How to Build a Laser Death Ray
Beam Pointer Telescopes
Normally, a telescope is a pivoting optical platform that can be directed at a target object to take light from that object and project it onto a sensor. Here, we reverse that - the scope takes light from the death ray and sends it backwards through the mirrors and lenses of the scope to project it into a focused spot on the target.
Below we show an example of how a beam pointer telescope might be set up. Shown on the left are two parabolas with the same focal point. Light traveling parallel to the parabola's axis will be reflected toward or away from the focal point, and vice versa, when the light hits the parabola. So, if you start out with light shining straight up and it hits the upper parabola, it will diverge as if coming from the focal point. Then, when it hits the second parabola, it is again reflected so that it is in a parallel beam. Shifting the parabolas slightly can make the beam slightly converging rather than parallel so it is focused at some distant point. Now rotate the parabolas around their axis to form curved reflective surfaces, and you have the beginning of a suitable telescope. On the right is an example of how this has been put into practice - with the light path for the beam pointer of the U.S. Air Force's AirBorne Laser. This allows the beam pointer telescope to rotate to point the beam in any direction.
Of course, stuff sitting in the beam path will both block part of the beam and cause some unwanted diffraction. Fortunately, there is a way to build a beam pointer telescope without anything getting in the way of the beam. One such method uses non-centered and non-coaxial parabolas. This is shown below, the smaller parabola is aligned with its axis perpendicular to that of the larger parabola. Both share the same focal point. So when light comes in perpendicular to the pointing axis, it bounces off the small mirror as if coming from the focal point, and is then bounced off the large mirror in a parallel beam. Examples of a beam pointer using this design are shown on the left of this page.
Mirrors are not, of course, the only way to direct and focus a beam. Any of the optical elements discussed, such as lenses or zone plates, can be used as part of a beam pointer.