JACA Scale

Like most people who own a SQM (Sky Quality Meter) I take measurements almost every night I observe. This is especially useful on holiday to assess the darkness of a site or when you’re determining whether its worth continuing to observe, or whether to call it a night.

During an astro-holiday in 2011 with some friends of the SSA (Almere Observatory), we found that on one particular night all our SQM’s were getting scores of around 21.3 instead of the usual 21.6. The difference in darkness was substantial. I’ve always found the SQM a very handy device but found the nonlinear-scale tricky to interpret. I started talking with Jan van Gastel (who was among our small group of observers) about how nice it would be to have a linear scale of 1-100 so you could assess one score vs another more easily. For instance what is the real difference between a 21.1 night in Holland vs a 21.6 night in the France. Jan (who is pretty good with numbers) thought creating a linear scale wouldn’t be all that difficult and actually made a scale the next morning using excel.

The results were somewhat surprising to me. There is a progressively larger difference in linear scores when SQM scores rise. For example in linear scores there is a less difference between SQM scores 20.8 and 21.1 than there is between SQM scores 21.4 and 21.7. Because the term “linear SQM scale” isn’t very catchy I’m calling it the JACA scale (for obvious reasons and also because its Just Another Calculation Approach).