet.

Why is measuring darkness important? As groups like the Blanco County Friends of the Night Sky work to decrease night time light pollution, it is helpful to know if we are actually achieving our goals. By taking periodic readings of the night sky darkness, we can determine if we are making progress (or losing ground).

SQM Cluster

The Blanco County Friends of the Night Sky is the first organization in the country to set up SQM monitoring stations strategically positioned to capture data on night time light pollution for an entire county. We are currently in the process of getting these monitoring sites established. The progress is reflected in the image below, and we will update this image from time to time. Once the units are installed, we will begin to feed collected data to Dark Sky Texas and the University of Hong Kong for their Globe At Night worldwide view of light pollution. As we progress with this venture, we will update this page as needed.