This calculates the angular field of view of a 35mm camera with a lens (or telescope) in front of it. It goes on to calculate the length of star trails assuming the camera is in a fixed position.
The image size is initialised in the last section of the form to be 1024 pixels by two thirds of that because that is the kind of size that would be seen on a computer screen. Change the size to that of your camera sensor or of a print if you want to see how big the trails would be in those cases.