We continue the research initiated in Andreev et al. (Computing conformal maps onto circular domains, 2010, submitted) on the computability of conformal mappings of multiply connected domains by showing that the conformal maps of a finitely connected domain onto the canonical slit domains can be computed uniformly from the domain and its boundary. Along the way, we demonstrate the computability of finding analytic extensions of harmonic functions and solutions to Neuman problems. These results on conformal mapping then follow easily from M. Schiffer’s constructions (Dirichlet Principle, Conformal Mapping and Minimal Surfaces, Interscience, New York, 1950).