[SQM] Clear Sky Detection experiment

[Anthony Tekatch]

Hello All,

We are currently experimenting with a technique to determine a "clear
sky" using the SQM-LE/LU.

A description by Dr. Doug Welch is here:
  http://unihedron.com/projects/sqm-le/csd/sqmle_feb2010.html

Basically, we have noticed that the light fluctuates more when clouds are
overhead.

An annotated example is here:
  http://unihedron.com/projects/sqm-le/csd/example.png
The Moon was visible for the first half of the night and hence the reason
for the non-zero slope.

A live experimental plot is shown here:
  http://unihedron.com/projects/sqm-le/csd/sqmleg.gif

Using the SQM-LE/LU like this will most easily detect low clouds when
either the moon is up or there is some light pollution present.

The formula we are using for the above plot is fairly simple. We use an
array of the last 10 readings which have been taken one minute apart. A
sum of the absolute differences between each reading is a pretty good
indicator of a clear sky (0=clear, above 1 = cloudy).

Best regards,
Anthony Tekatch
Unihedron