Light Pollution and Sky Glow

1. Set up a piece of white paper so that it is equally illuminated by indirect natural light. Choose a place where the light is bright. The most important thing is that the paper is illuminated evenly. Choose a time and place during which it is unlikely there will be any changes in how much light is present.
2. Set up your digital camera so that the white paper fills the entire field of view. The distance from the camera to the paper is not important.
3. Put the camera in manual mode and make the following changes:

• Adjust the focus so that the camera is focused on the white piece of paper. Once you set the focus, do not change it.
• Set the camera’s sensitivity to ISO 200.
• Set the aperture to f/2.8.
• Set the image resolution to a low setting.
• If your camera has a self-timer feature, set it so that the camera shutter opens a few seconds after you take a picture. This minimizes shake.

• Start ImageJ.
• Open the first photo using “File/Open”... command.
• Click on “Analyze” and select “Histogram”.
• A histogram of the pixel values in the photo will open in its own window. You will use the histogram to measure the average pixel gray value in each photo.
• Record the Mean, StDev, Min, Max, and Mode in your data tables. StDev is short for “standard deviation”. Min and Max are short for minimum and maximum, respectively. Mean is another name for the average. The mean of this histogram is the average pixel value.
• Click on “File” and select “Open Next” to open the next file.

Repeat steps 3.d.–3.f. until you have analyzed all images in both sets of calibration photos.

• This is a “semi-log” plot.