| Popis: |
Most antennas have strong gravitational astigmatism and vary as a function of the antenna position. We suggested that this kind of surface error could be corrected away if a subreflector is mechanically deformed in a proper way [l]. Analyses by ray trace method showed that the shape of the maln reflector can be exactly corrected by a similarly shaped subreflector if rays do not cross each other [2]. Analyses on the 140-ft. radio telescope structure and direct measurements with a elongated feed horn [3] illuminating a central strip of the aperture showed that the telescope antenna has strong gravitational astigmatism. A mechanical deformable subreflector (fig. 1) was built for the test [4] [5], providing a first order astigmatic correction. The deformations of the subreflector is antenna position dependent, actuated by four servo controlled motors. The 88 hours of test on the telescope by observing seven unresolved radio sources at water vapour line (22.3 GHZ, X = 1.345 cm) at various pointings and with different amounts of deformation gives a considerable amount of improvement on efficiency, especially far south and east (fig. 2). A fairly strong secondary beam disappeared. The beam shape became more symmetrical, narrow and round. At 20' elevation the aperture efficiency increased by factors between two and three (fig. 3). This mechanically deformable subreflector is proven to be an inexpensive means to improve the efficiency of large radio antennas. References |
