Exact Optics: A unification of optical telescope design
  A perfect focus telescope is one in which all rays parallel to the axis meet at a point and give equal magnification there. It is shown that these two conditions define the shapes of both primary and secondary mirrors. Apart from scale, the solution depends upon two parameters, $s$, which gives the mirror separation in terms of the effective focal length, and $K$, which gives the relative position of the final focus in that unit. The two conditions ensure that the optical systems have neither spherical aberration nor coma, no matter how fast the $f$ ratio. All known coma--free systems emerge as approximate special cases. In his classical paper, K. Schwarzschild studied all two mirror systems whose profiles were conic sections. We make no such a priori shape conditions but demand a perfect focus and solve for the mirrors' shapes. 
D. Lynden-Bell (Institute of Astronomy, University of Cambridge)