Why does e.g. Hubble's secondary mirror not block part of the picture? The SOHO observatory has an opaque object in the light path that blocks the direct light from the sun making it possible to observe the corona.
Hubble and many other telescopes have a big primary mirror in the bottom of a tube which bounces light on to a smaller secondary mirror at the opening of the tube. Why doesn't that other mirror cause a circular area of "no data" in the middle of the resulting images?
A particular point in the image is formed by the convergence of all the rays of light that leave from that point in the object, and bounce off the mirror.  As long as there are some rays that can do that, then the only effect of the secondary is to block some of the rays, dimming those parts of the image slightly.