Using a Digital Camera to Measure Skyglow
Julia Huang, Alina Khataw
Floor Location : M 018 V
In this experiment, we used a manually controlled camera and tripod to take pictures of the night sky to measure skyglow. The purpose of the experiment was to find out more about how light pollution and skyglow varies at different locations across Richmond. Our hypothesis was that if we compare the skyglow of some locations near Central Richmond and more rural areas on clear nights, the skyglow values of areas near Central Richmond would be many times higher than the rural areas. Our question was: How does distance away from Central Richmond affect the skyglow value of that location?
The procedure of our experiment consisted of three parts: calibrating the camera, taking the skyglow pictures, and measuring the skyglow values of those photos. We calibrated our camera by taking two sets of photos of a blank sheet of paper taped to a wall, and analyzing the average pixel values using a free analyzing program called ImageJ. All the photos were taken using the same settings, except for the shutter speed. We set the aperture at f/3.6, the sensitivity as ISO 200, the image resolution to a low setting, and the self-timer to two seconds, to minimize camera shake. For each photo, we decreased the shutter speed by a factor of two, starting from 30 seconds, to 15, 8, 4, 2, 1, ½, ¼, ⅛, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, and 1/100. After taking the second set of photos using settings identical to the first set’s, we analyzed all the photos, and recorded the Mean, StDev, Min, Max, and Mode in a chart. Then, we graphed the points using semi-log graph paper that we downloaded from the internet, with the exposure time as the x-axis and the average pixel value (Mean) as the y-axis.
We set out at 11pm to take the skyglow photos, starting at Richmond Center. We took many pictures using different shutter speeds, and decided that a shutter speed of 25 seconds was the best. Then, we traveled to Terra Nova, Thompson Community Center, the Walmart on Alderbridge Way, River Road, and Brighouse Elementary, and took pictures of the sky using the same settings.
To analyze the pictures we took, we first used ImageJ to measure the average pixel values of all the photos. Since all the images were taken using the same settings and shutter time, we were able use the calibration curve that we graphed to determine an “equivalent exposure time” (EET) for each photo. By converting average pixel values of all the skyglow photos into an EET, we were able to determine how much brighter or darker one location was when compared with another. Using this method, we found out that Richmond Center’s skyglow was 3.5 times brighter than Terra Nova’s, meaning that out of all our locations Richmond Center's was the brightest.