Where is the focus point dish network dish?

The parabolic dishes used by DirecTV and the Dish network are one side of a parabola. Picture a parabola graph opening upward with its center at the origin. Then cut off the whole parabola on its left side (x<0). The resulting section of a parabola still has its center (not the focus) at the origin. That is approximately what the dishes look like, but that makes it hard to find the focus since the usual ways of measuring the diameter and depth and applying a formula don't work.

So here is what I did. Go to a hobby store and buy a bunch of small mirrors, like 1/4 inch squares. Then use some glue to paste 4 of the mirrors around the edge of the dish and one mirror in the approximate middle of the dish you have. Take the dish into the sunlight and aim it approximately into the sun. (The area of the 5 mirrors is small enough that you won't overheat anything or burn yourself.) Take a small piece of paper, like a 3x5 inch index card, and move it around, and move the dish around, until you find the position of the dish and the position of the index card that results in all five mirrors reflecting the sun onto the card. Keep adjusting to move the 5 spots closer and closer together. When you finally get the 5 spots as close together as possible, the spots should be overlapping, and the center of the spots will be at the focus of the parabola. This won't be perfect: the dish is not perfect. But it will be good enough for any RF work you want to do.

You can put more than 5 mirrors on the dish to check how good the dish really is, but start with 5: more is just confusing at the beginning. And you don't need to take the mirrors off unless you want to (depends on type of glue you used) since the mirrors won't affect the RF (which goes through glass and, anyway, the area is very small).