The focal plane in this case is limited by the size of the mirrors. The objective lens doesn't care if the beam hits in in the center or the edge - if it comes from the center of the focal plane it's going to be projected perpendicular to the focal plane. It may work after a fashion if you need a truely tiny angle...
"There are painters who transform the sun into a yellow spot, but there are others who, with the help of their art and their intelligence, transform a yellow spot into the sun." Pablo Picasso