To convert the SQM mpsas reading to cd/m2, use Equation (2.1): [cd/m2] = 10.8 × 104 × 10(−0.4×[mag/arcsec2]) (2.1) Naked eye limiting magnitude (NELM) Quite often astronomers will refer to a sky by the darkest star they can see, for example a "6th magnitude sky", in...