Introduction of a twist phase into the cross-spectral density function
of a gaussian Schell-model (GSM) field is known to increase the beam
divergence and to rotate the beam upon propagation. The focusing of th
ese partially coherent, twisted wave fields is investigated by derivin
g explicit expressions for the beam propagation parameters in the imag
e space of an arbitrary rotationally symmetric paraxial optical system
. The presence of the twist phase is shown to increase the size of the
best focus, and to move it towards the image of a completely incohere
nt source. The results are verified experimentally using an acousto-op
tically coherence-controlled twisted GSM beam.