Published on Nov 30, 2023

The Objective : Skyglow caused by excess light from urban centers obscures the visibility of stars and is an increasing problem for astronomical observations. This experiment determines whether the amount of skyglow can be predicted based upon the angle of observation and a site's distance from an urban center.

I used a digital (CCD) camera to take over 300 60-second time exposures in similar weather and moonlight conditions between September 2003 and March 2004 from sites around San Diego County at distances of 30, 45, 60, 75, 100, and 124 kilometers from the urban center and at angles of 45, 60, 90 (zenith), 120, and 135 degrees.

The images were downloaded and converted into bmp files. I developed a custom computer program to isolate skyglow pixel values by removing CCD noise and star pixels from the images and to compute the average intensity of the skyglow pixels, from 0 to 765, for each image.

Resulting intensities for each site were averaged, graphed, and compared to known functions to determine a best-fit mathematical correlation to the intensity (skyglow) as a function of a site's distance from the urban center and the angle of observation.

The average intensity of the zenith images varied greatly at the six sites, from 32.4 at 30 km from the urban center to 13.6 at 45 km, 7.1 at 60 km, 5.8 at 75 km, 3.9 at 100 km, and 3.1 at 124 km.

Based upon the data, I derived an approximate formula for zenith skyglow value, "S", as a function of distance, "d": S = 2.4 X 10^2 X d^-1.9 per one degree of sky.

The amount of skyglow at 45 to 60-degree angles of observation averaged up to 220% more than zenith skyglow values for the same distance and up to 87% average increase at 120 to 135 degrees.

The amount of skyglow (S) decreased inversely with the distance (d) from the urban center, as approximated by the equation: S = 2.4 X 10^2 X d^-1.9 per one degree of sky, with significantly greater amounts of skyglow for non-zenith angles of observation both towards and away from the urban center.

Applying the formula reveals that observable visible light from stars remains below 50% until over 50 km from a city the size of San Diego and does not improve to 90% visibility until over 115 km from the urban center, indicating an increasing threat to astronomical observations at the nearby Mt. Laguna and Palomar Observatories.

This project examines the effect of urban skyglow on the visibility of stars using computer analysis of CCD pixel data and derives a formula for skyglow as a function of a site's distance from an urban center and the angle of observation.

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