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What is it?
It is a hand-held sky brightness measuring meter that gives a reading of the night sky's brightness in magnitudes per square arc-second, the most common measurement figures used by professional astronomers.
Why would I want one when I can just look at the sky and estimate the naked-eye limiting magnitude using star counts in selected areas?
Well, the problem is your vision and mine will vary. We may determine different limiting magnitudes because of visual acuity, yet the night sky has a true, objective, brightness that is independent of our abilities to estimate it. The Unihedron SQM-L doesn't get tired or give different results if you forget your glasses.
Other than measure the sky brightness, what can I do with it?
Here are some ideas:
How does it work, and how can I best use it?
The SQM-l measures brightness in a 20° wide cone. If pointed at the zenith, it will measure the average sky brightness down to +80 degrees off the horizon in every direction. This is its strength and its weakness. It truly gives you an idea of how bright your ZENITH sky is. You can aim the unit at +40 degree altitude and not have the horizon cut off the light cone, giving a good reading. If you have more light pollution in one direction than another, the SQM-L can effectively measure the degree of it.
How accurate is it?
An ISTIL International report by Cinzano shows that it is very accurate. If you wish to convert to V magnitude (for certain magnitude comparisons), you have to subtract 0.17 magnitude from your reading (21.0 becomes 20.83), +/- 0.07 magnitudes. In Moonlit or light polluted skies, you'd subtract 0.11 magnitudes to yield the V magnitude (21.0 becomes 20.89) +/- 0.14 magnitudes.
More importantly, to convert to a point-source brightness, you need to add 0.3 magnitudes to the reading (21.0 becomes 21.3). This is the most important characteristic to remember when figuring out the NELM at your site.
Are there problems with the figures you get?
To a certain degree. A truly dark site with clouds (no light nearby to reflect from the clouds) will read darker than a site without clouds. The Milky Way overhead will add 0.1 to 0.2 magnitudes (read brighter, or a lesser number) to the brightness compared to a period on the same night when the Milky Way has rotated out of the way. Duh. Anyone could have told you that, because we all know the Milky Way makes the sky brighter at a dark site. This is probably a reason to note the magnitude measured on an hourly basis when using the meter, and noting where the Milky Way was in the sky.
Also, brightness does not equal transparency. I've measure nights of relative darkness that had more horizon extinction and muted Milky Way appearance than other nights of the same measured brightness. It's obvious that we need both darkness AND transparency (or Clarity, as I call it) in order to have superior views. But, in normal, slightly light-polluted sites, the presence of water vapor in the sky causes a general sky brightening and a diminution of transparency at the same time. In such sites, the measured brightness will more directly correspond to what you can see through a scope.
Any final comments?
I have found the SQM-L to be easy to use and its readings to correlate with my personal evaluations of the darkness of my observing site. The more of us who have and use one, the more information we will have about the nature and growth of lighting that surrounds us. Having objective measurement figures for a host of sites all over the world will allow us to more effectively target the sources of light pollution and to effectively avoid the worst of it when we travel to darker observing sites.
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