ORM sky brightness

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The Sky Quality Meter (SQM) is a small device used to measure the luminance of the night sky. The IAC has installed two of them (SQM-L model) at El Teide (Tenerife) and Roque de los Muchachos (La Palma) observatories, Canary Islands (Spain). The  installation and callibration of the SQM, as well as the tools needed for the automatic data acquisition and analysis were developed by the IAC’s Support Astronomer Group, supervised by Rafael Barrena and with the invaluable help from the IAC summer student Miguel Nievas.

A plot with the data collected from the previous night is created every morning. The SQM starts taking data only when the Sun is 10 deg below the horizon, with a frequency of one measurement every 20 seconds, although only a 3 minutes average is plotted.

The plot appearing in the upper panel shows sky brightness (magnitudes per square arcsecond) vs. Solar altitude above the horizon (in degrees). On the top-right corner some information on the Moon phase and labels about data collected before midnight (green) and after midnight (blue) are presented. The bottom plot represents the sky brightness vs. time (UTC) and the discontinous grey vertical lines indicate the sunset and sunrise times while a red shadow shows the time lapse when the Moon is above the horizon.

In general, a global preliminary study made by M. Nievas and R. Barrena concluded that spring and summer nights are darker than autumn and winter nights.

Astronomical effects

Influence of the Sun: The dependence of the sky brightness with the Solar altitude (negative values indicate that the Sun is below the horizon and positive values that is above the horizon) is shown on the top plot. Due to the scattering and diffusion of solar photons by the atmosphere, on the darkest nights, the minimum sky brightness is gotten at the minimum Solar altitude.

Moon: The effect of moonlight is clear on the sky brightness. On bright nights no information appears on the plots, as the sky magnitude is so bright that it is outside the limits of the plot. On grey nights one can see the plots only close or before the Moon rising and close or after Moon setting. Therefore, the sky brightness is higher when the Moon is at higher altitude. Sometimes the moonlight is collected on the detector without shadows and produces some internal reflections that translate into a deeper magnitude decrease.

Milky Way: The high field of view of the SQM (+/-20 deg) allows the detection of the Milky Way when it is close to the cenit. On nights when the Milky Way is at high altitude the effect is maximum and it translates into a decrease up to 0.5 mag in some nights of the year. The effect is larger on summer nights, when the Galactic plane is at its highest altitudes over the horizon.

Atmospherical effects

Thin cirrus: The global effect of thin cirrus is that the sky brightness magnitude decreases significantly because they reflect the moonlight (when present) and/or the city light pollution. On typical nights of thin cirrus small oscillations on the sky brightness can be appreciated. The effect of light pollution is smaller but still evident.

Calima (dust): The effect of high dust layers on these plots is similar to the one produced by thin cirrus. The more dense and inhomogeneous are the dust layers the bigger is the effect on the sky brightness.

Clouds: The effect of high clouds is similar to the effect of cirrus but, due to the fact that they are denser than the cirrus, the reflection of moonlight and/or light pollution is larger. High clouds can affect the sky brightness up to 2 mag.

Fog: On dark nights (or on the dark part of grey nights) a dense fog translates into darker magnitude values (more than 21.9 mag) than what expected in a clear night. On bright nights (or on the bright part of grey nights) a dense fog translates into brighter magnitude values than without fog, owing to moonlight reflections into the fog.