spirmf performs a 2-dimensional interpolation of an existing RMF onto a desired grid in (input energy) x (output energy) channel space. This accommodates spectral analyses invoking essentially arbitrary binning of the data. The current limitations are that 1) the input and output matrices must be square (i.e. number of detector channels equals number of input energy bins), 2) the input matrices cannot be compressed (compression here referring to the OGIP condensed matrix file format), although the output matrices are automatically compressed, and 3) the output energy grid must fall within the span of the input grid (or very nearly so). spirmf can be used for either XSPEC-12 compatible component RMFs (see appendix B.0.3), or for single RMFs for use with spiros extracted spectra. Elements above the diagonal are treated as zero. spirmf can optionally update a PHA-II data file to facilitate ease of use with XSPEC.
Both the input and output RMFs will be in the condensed file format, with zero-valued elements ignored.