Crossed Dragone

The Crossed Dragone Telescope is an off-axis telescope design consisting of a parabolic primary mirror and a large concave secondary mirror arranged so that the focal plane is at right angles to the incoming light. In this configuration the polarization of light is preserved through the optics.

Other advantages of this design are a large field of view in a compact volume. Due to its off-axis nature the secondary mirror does not block any of the incoming light. At millimeter and submillimeter wavelengths this greatly decreases systematic effects due to diffraction. The main disadvantage is that the size of the secondary mirror is of similar size to the primary mirror making it expensive to make and heavy (requiring large supports). However, for professional applications where low systematic effects are critical (for example in cosmic microwave background experiments), the benefits of low systematics across a large field of view can far out-weight these disadvantages.