2.1 Getting accurate readings
3. There are 60 arcseconds in an arcminute, and 1296000 arcseconds in a circle.
Square arcsecond
(arcsec
2
) is the area covered by a square measuring 1
arcsec
×
1
arcsec
.
Magnitude per square arcsecond
is the definition of brightness in magnitudes spread out over one square arcsecond
of the sky. For example; if the SQM provides a reading of 20.00 mpsas, that would be like saying that a light of a 20
th
magnitude star brightness was spread over one square arcsecond of the sky.
The ”magnitudes per square arcsecond” numbers are commonly used in astronomy to measure sky brightness. More
details can be found at www.stjarnhimlen.se/comp/radfaq.html
Each magnitude lower (numerically) means just over 2.5 times as much more light is coming from a given patch of sky.
A change of 5 mags/sq arcsec means the sky is 100x brighter.
Also, a reading of greater than 22.0 is unlikely to be recorded and the darkest we’ve personally experienced with the
SQM is 21.80.
2.1 Getting accurate readings
Various factors will cause the night sky brightness to fluctuate. Taking more readings will be useful in ruling out spurious
events. The SQM gathers light for at least a one second period, and the brightness report is based on the light that was
accumulated during that time.
At the darkest sites, natural variations in conditions such as airglow and the brightness of the zodiacal light are limiting
factors.
Prevent artificially high (dark) readings
by ensuring that there is nothing blocking the view of the sensor. Avoid taking
readings near trees or buildings that may block the sensor.
Prevent artificially low (bright) readings
by ensuring that there are no lighted objects (street lamps, the moon, etc.)
that shine into the sensor at any angle.
2.1.1 Seeing conditions
The apparent blurring/transparency and twinkling (scintillation) of stars is due to wind in the upper atmosphere that
causes water molecules to distort the light from space.
Stars are too small in comparison to the entire SQM field of view, so scintillation is not expected to alter the SQM
reading significantly.
2.1.2 Light pollution
Undesirable artificial light that reaches you is considered to be light pollution. Much of this light comes from outdoor
illumination of parking lots, street lamps, office buildings, advertising signs, etc..
Other causes of extra light in the night sky are listed below:
Aurora
Charged particles emitted from the Sun are directed to the poles of the earth by the Earth’s magnetic field. These particles
collide with atoms in the atmosphere and cause light to be emitted. Aiming the meter at the polar regions during Aurora
Borealis (in the North) or Aurora Australis (in the South) will reduce the reading (lighter). Aiming the meter towards
the equator will increase the reading (darker) under these conditions.
Airglow
Airglow is light produced by various phenomenon in the atmosphere which prevent the sky from being totally dark. Effects
of the magnetic poles of the Earth may cause airglow to be brighter near the poles.
Unihedron SQM-LU Operator’s Manual -
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