Often one does not want to use the full resolution of a spectrum, either because the channels over-sample the spectral resolution or because the S/N is low. XSPEC and the associated programs provide a number of ways of handling this. Firstly, the XSPEC command setplot rebin can be used to add channels together in the plot. It is important to realize that this affects only the plot and not the data being fitted.
Two FTOOLS are available to bin and group data for fitting purposes. FTRBNPHA bins up the data in a non-reversible manner and should only be used to ensure that the number of bins in the spectrum is the same as that in the response. FTGROUPPHA is the tool of choice for grouping the data to get adequate S/N or number of counts in each channel. FTGROUPPHA does not actually add together channels, but instead sets a flag which is read by XSPEC and causes XSPEC to sum the appropriate channels. If a data file is read with some grouping then XSPEC will apply the same operation to any background or response files used.