subreflector

A subreflector is a secondary mirror used in many dish antennas and radio telescopes. Placed between the feed and the primary reflector, it redirects and reshapes the radio beam produced by the feed so that the primary reflector forms the desired beam. This two-mirror arrangement allows a more compact structure, improved aperture illumination, and the ability to achieve higher effective focal lengths without lengthening the overall antenna.

In common two-mirror configurations, the subreflector boundaries differ. In a Cassegrain arrangement, the subreflector is typically a convex hyperboloid located near the primary’s focus; it reflects the energy back toward the primary, which then reflects it through a hole in the primary to the feed. In a Gregorian arrangement, the subreflector is concave and reimages the beam to a focus typically beyond the primary, yielding a different optical path. The exact shapes and placements are chosen to meet antenna performance goals such as bandwidth, gain, and spillover suppression.

Subreflectors can be fixed or actively controlled to adjust illumination patterns, correct aberrations, or steer beams. Shaped or offset subreflectors tailor the aperture illumination to maximize efficiency and minimize sidelobes. Some systems use movable or phased subreflectors to achieve beam steering without moving the entire dish, enabling rapid scanning or multi-beam capabilities.

Beyond astronomy, subreflectors are employed in radar and satellite communication antennas to create compact, high-performance RF optics suitable for various frequency bands.