[Simon Strauss]

Just because you have a hole in the center of the mirror that is reflecting back from the focal point does not mean you will get a straight beam. The light will simply disperse from that point due to the variety of rays exiting through it from many different points on the mirror. There is a reason why lasers were not invented hundreds of years earlier, without inducing your light through a gas you will never be able to get a beam with an almost infinitely straight path.

In that case my suggestion to you is to review the optics chapter from your high school physics and put a convex lens where you plan on having the hole, that way you can further control where you want your focus to be.

Hope that helps
-Simon Strauss

P.S. how do you plan on getting the necessary amount of light from the sun if you are heating an object that is directly in the path of the mirror's axis of reflection?