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 should be proportional to the cosine of the angle, but I can't see why it's physically happening. Shouldn't every point on the mirror collimate a cone of small radius from the point source? Here is an image that gives a better idea of what's happening - note that the light source P is at the focal point of the mirror.