Strong coma lobes from small gravitational deformations

Beam mapping at 1.3-cm wavelength has shown very strong sidelobes at the 140-ft telescope, up to four lobes in a row when pointing far west. Different types of observations and the theory of coma lobes lead in good agreement to the conclusion that the telescope suffers a large lateral defocusing, varying by 7.2 cm east-west (EW) and 2.8 cm north-south (NS) for pointing changes of 90\deg . Since the lateral deformation of the feed legs is only 0.5 cm, it must be mainly the optical axis which moves. This is achieved by a gliding rotation of the best-fit paraboloid, gliding along a slightly deformed surface, while rotating about the center of the average surface curvature. It is shown that the resulting lateral focal offset can be much larger than the surface deformation which causes it. The gliding rotation is also confirmed by a structural analysis. The observational and analytical methods described can also be applied to other telescopes. Polar mounts such as the 140-ft may show the effect in two directions: east-west and north-south, while alt-azimuth mounts can show it in only one direction: up-down. The effect of lateral defocusing is to be expected at other telescopes as well whenever the rim is more flexible than the center; and the resulting degradation of efficiency and beam shape will be significant whenever gravity is important in the error budget. In these cases the telescopes should be supplied with a variable lateral shift of the mount at the prime focus, computer-controlled as a function of the pointing, following the axial movements. This additional degree of freedom may improve the short-wavelength performance considerable for telescopes whose surface panels are more accurate than the gravitationally deformed backup structure.