A 408-MHz aperture synthesis radio telescope

A 408-MHz earth rotation synthesis telescope is described, which has been constructed as a second frequency channel on an existing 1420-MHz telescope. Both frequencies are received simultaneously. The angular resolution at 408 MHz is 3.5 arc minutes, and the field of view is 7°. Sensitivity is 3 mJy/beam area (1 Jy = 10⁻²⁶W m⁻² Hz⁻¹). A dual-frequency, prime focus feed receives both hands of circular polarization at each frequency. The signal transmission system is designed so that the signal phase from its 408-MHz input to its 30-MHz output is entirely unaffected by changes in the length of the single coaxial cable which joins each antenna to the central processor. The functions of phase rotation, signal delay equalization, and correlation are performed in a digital signal processor, using microcomputer software rather than hardware. The quadrature output is also produced by computation in real time, using a band-limited form of the Hilbert transform. Some observational results are presented.