To obtain the observed spectrum (C(I)), for a given observation,
XSPEC uses two files: the data file, and the background file (for
FITS file format see Arnaud, George & Tennant 1992
).
The data file tells XSPEC how many total photon counts were detected
by the instrument in a given channel. XSPEC then uses the background file
to derive a background-subtracted C(I) in units of counts per second.
The background is scaled to the data by the ratio of the BACKSCAL
keywords in the data and background files. It also is scaled for
exposure times (EXPOSURE keyword). The background-subtracted count
rate is given by :
where D(I) and B(I) are the counts in the data and background files;
and 
are the exposure times in the data and background
files; and 
and 
are the BACKSCAL values from the data and
background files.
When this is done XSPEC has an observed spectrum to which the model spectrum can be fit.