Abstract: An offset Gregorian antenna used in radio astronomy. The antenna may include a primary reflector having a substantially paraboloidal shape and a secondary reflector having substantially an ellipsoidal shape. The secondary reflector may be displaced from the optical axis of the primary reflector. The antenna may also include a feed located substantially coincident with the second focus of the secondary reflector. The feed may be located within a vacuum sealed enclosure, and a cryo pump may be used to cool the feed within the enclosure. Also, a shroud may partially surrounding the feed.