Evening 16 July - SQM readings

Our library has a Starry Sky kit supplied through the Montana Learning Center. It contains a Unihedron Sky Quality Monitor (SQM), a small Planisphere, a small red flashlight, and some documentation Didn't set up a telescope tonight, but took the SQM from the libraries Starry Sky kit out to measure the skies brightness. The observatory walls are up, so I took readings from in there. The walls are 5' tall, so for the first readings I held the SQM just low enough in the by the pier to be sure no outside light. Three consecutive readings at zenith were 21.36, 21.37, and 21.35, for an average of 21.35 mpas. I then did three readings pointed at zenith from the top of the telescope, which is about 6” higher than the wall. These readings were 21.33, 21.27. and 21.28 for an average of 21.29 mpas. I pointed it about 20º above the north horizon and it read 21.14 mpas. Pointed about the same above the south horizon it was 20.06 mpas. Our small town is 1 mile south of us, and there is a truss company just under half a mile south across open fields. They have security lights that are on all night. They do have shields that keep them from shining upward, but since they are mounted above my level the glare is directly visible to me.

I then moved into the backyard where I was completely screened from lights by trees. This gave readings of 21.33, 21.29, and 21.35 at zenith, averaging 21.32mpas. According to the guide in the Starry Sky kit, this is Bortle 4.

The SQM measure brightness in magnitude per arc-second. It uses the same magnitude scale used for stars, so the higher the number the dimmer, or darker, the sky glow is. It averages this reading for about an 80º circular area of the sky.

The Bortle scale is a subjective scale developed by John Bortle and published in Sky & Telescope magazine in 2006. It is an attempt to give amateur astronomers a rough way to gauge how bright the sky is. Because the scale is subjective and the class descriptions are very wide and in some cases overlap, it is a very rough guide. It is still widely used at least in part because it doesn't require any additional equipment.

There are light pollution maps that show estimated SQM and/or Bortle values. Some astronomy weather applications such as Clear Outside will also show them. One issue with the apps is they tend to be a little optimistic and in some cases rely on data that is a year or more old. Light pollution is growing at a phenomenal rate. Ten year ago I didn't have access to an SQM, but the sky was easily a Bortle 2-3.

There is an ongoing discussion about Bortle vs SQM values on the forums. Do a search for “Bortle vs SQM and you will find a lot of interesting discussion.