Unihedron SQM-LU-DL, Operator'S Manual

Page: 12 / 101

2 Measurements
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.
The darkest we’ve personally experienced with the SQM in a natural clear sky was 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.
A reading of greater than 22.0 is unlikely to be recorded, however there are reasons for extra dark readings:
•
Cloud or ash covered sky in a remote area where little natural sky brightness can be recorded.
•
Uncorrected readings from covered meter like a weatherproof housing.
•
Obstructed view like inside a forest, or where large buildings are located within the field of view, or a person or bird
blocking the view.
When the sky is very dark, the meter takes longer to gather enough light to produce a reading. During this time
of light gathering, the reading will remain constant until the meter can start gathering readings fast enough for new
reading-requests.
The readings are averaged in an 8-cell rolling buffer, so it may take a while for the meter to produce a new value of
darkness when obstructions leave the field of view.
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
Scintillation is due to refractive changes in the atmosphere caused by temperature changes at differing heights.
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:
12
Unihedron SQM-LU-DL Operator’s Manual -
20220119