Diffraction-Limited Imaging II. Optical Aperture-Synthesis Imaging of Two Binary Stars

We have used the technique of nonredundant masking at the Hale 5 m telescope and radio VLBI imaging software are used to make optical aperture-synthesis maps of two binary stars, β Corona Borealis and σ Herculis. The dynamic range of the map of β CrB is about 50:1. For σ Her, we find a separation of 70 milliarcsec. These maps demonstrate the potential of the nonredundant masking technique for diffraction-limited imaging of astronomical objects with high dynamic range. We find that the optimal integration time for measuring the closure phase is found to be longer than that for measuring the fringe amplitude. There is not a close relationship between amplitude errors and phase errors, as is found in radio astronomy, and amplitude self-calibration is less effective at optical wavelengths than at radio wavelengths. The primary beam sensitivity correlation made in radio astronomy is not necessary in optical interferometry