INTERFEROMETRIC IMAGING WITH ARRAYS OF LARGE OPTICAL TELESCOPES IN THE MULTI-SPECKLE MODE

We present a method for interferometric imaging with arrays of large o ptical telescopes in the multi-speckle mode. The raw data were produce d by simulating light propagation in the atmosphere, various pupil fun ctions similar to the pupil function of the ESO Very Large Telescope I nterferometer (four 8-m telescopes), earth rotation, and photon noise. The generated data sets consist of up to 48 000 interferograms per ex periment with 100 to 80 000 photoevents per interferogram. Since a Fri ed parameter r0 smaller than the telescope diameter was chosen, multi- speckle long-baseline interferograms were obtained which consist of ma ny speckles with interference fringes in each speckle. This experiment al condition is called the multi-speckle mode, which is typical for in terferometric imaging with large telescopes at optical wavelengths. Fr om the various data sets diffraction-limited images were reconstructed by the speckle masking method (bispectral analysis) and the iterative building block method. An advantage of speckle masking is that it can directly be applied to multi-speckle long-baseline interferograms in spite of the high redundancy of baselines in the input and output pupi l of arrays of large optical telescopes (each r0-subpupil can be regar ded as an individual r0-telescope with different phase error). Image r econstruction is possible without the use of non-redundant masks since speckle masking is a generalization of phase closure imaging to highl y redundant arrays (or large optical telescopes). The reconstructed im ages show the dependence of the signal-to-noise ratio on photon noise and other parameters. The proposed method can also be applied to radio interferometric data (especially, mm- or sub-mm-observations).