Optical processor for the very large array (VLA) radiotelescope: encoding- dependent errors

An optical Fourier transform processor converts visibility data at its input to a sky brightness function at its output. Scattered light and spurious terms resulting from the method of encoding the complex visibility function as a real -valued and nonnegative transmittance result in errors in the processor output. In this paper, we discuss the degree to which each of these undesired terms degrades the signal -to -noise ratio of the output for different encoding methods and classes of images. We suggest alternative methods of encoding the input that greatly reduce the errors. Two particularly powerful methods of minimizing noise are "complementary weighting" and "bias equalization ". Two data dependent parameters are found to be most important: (1) the ratio of the meansquared visibility magnitude to the square of its maximum, and (2) the ratio of the squared maximum of the brightness function to its mean squared value.