Look at the diagram...you'll see the intersection plane at 45 deg to the optical axis needs the elliptical mirror positioned with it's centre away from the axis to full reflect the conical beam coming from the primary mirror.
http://www.lcas-astronomy.org/articl...ory=telescopes
BTW all commercial secondary mirrors are NOT elliptical! They are a section of a cylinder cut at 45 deg - this can reduce the amount of light to the final image by up to 4%....they don't tell you that.