Mills Cross

Mills Cross. The Mills Cross is a pioneering type of radio telescope array, named for its inventor, the Australian radio astronomer Bernard Mills. Its innovative design, consisting of two long, perpendicular lines of antenna elements, forms a giant cross shape on the ground, enabling it to synthesize a high-resolution beam for mapping radio sources in the sky. This instrument was fundamental in the early development of radio astronomy, providing the first detailed surveys of the radio sky and catalogs of extragalactic objects. The cross design directly influenced later generations of synthesis telescopes and aperture synthesis techniques.

History and development

The concept was conceived by Bernard Mills in the early 1950s at the CSIRO's Radiophysics Laboratory in Sydney. It was developed to overcome the resolution limitations of early single-dish radio telescopes like the one at Jodrell Bank. The first major prototype, often called the "Sydney University" or "Fleurs" cross, was constructed at the Fleurs field station near Sydney. This work was contemporaneous with other foundational radio astronomy projects, including the Cambridge Interferometer and the Ohio State University radio telescope. The success of the initial design led to the construction of the larger and more famous instrument at the same site, which became a cornerstone of Australian astronomy in the late 1950s and 1960s.

Design and operation

The classic Mills Cross employs two long, linear arrays of dipole antennas aligned north-south and east-west. Each arm acts as a phased array, producing a fan-shaped beam. The intersection of these two fan beams from the perpendicular arms creates a single, high-resolution pencil beam through a process of multiplication, a technique related to Earth rotation synthesis. This design provided a much larger effective aperture than any single dish of the era at a fraction of the cost. The original telescope operated at a frequency of 85.5 MHz, studying metre-wave emissions. The electronics involved delay lines and correlators to combine signals, a precursor to the technologies used in modern radio interferometers like the Very Large Array.

Scientific contributions

The Mills Cross produced the seminal "Mills, Slee, and Hill" survey of the southern sky, cataloging thousands of discrete radio sources. This survey was crucial for identifying and studying powerful extragalactic objects such as radio galaxies and quasars, including famous sources like Centaurus A and Virgo A. Its data helped establish the log N–log S relation, a key cosmological test, and contributed to the historic conflict between the Cambridge and Sydney source counts. The telescope also mapped large-scale galactic emission and studied supernova remnants, providing foundational data for understanding non-thermal synchrotron radiation processes in the universe.

Notable installations

The principal and most influential installation was the Fleurs Mills Cross in New South Wales. A significantly larger and more advanced version, known as the Molonglo Cross, was later built at the Molonglo Observatory near Canberra; it was subsequently rebuilt into the Molonglo Observatory Synthesis Telescope. Another important cross-type instrument was the Bologna Cross in Italy, constructed at the Northern Cross Radio Telescope site. The design philosophy also influenced the Clark Lake Radio Observatory's Teepee-tee telescope in the United States and the initial design concepts for the Square Kilometre Array.

Legacy and influence

The Mills Cross demonstrated the power of using fixed, inexpensive arrays to achieve high resolution, directly paving the way for the development of aperture synthesis and modern synthesis telescopes. Its principles are evident in instruments like the Very Large Array, the Giant Metrewave Radio Telescope, and the Murchison Widefield Array. The cross design itself evolved into the Christiansen Cross, used for solar observation. Bernard Mills was awarded the Edgeworth David Medal for this work, and the telescope remains a landmark in the history of astronomy, symbolizing the innovative, cost-effective engineering that characterized the rapid expansion of radio astronomy in the mid-20th century.

Category:Radio telescopes Category:History of astronomy Category:Astronomical observatories in Australia