Off-Axis Parabolic Mirrors

A parabolic mirror focuses a collimated beam into a focal point, irrespectively of the wavelength of the incident light, thus allowing achromatic focusing. Parabolic Mirrors also allow light from a point source located at the focus to create a collimated beam with plane wavefronts. An on-axis parabolic mirror is often not useful because it overlaps with part of the incoming beam, thus by designing a surface that only a portion of the parabolic surface is being considered, the light can focus in an off-axis way, offering a way to use the advantages of parabolic mirrors without interfering with the assembling task.

The important parameters that control the functionality of an off-axis parabolic mirror are:

  • The parent focal length (PFL), which is the focus-length of the equivalent on-axis parabolic mirror (i.e. the focal length from the center of the complete parabola).
  • The effective focal length (EFL), sometimes known as the reflective focal length (RFL), which is the measured focal length from the surface of the mirror to the focal length.
  • The angle of focusing, denoted here as , which is the angle between the PFL vector to the EFL vector.

In the following images, we present various examples of common parameters of off-axis parabolic mirrors.

With the following Simulation engine, one can examine the parameters of various off-axis parabolic mirrors


As those configurations are sensitive to the positioning of lenses and the curved mirrors in relation to the optical axis as well as in relation to each other, the alignment of such apparatus is quite tedious.  We offer a software simulation tool based many standard mounts as well as the 3DOptix Breadbox™ platform with its modular and versatile optomechanical mounts


90851/61  - Inline Folded Tunable telescope configuration \ Beam expander- 1''/25mm-1''/25mm Parabolic mirrors based