Focus Scientific

Unihedron Sky Quality Meter L

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.

Uses

  • Find out how good the night or site REALLY is.
  • Compare the sky brightness at different sites quantitatively.
  • Document the evolution of light pollution in your area.
  • Set planetarium dome illumination to mimic the skies people are likely to experience elsewhere in the city.
  • Monitor sky brightness through the night, night-to-night, and year-to-year.
  • Determine which nights show the greatest promise for finding the 'faintest fuzzies'.
  • Calibrate the effect of sky brightness on qualitative measures such as the Bortle Scale.
  • Investigate how sky brightness correlates with the solar cycle and month-to-month sunspot activity.
  • Help provide local ground truth for future sky brightness prediction with the Clear Sky Clock.
  • CCD users can make a correlation between the SQM reading and when the background reaches some ADC level.

Features

  • Audible signal while measurement is in progress.
  • Sky brightness displayed in visual magnitudes per square arcsecond.
  • Infrared blocking filter restricts measurement to visual bandpass.
  • Temperature in both Celsius and Fahrenheit as well as model number and serial number can be displayed with different button press sequence.
  • Precision readings at even the darkest sites.
  • Power-saving features designed in for maximum battery life.
  • Reverse battery protection.

Specifications

  • The Half Width Half Maximum (HWHM) of the angular sensitivity is ~10°. The Full Width Half Maximum (FWHM) is then ~20°. The sensitivity to a point source ~19° off-axis is a factor of 10 lower than on-axis. A point source ~20° and ~40° off-axis would register 3.0 and 5.0 magnitudes fainter, respectively. SQM vs. SQM-L comparison chart.
  • Maximum light sampling time: 80 seconds
  • Power: one 9V battery (included)
  • Dimensions: 92 x 67 x 28mm (3.6 x 2.6 x 1.1")
  • Weight: 140g (0.31 lbs)
  • Specifications subject to change without notice