Unihedron SQM-LU-DL, Operator'S Manual

Page: 14 / 101

2 Measurements
Nobody has performed the task of defining a relationship between the two methods of sky brightness ( x magnitude
skies and magnitudes per square arcsecond) -- probably because one is subjective and the other is objective and a wide
variety of people would have to be polled.
An approximation exists for the conversion between NELM and MPSAS. You can use an NELM converter[5] created
by SQM user K. Fisher to do that conversion, or the chart shown in Figure 2.2 and Equations (2.2) and (2.3).
Figure 2.2: Mpsas vs NELM
Convert NELM (V mags) to MPSAS (B) sky brightness [3]
B
mpsas
= 21 . 58
−
5
×
log (10
(1 . 586
−
N ELM/ 5)
−
1) (2.2)
Convert MPSAS (B) sky brightness to NELM (V mags) [4]
N ELM = 7 . 93
−
5
×
log (10
(4 . 316
−
( Bmpsas/ 5))
+ 1) (2.3)
NSU
Natural Sky Units [1] (NSU) is radiance relative to an assumed natural radiance of 21.6 magSQM/arcsec
2
.
NSU values indicate how much brighter or darker the sky is compared to a typical historic clear night sky. It is defined
here as N SU = 10
0 . 4∆
where ∆ is 21.6 minus the observed value in magSQM/arcsec
2
.
14
Unihedron SQM-LU-DL Operator’s Manual -
20220119