At a recent meeting of the Loss of the Night Network (
http://www.cost-lonne.eu/), the participants produced a recommendation for the best practice in taking SQM observations. I've slightly revised the instructions for clarity, and added some additional information below:
We suggest that you only take observations on totally clear or totally overcast, moon-free nights. There is too much variation on partly cloudy or moonlit nights for the data to be analyzed. If you are going out into the cold from a warm room, give the SQM a few minutes to cool off before you begin your observations.
First, push the measurement button 3-4 times before you begin the actual measurement. This is recommended by Unihedron, because the first measurements are sometimes biased as the electronics turn on after a long period of being off.
Next, arrange your body so that you are looking in one of the four compass directions, and take a measurement with the SQM pointing towards zenith. Record the measurement, then turn 90 degrees and repeat the observation. Take four measurements in total, one for each compass direction. If the SQM-L is being affected by stray light or EMF, this may minimize or reveal the effect.
If the four observations are not self-consistent (maximum range about 0.2 mag
SQM/arcsec
2), then it is probably not a good measurement location, and the data should not be recorded.
If the measurements are consistent, record the sky brightness as the mean (average) of the four measurements.
Please share your observations with the scientific community! There are several easy ways to do this:
1) Smartphone apps: You can submit your observations using the free "Loss of the Night" app:
Android:
https://play.google.com/store/apps/details?id=com.cosalux.welovestarsiOS:
https://itunes.apple.com/en/app/loss-of-the-night/id9284405622) GLOBE at Night via smartphone or regular computer:
http://www.globeatnight.org/webapp/3) GLOBE at Night via spreadsheet: If you are planning on taking a lot of handheld observations, you may prefer to record them all and then pass the data on to GLOBE at Night at once. If you do this, please ensure that your sheet includes the following data, preferably in this order:
latitude, longitude, day, month, year, hour, minute, SQM value, SQM serial number, cloud cover, (optional sky comment), (optional location comment), country
Please record cloud cover as "clear" or "overcast".
Send your spread sheet to Dave Bell at GLOBE at Night. His email address is dbell /at/ noao.edu
Why is it important to record observations? You probably already know this, but the way the world is being lit is changing, and many communities are adopting LED lights. No one knows whether this is going to make the sky brighter or darker, and if we have information about how skyglow is changing in different communities, we may be able to influence future lighting changes for the better.
Notes:
1) The technique of making 4 measurements with different orientations was suggested to us by Andreas Hänel.
2) If you're curious about whether or not handheld measurements from different people and meters are useful for science, you can read our report analyzing 4 years of GLOBE at Night handheld SQM data:
http://www.nature.com/srep/2013/130516/srep01835/full/srep01835.html?WT.ec_id=SREP-704-20130603
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