Focusing a sun-powered death ray There is an abundance of Youtube clips showing vloggers using parabolic mirrors to focus the sun's energy to a point that can burn wood and melt metal.  These projects all have a pretty glaring (pun not intended) flaw: the range of the "death ray" is fixed.  Objects will only burn if they are placed in exactly the focal point of the mirror.
My friend and I have been debating how/whether a system of lenses could be used to adjust the focus of the solar death ray.  We have been sketching and googling diagrams of projectors and telescopes, but we're at an impasse.
Is it possible to build a solar collector parabolic mirror that can be adjusted to burn objects at different ranges?  If so, what arrangement of lenses would make this possible?
 A: You could put a diverging lens in between the concave mirror and its focal point. Depending on the power of the diverging lens, you should be able to chose a point on the optic axis so that original parallel rays coming onto the mirror, rather than converging toward its focal point, can be made to pass through the diverging lens more or less parallel on its other side.
The idea then would be to put another convex lens on the other side of the diverging lens, so that these now parallel rays can be used to focus on the focal point to the right of the convex lens.
The main idea is that the convex lens should be able to move back and forth on some track or rail controlled by you, so that you can move the actual point where you need the suns light rays converge.
This way the light should be able to be focused on any point.
A: I know you aren't looking to make a "laser" out of the sun's rays, but if you assume the sun's rays are parallel (which is a good approximation), then you could theoretically collimate them into any width you want using the below arrangement of mirrors. The light rays would come directly downwards, from North to South, and each mirror is a parabola with a shared focus of the black dot.

