A full description of the algorithm can be found in
"Generation of Energy Dependent Exposure Maps for
ROSAT PSPC analysis", Mendenhall, J.A., Snowden, S.L., Burrows, D.N.
presented at AAS meeting, Summer 1992, Columbus Ohio.
A discussion of the accuracy of the algorithm can be found in
"The Flattening Accuracy of the Exposure Map Software",
Schlegel, E.M., OGIP Calibration Memo, CAL/ROS/93-026