The Australia Telescope Compact Array uses a Cassegrain optics design, with radio waves reflected from the dish surface to a sub-reflector and then to the receiver. The 2.75m-diameter subreflectors are mounted near the prime focus, about 7m above the dish surface, and have a specially designed shape, differing slightly from the hyperbolic secondary of standard Cassegrain optics. The receiver feedhorn being used is rotated into position in the turret so that it is mounted on the optic axis, which enables very accurate polarisation measurements to be made. Subreflector height can be adjusted to achieve optimal focus for the different frequency bands. (Image credit: Leanne Edwards)