Quality Meter (SQM)
A Sky Quality Meter (SQM)
from Unihedron is a hand-held instrument that allows individual observers to quantify the sky brightness.
To use the meter, simply hold
the SQM overhead while it is aimed at the zenith and press the red button.
After a few seconds the unit will beep and a number is displayed.
Basically, the larger the number the darker the sky. A meter reading of 21.00 would indicate a very dark site, while a
reading of 16.00 would indicate a degraded, light polluted sky. It's that easy.
The SQM measures how much light,
coming in from a cone of sky, strikes a sensor. For older models, the
"funnel" is about 80
degrees wide; less than 30 degrees for newer models. The meter then converts that amount
of light into units of magnitudes per square
arc-second, which are admittedly not easy to visualize. For the
purpose of Night Vision, we will create a simpler
name for the SQM units, to be called "squims." For
example, if the meter reads 18.75 magnitudes per square arc-second, we will
describe it as "18.75 squims."
Observers must insure that nearby lights are not within the detection cone of
the sensor. Similarly, the light
of the moon should not influence the sky's darkness, which is indicated by
the Darkness line of a Clear Sky Clock.
readings should be taken after astronomical
twilight, when the sun does not contribute any light to the night
Brightness with an SQM
magnitudes per square arc-second, or "squims")
SQM readings paired with a Clear Sky Clock are at:
See also the SQM
Database of global measurements ("Suburb back yard, IN").
is more information on "magnitudes per square arc-second," including
the excerpts below:
"The practical minimum of the natural background radiation is ~21.6 magnitude
per square arcsecond," per authors
Kohei Narisada and Duco Schreuder. Source:
Light Pollution Handbook,
How Dark Can the Night Sky Get? by Brian Skiff
"The widely accepted value for sky brightness at the zenith at a site
completely free of man-made light sources and near solar activity minimum is: V
mag. 22.0 per square arcsecond = mag. 13 per square arcminute. In other words, a
perfect site has a sky brightness equivalent to having a mag. 22 star in every
square arcsecond box (hardly bigger than the star image itself) over the entire
Light Pollution in Southern Ontario by Richard Berry includes Tables I
and II, which describes the appearance of the sky for given SQM values (in magnitudes per arcsecond
squared) and suggests an equivalent unit (tenth magnitude stars per square
degree) for visualizing magnitudes per arcsecond squared.
From the Journal of the Royal Astronomical Society of Canada, Vol. 70, No. 3,
June 1976, Whole, No. 540; reprinted courtesy of Toronto Centre, R.A.S.C.
Astronomer Ferdinando Patat notes the value in measuring sky glow with an SQM in
the late evening after astronomical twilight.
|From the PHM Clear Sky Clock at http://cleardarksky.com/c/PnnHrMPINkey.html:
The line labeled darkness is not a weather forecast. It shows
when the sky will be dark, assuming no light pollution and a clear sky.
Black is a dark sky. Deep blue shows interference from moonlight. Light
blue is full moon. Turquoise is twilight. Yellow is dusk and white is
daylight. For those who prefer numbers, the scale is also calibrated.
The numbers are the visual limiting magnitude at the zenith. (The
brightness of the faintest star a standard observer can see straight
up.) Mouse over a darkness block for details.
It is based on Ben Sugarman's Limiting
Magnitude calculations page. It takes into account the sun's and
moon's position, moon phase, solar cycle and contains a scattering model
of the atmosphere. It doesn't consider light pollution, dust, clouds,
snow cover or the observer's visual acuity. So your actual limiting
magnitude will often be different.
A. Danko, creator of the Clear Sky Clock, writes:
"Accurate SQM readings should be had only when the clock's darkness
line reads 6.0...The darkness line calculates a theoretical sky brightness
that does not consider light pollution. But it does consider the phase of
the moon, altitude of the moon and altitude of the sun...It clearly shows
that even a gibbous moon can have very little effect when it is at only a
few degrees altitude. The altitude at which a given phase of the moon
affects the brightness at zenith varies in a complex way which the darkness
model takes into account...While the 6.0 number the
"darkness" line calculates for a sky free of scattered moonlight
and sunlight is not quite right (mostly because vision varies hugely from
person to person), it's a very good indication of when the sky will be at
minimum brightness from light from the sun and moon."
Copyright ©2006 A. Danko; used with permission.)
SQM, courtesy of Unihedron, is featured at a light pollution workshop.