The http://en.wikipedia.org/wiki/Barn_door_tracker is a device to track moving objects. In this application, it tracks stars, compensating the Earth rotation. This allows one to obtain astronomical images where most of the light from stars are concentrated in a small regions of the detector.
This image shows the Small Magellanic Cloud seeing from the window of my apartment. It is a ~700 thousand people city called Sao Jose dos Campos, SP, Brazil.
I made a stack of 30 images of 6 s exposure each one, all subtracted from a very well estimated sky background image.