Unihedron SQM-LU, Operator'S Manual

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2 Measurements
The Milky Way
As one goes to sites with darker surface brightnesses, the fraction of the total light received by the SQM-LU which can
be attributed to the Milky Way bulge increases and so the “offset” in mpas will be larger (due to the Milky Way.)
The northern view of the Milky Way contributes about 0.10 mpsas under 21.5 mpsas (moonless) skies.
The southern view of the Milky Way might be as big an effect as 0.56 mpsas in dark skies where it goes near-overhead.
For more information, see Surface Photometries of the Milky Way (Sc 1997)
vizier.u-strasbg.fr/ftp/cats/VII/199/ReadMe
Moisture
Clouds, fog, and mist will reflect artificial light back down to the Earth causing a brighter (lower) reading. If there is
no artificial light, then clouds may prevent starlight from coming to you and the reading will be darker (higher). This
extra-dark situation can occur in very isolated areas like mountain tops, the ocean, or the desert. You will have to be
aware of this special situation when analyzing readings.
Volcanic eruptions
Dust released into the atmosphere by volcanoes can reflect light from the surface of the earth back down. In a dark
location this dust will prevent the light from stars and Milky Way and produce a darker (higher) reading.
Zodiacal light
The sunlight reflected of off dust particles in the ecliptic plane of our solar system is called zodiacal light.
The amount of light will be different depending on whether the meter is pointed to the poles or plane of the solar
system. It is likely to have less than 2% effect. The primary reason for this small effect is that the brightest and widest
part of the zodiacal light is nearest the horizon where the SQM has almost no sensitivity (due to it being a primarily
zenith-looking device). The portions at higher altitude are the narrowest and faintest and they would barely creep into
the sensitivity cone of the SQM.
2.1.3 Other luminance scales
Candela per square meter (cd/m
2
) is commonly used by lighting engineers.
To convert the SQM mpsas reading to cd/m
2
, use Equation (2.1):
[ cd/m
2
] = 10 . 8
×
10
4
×
10
(
−
0 . 4
×
[ mag/arcsec
2
])
(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 that
case you can see 6th magnitude stars and nothing dimmer like 7
th
magnitude stars. The term “6
th
magnitude skies” is
very subjective to a persons ability to see in the night, for example an older person might say “5
th
magnitude skies” but
a young child with better night vision might say “7
th
magnitude skies”.
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).
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Unihedron SQM-LU Operator’s Manual -
20170728