Asymmetric Sub-Reflectors for Spherical Antennas and Interferometric Observations with an FPGA-Based Correlator

The Nasu Radio Interferometer, consisting of eight equally spaced, 20-m diameter fixed spherical antennas, was developed for the purpose of surveying unknown variable radio sources at 1.4 GHz. An asymmetrical Gregorian sub-reflector was designed and installed on each antenna for the purpose of correcting aberrations caused by the spherical reflector. The total collecting area is $2512 \,\mathrm{m}^2$ and the field-of-view of each antenna is $ {0\rlap {.}{}^{\mathrm {\circ }}6} \times {0\rlap {.}{}^{\mathrm {\circ }}6}$ . In survey observations a spatial-fast Fourier-transform (FFT)-type multi-beam system will be used. We report on the design of spherical reflectors and a digital back-end system, the basic principle of spatial-FFT image forming, and a result of interferometric observations with an FPGA-based digital correlator.