An affordable meter for measuring sky brightness for astronomers.
The Sky Quality Meter L (Lens model) measures the brightness of the night sky in magnitudes per square arcsecond (MPSAS), and offers unprecedented sensitivity in a handheld meter.
Some interesting facts...
A perfectly dark country sky of 22 MPSAS is 4.2 magnitudes per square degree. Integrated over the full 180 degree hemisphere of the sky, and taking into account the brightening toward the horizon, the light from such sky glow alone amounts to just about -8 magnitude! That's nearly 30 times brighter than Venus.
How does the light from all the naked-eye stars (to 6.5 magnitude) compare? Collectively, a 180 degree hemisphere of such stars amount to about -4.2 magnitude (just about as bright as Venus). As we've already seen, a perfectly dark sky approaches 30 times brighter. And so we see that the light from resolved stars--even in a perfectly dark sky--is but a small fraction of that from just the sky glow alone.
Under a full Moon, or from the downtown core of a large city like Toronto or Montreal, the sky brightness is around 17 MPSAS. This is 5 magnitudes, or a factor of 100 times brighter the ideal 22 MPSAS sky. Which is why light pollution filters are practically mandatory when seeking out even the brightest nebulae, for example.
Having seen how comparatively minor is the contribution of starlight to sky brightness, we see that when even as comparatively restricted sensor a field of ~20 degrees for the SQM-L is employed, our measurements of sky brightness can be used with some confidence. Even when the field includes a few of the brighter stars.
Now, the milky way is another matter. Its areal extent is considerable, and often can fill much of the meter's field of view. As can be appreciated, that it is visibly brighter than the adjacent sky necessarily implies something of an increase in measured sky brightness. Hence the good practice to strive to keep the milky way well enough out of the field.
