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 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:
- Measure different remote sites to determine which is better.
- Check new sites to compare to your usual site.
- Check from night to night and hour to hour to steer your observing session. If you discover your site is darker after midnight, wait until then to push your scope to its limits. If the night is poor, spend it observing the brighter, more easily seen, objects and leave the faintest DSOs for better nights.
- Compare your readings over time with the Bortle Scale to see how it compares to your site and to put actual numbers on the scale. I've determined that, at my site, mag.21.3 corresponds to about the Bortle Class 3.
- Send your data to the International DarkSky Association. With enough data, they can target the worst offenders to make the biggest difference in sky brightness.
- You can accurately gauge how bad your home is, compared to your favorite dark site. If the difference is small, push yourself to observe at home in between outings to the dark site. If, like me (3.5 to 3.9 magnitudes brighter skies at home), your home is really poor, start a lunar observation program at home that you wouldn't waste your time on when you travel to that dark site.
- Keep a log of your site over the years. This will document the encroachment of lights, and may also display a Solar Cycle variation in readings.
- Eliminate the subjective evaluations of night sky brightness and make your notes more useful. If the brightness is noted in your observation notes, this may tell you what's visible in certain skies.
- Compare your data with the Light Pollution maps on sites such as the ClearDarkSky Clocks site. You may be able to put a number with the color that is better defined than the estimates they used, and you may be able to see a result from altitude changes.
- By using a technique (soon described), you can measure your site in different directions of the compass. This can tell you if you need to find a site a little farther in one direction, or even as little as tell you where you need to set up light shielding.
- Save time in observing by getting a sky brightness measurement in only a few seconds, rather than waste precious observing time counting stars.
How does it work, and how can I best use it?
The SQM measures brightness in an 80 degree wide cone. If pointed at the zenith, it will measure the average sky brightness down to +50 degrees off the horizon in every direction. This is its strength and its weakness. It truly gives you an idea of how bright your overall sky is. You cannot aim the unit lower than +40 degree altitude and not have the horizon cut off the light cone, giving a false reading. But, if you have more light pollution in one direction than another, the inclusion of the brighter section can penalize the section of the sky that is darker.
You can measure the brightness of a smaller section of sky by doing the following:
- Use a paper-towel tube cut in half (or a TP tube).
- measure the sky brightness without it.
- hold the tube in front of the sensor and make another sky reading, pointing in the exact same direction.
- subtract the difference. You will have to add this amount to the readings you take, wherever the SQM is pointed, to convert to the calibrated reading at the zenith with the full light cone.
For example, say I measure 21.1 at the zenith with a full light cone, and 22.1 at the zenith with the tube in place. Every direction I point the SQM, I will have to add 1.0 magnitudes to my readings to be able to compare it with the calibrated reading. If you never point lower than, say, +50°, the tube will be unnecessary.
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.
Careful to not let nearby trees or buildings cut off some of the field or the reading will not be accurate.
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 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.
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.
Why did I avoid getting this? Its great! 
