  Extending the transfer matrix DMRG algorithm, we are able to calculate
imaginary time spin autocorrelations with high accuracy (absolute error
$<10^{-6}$) over a wide temperature range ($0<\beta J<20$). After analytic
continuation using the rules of probability theory along with the entropic
prior (MaxEnt), we obtain real frequency spectra for the XY model, the
isotropic Heisenberg and the gaped Heisenberg-Ising model. Available exact
results in some limits allow for a critical evaluation of the quality of
answers expected from this procedure. We find that high precision data are
still insufficient for resolving specific lineshapes such as low frequency
divergences. However, the method is appropriate for identifying low temperature
gaps and peak positions.
