An exponential table model. The filename to be used should be given
immediately after etable in the model command. For example,
XSPEC> model etable mymod.mod
uses mymod.mod as the input for the model. XSPEC will multiply the contents
of the model by -1 then take the exponential ie this model is for calculating
absorption functions. For specifications of the table model file, see the
OGIP memo 92-009 on the FITS file format for table model files
(available on the WWW or by anonymous ftp from legacy.gsfc.nasa.gov
under caldb/docs/memos).