detector. Either way, the instrument design must take into account scattering and diffraction to make sure that as little unwanted light as possible reaches the final detector. Lyot's key invention was an arrangement of lenses with stops, known as Lyot stops, and baffles such that light scattered by diffraction was focused on the stops and baffles, where it could be absorbed, while light needed for a useful image missed them. As examples, imaging instruments on the Hubble Space Telescope and James Webb Space Telescope offer coronagraphic capability. Band-limited coronagraph A band-limited coronagraph uses a special kind of mask called a band-limited mask. This mask is designed to block light and also manage diffraction effects caused by removal of the light. The band-limited coronagraph has served as the baseline design for the canceled Terrestrial Planet Finder coronagraph. Band-limited masks will also be available on the James Webb Space Telescope. Phase-mask coronagraph A phase-mask coronagraph (such as the so-called four-quadrant phase-mask coronagraph) uses a transparent mask to shift the phase of the stellar light in order to create a self-destructive interference, rather than a simple opaque disc to block it. Optical vortex coronagraph An optical vortex coronagraph uses a phase-mask in which the phase shift varies azimuthally around the center. Several varieties of optical vortex coronagraphs exist: the scalar optical vortex coronagraph based on a phase ramp directly etched in a dielectric material, like fused silica. the vector(ial) vortex coronagraph employs a mask that rotates the angle of polarization of photons, and ramping this angle of rotation has the same effect as ramping a phase-shift. A mask of this kind can be synthesized by various technologies, ranging from liquid crystal polymer (same technology as in 3D television), and micro-structured surfaces (using microfabrication technologies from the microelectronics industry). Such a vector vortex coronagraph made out of liquid crystal polymers is currently in use at the 200-inch Hale telescope at the Palomar Observatory. It has recently been operated with adaptive optics to image extrasolar planets. This works with stars other than the sun because they are so far away their light is, for this purpose, a spatially coherent plane wave. The coronagraph using interference masks out the light along the center axis of the telescope, but allows the light from off axis objects through. Satellite-based coronagraphs Coronagraphs in outer space are much more effective than the same instruments would be if located on the