When you collimate a point source using an off-axis parabolic mirror (OAP) with a circular shape, the beam area of the collimated light becomes more and more elliptical (x-dir. is smaller than y-dir.) as the off-axis angle is increased. Is there a reason why this happens? And is there a way to quantify this? My best guess is that it has something to do with the magnification because the beam spread is further in the x-dir. than the y-dir, but what does magnification mean when speaking of collimated light (infinite focus distance).