Star Trails from the City 2005/2/6

The place I live (Green Tree, Pittsburgh) has severe light pollution. Figure 1 is a 20-second exposure of the night sky from my room. Sirius is the brightest star at left and Orion is at center. Experienced astrophotographers would know from the photo that any longer exposure the sky will be washed out.

Figure 1. A 20-second exposure shows the light pollution.

Normal long exposure star trail photo is impossible. However the Canon 300D camera I use can automatically take photos at specified time interval with a computer. I let it take one 20-second photo per minute for several hours. I set up the camera on a tripod in the room looking out the window. (The scene can be more artistic without the wires!). After one night and a drained battery, I had around 300 photos.

To put them together, I perform a 'max' operation on the pixels across the images (standard stacking programs won't work since stacking is averaging which will dilute the stars). This is done with a few lines of code in Matlab. The result is quite descent. Figure 2 shows the max of 55 photos spanning 55 minutes. Sirius and Orion were setting. Figure 3 shows the max of 210 photos.

Figure 2. Max of 55 photos. Sirius and Orion setting.

Figure 3. Max of 210 photos (3 and a half hours)

S&T had a recent article discussing the idea.

One can also made a movie out of the photos.