Even with the grouping/thresholding mechanism described above, the RMF sizes can still be fairly large. Another possiblity is to resample the input spectrum, using an external task such as evselect. rmfgen will read in the attributes in the spectrum dataset describing that rebinning and offset (SPECPIX, SPECVAL and SPECDELT), and resample the response matrix accordingly. The resampling affects the channel axis of the matrix only; the energy axis is unaffected.
It is expected that a rebinning factor of typically 10 is possible with EPIC MOS spectra, which corresponds to a similar size reduction in the RMF.