1998A&AS..132..261L

We discuss a technique for mapping the synchrotron turnover frequency distribution using nearly simultaneous, multi-frequency VLBI observations. The limitations of the technique arising from limited spatial sampling and frequency coverage are investigated. The errors caused by uneven spatial sampling of typical multi-frequency VLBA datasets are estimated through numerical simulations, and are shown to be of the order of 10%, for pixels with the deconvolution SNR ∼7. The fitted spectral parameters are corrected for the errors due to limited frequency coverage of VLBI data. First results from mapping the turnover frequency distribution in 3C 345 are presented.