The Mills Cross Telescope was a two-dimensional radio telescope built by Bernard Mills in 1954 at the Fleurs field station of the Australian Commonwealth Scientific and Industrial Research Organisation in the area known now as Badgerys Creek, about 40 km west of Sydney, New South Wales, Australia.
Each arm of the cross was 1500 feet (450 m) long, running N–S and E–W, and produced a fan beam in the sky. Mills said it "consists of two rows of 250 half-wave dipole elements backed by a plane wire mesh reflector; the individual dipoles are aligned in an E-W direction." The cross operated at a frequency of 85.5 MHz (3.5m wavelength), giving a 49 arcminute beam.
When the voltages of the two arms were multiplied a pencil beam was formed, but with rather high sidelobes. The beam could be steered in the sky by adjusting the phasing of the elements in each arm.
Between 1954 and 1957, Bernard Mills, Eric R. Hill and O. Bruce Slee, using the Mills Cross, carried out a detailed survey of the sky and recorded over 2,000 sources of discrete radio emission, publishing results in a series of research papers in the Australian Journal of Physics. The differences between these sources and the Cambridge 2C survey were a cause of scientific disquiet until serious questions about the 2C survey results were resolved several years later.
In 1963, the Fleurs site was transferred to the School of Electrical Engineering at the University of Sydney. The observatory was effectively closed in 1991. The 18 m dish antenna installed at Fleurs in 1959 was transferred to the Parkes Observatory.
Two of the old 13.7m dish antennas were relocated from the University of Sydney site to the CSIRO at Marsfield in 2005, as part of a precursor study into the Square Kilometer Array (SKA) development.
Fleurs was also the site of:
Following the success of this design, Mills built another large cross antenna, the Molonglo Cross Telescope, near Canberra.
Other large cross-type radio telescopes were later built in the United States, Italy, Russia, and Ukraine.