Light Pollution – Perhaps the SQM-LU isn’t the best choice for a mobile setup!

I was considering getting the SQM-LU, the USB lensed SQM meter, for using as a mobile dark sky setup. This would be very similar to my previous attempt where I used a GPS and the IYA lightmeter around Wicklow.

There are a few issues with using the SQM-LU in this way.

The sampling time of the SQM-LU can be between 1 to 10 seconds depending on the darkness of the sky. Used on a mobile platform on a typical road (60 km/h speed limit) a single reading may take up to 170 meters. This poses 2 problems. Firstly where do we define the reading to have been taken from, the center of the range of GPS coordinates sampled over this time would seem to be the best option here. Secondly, we have the possibility of sky brightness varying over this distance due to terrestrial effects; by that I mean oncoming headlights, houses, trees etc., which would skew the result towards a brighter value.

When climbing or descending the meter will not be pointed directly at the zenith. Provided the gradient was constant that would not be a problem, but that’s unlikely to be the case in reality, so the entire sample will not be pointed at the same point of sky. Roads tend to not be that steep in wicklow so the variance should not be much, but it would be there given the possible sample time.

The guide for the previous units whereby the initial result is best discarded should not apply to the SQM-LU. Given the constraints above it would be possible to take single readings every 200m or so. Stopping in lay bys etc. at best tends to give a resolution of 1km, the benefit being that you can pick good locations to sample.

Best practice for using the SQM is to take a set of readings and average them. Taking say, 4 readings, in the mobile set up could take approximately 700m. Interference from cars, trees etc. may pollute some results but these could be determined and disregarded. Determining the location is a bigger problem, as we would need to find the center of the used sample range, which will give us a point with a radius of 350m.

Thinking about these issues it seems that just stopping every kilometer or so and taking proper readings makes more sense, and does not involve spending €200 on a new device that while it seems like a good way to map large areas may not deliver adequate results in practice.

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