Yolo telescope

A Yolo telescope is a type of off-axis reflector telescope that has concave primary and secondary mirrors. The path of light from the secondary to the focal plane crosses the incoming light. The design eliminates coma but has astigmatism, which is often reduced by use of toroidal reflectors (like the concave inner-surface of the outer part of a torus, with a different degrees of curvature in different directions) for the primary and/or secondary mirrors.