Ken, a Sky Quality Meter measures the sky darkness in magnitudes per square arc-second. More info here on the manufacturers website: http://unihedron.com/projects/darksky/ Peter ----- Original Message ----- From: "Ken Reeves" <kreeves@xxxxxxxxx> To: <az-observing@xxxxxxxxxxxxx> Sent: Monday, March 19, 2007 8:08 PM Subject: [AZ-Observing] Re: SQM readings Pardon my ignorance, but what is an SQM and what is the scale of it? Ken Reeves -- See message header for info on list archives or unsubscribing, and please send personal replies to the author, not the list.