We have used the technique of non-redundant masking at the Palomar 200-inch telescopeand radio VLBI imaging software to make optical aperture synthesis maps of two binary stars, βCorona Borealis and σ Herculis. The dynamic range of the map of β CrB, a binary star with aseparation of 230 milliarcseconds is 50:1. For σ Her, we find a separation of 70 milliarcsecondsand the dynamic range of our image is 30:1. These demonstrate the potential of the non-redundantmasking technique for diffraction-limited imaging of astronomical objects with high dynamicrange.
We find that the optimal integration time for measuring the closure phase is longer than thatfor measuring the fringe amplitude. There is not a close relationship between amplitude errorsand phase errors, as is found in radio interferometry. Amplitude self calibration is less effectiveat optical wavelengths than at radio wavelengths. Primary beam sensitivity correction made inradio aperture synthesis is not necessary in optical aperture synthesis.
The effects of atmospheric disturbances on optical aperture synthesis have been studied byMonte Carlo simulations based on the Kolmogorov theory of refractive-index fluctuations. Forthe non-redundant masking with τ_c-sized apertures, the simulated fringe amplitude gives an upperbound of the observed fringe amplitude. A smooth transition is seen from the non-redundantmasking regime to the speckle regime with increasing aperture size. The fractional reduction ofthe fringe amplitude according to the bandwidth is nearly independent of the aperture size. Thelimiting magnitude of optical aperture synthesis with τ_c-sized apertures and that with apertureslarger than τ_c are derived.
Monte Carlo simulations are also made to study the sensitivity and resolution of the bispectralanalysis of speckle interferometry. We present the bispectral modulation transfer functionand its signal-to-noise ratio at high light levels. The results confirm the validity of the heuristicinterferometric view of image-forming process in the mid-spatial-frequency range. The signal-to-noise ratio of the bispectrum at arbitrary light levels is derived in the mid-spatial-frequencyrange.
The non-redundant masking technique is suitable for imaging bright objects with highresolution and high dynamic range, while the faintest limit will be better pursued by speckleimaging.
