A general Sobelov optical depth formula and some related results are d
erived for time-dependent spherically symmetric relativistic systems.
This formula and the other results are specialized for the case of hom
ologous expansion. The homologous expansion results will be useful for
supernova calculations. Remarkably, the Sobolev optical depth formula
for relativistic homologous expansion is angle independent. This new
formula corrects an erroneous angle-dependent formula given in an earl
ier paper.