To allow data from different length spectra to be added the option allowHEdiff=yes should be used. For example, this will allow a standard EPIC-MOS spectrum from 0-12 keV to be added to an EPIC-pn spectrum from 0-15 keV. This is allowed as long as the starting channel of the spectra are the same and the bin width of each spectrum is the same. It is essential that the order of spectra and response matrices is consistent. e.g.
epicspeccombine pha='pnspec15k.fits m1spec12k.fits' bkg='pnbck15k.fits m1bck12k.fits' rmf='pnspec15k.rmf m1spec12k.rmf' arf='pnspec15k.arf m1spec12k.arf' filepha=epic_src_152k_p1.ds filebkg=epic_bkg_152k_p1.ds filersp=epic_src_152k_p1.rsp allowHEdiff=yes
