Observe that the light from infinity strikes the first mirror at a 30 degree angle (angle of incidence)and that the angle of reflection equals the angle of incidence. This means that the ray is now at a 30 degree angle with respect to horizontal.
The rays passing between the mirrors are parallel and the distance between the rays as they hit the second mirror is the same as the distance between the incidence points when they hit the first mirror. Now assume that a ray hitting the very top of the first mirror will travel a vertical distance of sqrt(3)/4. This is one side of an right triangle with base angle 30 degrees. Since the long side of the triangle is sqrt(3)/2 the other leg of the right triangle is 3/4. This places the reflected ray at the apex of the opposite triangle.
Using this fact we see that corresponding rays also hit the opposite triangle at corresponding points. That is: if a ray hits the first triangle at a distance x from base, then the ray hits the second mirror at a point 1/4 - x from the base of the second mirror.
The reflected rays hit the third mirrored surface at precisely the same horizontal points and these points will reflect to horizontal points on the fourth mirrored surface directly below the original ray position.
