Experience of high resolution VLBI imaging using generalized maximum entropy method

The generalized maximum entropy method (GMEM) is a special modification of the standard maximum entropy method (MEM) which seeks solutions in the space of complex functions. In this work a reduced version of the GMEM intended for reconstructing real images with positive and negative values is used. As compared with the standard MEM, intended for the reconstruction of only non-negative images, the GMEM allows us to obtain higher-quality images with a much lower level of nonlinear distortions caused by errors in the data. Here, we present the results of the GMEM imaging of 36 selected extragalactic radio sources with a resolution of 0.3-0.5 mas on astrometric and geodetic VLBI observations at 8.2 GHz, obtained with a global array in the period from 1994-1996. In VLBI mapping practice this is the first experience of imaging with such a high resolution using maximum entropy technique. A differential maximum entropy method intended for increasing the dynamic range of images is demonstrated on the radio source 0059+581. In the case of unreliable `closure' phases, completely `phaseless' methods of mapping are recommended. Maps of two sources 0615+820 and 0642+214 are obtained using one such method.