J. Zamorano, A. Sánchez de Miguel, F. Ocaña, B. Pila-Díez, J. Gómez Castaño, S. Pascual, C. Tapia, J. Gallego, A. Fernández, M. Nievas"Testing sky brightness models against radial dependency: A dense two dimensional survey around the city of Madrid, Spain" (2016) JQSRT in press doi:10.1016/j.jqsrt.2016.02.029

Night Sky Brightness and Light Pollution in Comunidad de Madrid Poster in PDF
J. Zamorano, A. Sánchez de Miguel, J. Gómez-Castaño, F.Ocaña, J. Gallego, B.Pila, M. Nievas, C. Tapia, A. Fernández and S. Pascual
Light Pollution : Theory, Modelling, and Measurements LPMMT2013
April 15-18 2013, Smolenice, Slovak Republic

Abstract

We are carrying on a study of the night sky background brightness around the city of Madrid using Sky Quality Meter (SQM) photometers.

Data retrieval methodology includes an automatic procedure to measure from a moving vehicle which allows to speed up the data gathering. The night sky brightness, an astronomical quality parameter that accounts for luminous flux from the sky, is closely related with the light pollution.

The map with the spatial distribution of the night sky brightness around Madrid has been compared to the light pollution as measured with calibrated satellite imagery and nocturnal images taken by astronauts aboard the International Space Station (ISS).

The community of Madrid (Spain) is located at the center of the Iberian Peninsula (Castilian Central Plateau). Its capital is the city of Madrid. It has a population of 6.4 million mostly concentrated within the metropolitan area of Madrid.

Observations

We are using the Sky Quality Meter (SQM) photometers that have been cross-calibrated to determine zero offsets at the Laboratorio de Investigación Científica Avanzada (LICA) of the UCM.

Time and spatial coordinates of the data are obtained with computer linked GPS. The photometer measures the night sky brightness at zenith from the top of moving vehicle. SQM and GPS readings are recorded with a laptop.

The results of every track is archived in a CSV file with brightness data, coordinates and time (one point every 10-15 seconds). Sociedad Malagueña de Astronomía has developed a program (Road Runner) that records all the information in one CSV file and creates a KML file on the fly.

Data and Processing

Sky brightness measurements are obtained during absolutely clear nights (no clouds of any kind) and with no Moon over the horizon. To date we have more than 15,000 points. SQM magnitudes are transformed to fluxes and averaged in square cells 2 arcmin wide. The SQM data is continuously recorded even inside villages or when the light from lamp posts is contaminating the sky brightness measure. We do not use any kind of light baffle. We filter out spurious data with the information recorded by the observer and that obtained from KML maps. We check visually each track on Google Earth and flag rejected data points.

Caveats

• The final map is built with data taken in different nights with three SQM photometers that have been cross-calibrated to remove zero offsets.
• The SQM band covers Johnson B & V bands.
• Transformation from SQM magnitudes to astronomical bands depends on the sky color (see Cinzano report “Night Sky Photometry with Sky Quality Meter”).
• Sky color is related with the type of light sources that contribute to light pollution. Sky brightness varies during the same night both in color and intensity.

Results

  Figure 5: Night Sky Brightness map around Madrid. The survey is still in progress. Main villages and cities are labeled. The magnitudes are displayed in the SQM band which comprises the Johnson B and V astronomical photometric bands.

Work in progress