The calculation of uncertanties for flux measurements is going to be included in a future CIAO release. The proper derivation of flux errors doesn't directly follow from the uncertainties of the model parameters, for most models.

However, there are a few models where the flux error can be directly derived from the normalization of the model. For the models ngauss1d, nbeta, lorentz1d, and delta1d, we can compute flux errors based on the error of the normalization.

There isn't a Sherpa command to do this, but the information is written out when an MDL file is created. If the user uses one of the models listed above and writes out an MDL file, the flux error will be in the fluxerr column of the output file.