Presently, a typical large mm-wave radio telescope costs more than $100M but state-of-the-art arrays of focal-plane heterodyne receivers cover less than 10% of the useful field of view with large gaps between sensitive areas. Moreover, individual components are not efficiently coupled to the energy concentrated by the antenna, reducing the telescope's resolution on the sky by 20% and typically losing about half of the available signal. The key new development proposed by collaborators Drs. N. Erickson and B. Jeffs of the University of Massachusetts and Brigham Young University, respectively, is the construction of focal-plane arrays utilizing phased array feed techniques. This approach differs from traditional feed horns by using a cluster of small antenna feeds to synthesize a single feed for the main antenna. Such an array collects all of the energy in the focal plane. Phased combination of the elements of this focal plane array allows one to synthesize horns of optimal size and amplitude/phase distribution to enhance the telescope resolution and compensate for telescope surface errors. At the same time they produce nearly as many beams on the sky as elements in the array.
While phased focal-plane arrays hold great promise for revolutionizing the field of radio astronomy, successful deployment of large two-dimensional arrays is still several years off, for it requires hurdles to be overcome in manufacturing, processing, as well as packaging. Drs. Erickson and Jeffs plan to test concepts for producing receiver front-ends with very small element spacings, demonstrate the utility of modern digital signal processors to do the vector combination of beams, and develop computational algorithms for combining the beams, calibrating the array, and processing the final data stream. The goal of this 3-yr project is an 8 x 8 array covering the frequency range 70 - 95 GHz with 100 MHz bandwidth, cryogenically cooled and installed on the 100-m Green Bank Telescope of the National Radio Astronomy Observatory.
In addition to paving the way for future large, efficient, two-dimensional mm-wave receiver arrays, the Erickson/Jeffs project will involve engineering and astronomy graduate students at the two collaborative institutions and provide advanced research topics for talented undergraduate students in the Five College Astronomy Undergraduates Internship Program. It will thereby educate members of a new generation of researcher in the complex art of mm-wave array instrumentation. Funding for this work is being provided by NSF's Division of Astronomical Sciences through its Advanced Technologies and Instrumentation program.
