THE APPLICATION OF FOURIER TRANSFORM HETERODYNE TO ASTRONOMICAL INTERFEROMETRY

New, general techniques exist for directly measuring the phase and magnitude of electromagnetic fields. These direct field imaging (DFI) technologies facilitate the measurement of the amplitude and phase of electromagnetic fields in wavelength regimes shorter than radio wavelengths. In principle, DFI can be used over the entire electromagnetic spectrum because the concept is inherent in the nature of electromagnetic fields. In practice, an experiment demonstrating imaging of the amplitude and phase of in electromagnetic wave at HeNe laser wavelengths (633 nanometers) has been carried out. The ability to measure (image) the two components of the electric field opens up the possibility of accomplishing ground-based or space-based astronomical interferometry (AI) in the visible and infrared regime in direct analogy with very long baseline interferometry (VLBI), which is routinely carried out at radio wavelengths. In such a scenario, the magnitude and phase of the image field is coherently measured at each telescope in the array. Later the data from each telescope is brought together by coherently combining the properly phase matched signal data. The resulting image possesses resolution characteristic of the widest baselines between elements of the array: that is a property of synthesis imaging. As in current AI techniques, DFI requires detailed knowledge of the baseline distance between the separate telescopes. However, unlike current AI techniques, DFI does not require that the baseline distance be maintained to a fraction of a wavelength as long as the knowledge of the baseline distance is recorded.