Before XSPEC can take a given set of parameter values and predict the spectrum that would be
detected by a given instrument, XSPEC must know the specific characteristics of the instrument.
This information is known as the detector response. The response (R(I,E)), if you recall,
is a measure of the probability that an incoming photon of energy E will be detected in
channel I. As such, the response is a continuous function of
E. This continuous function is converted to a discrete function by the creator of a
response matrix who defines the energy ranges (
) such that:
XSPEC reads both the energy ranges (
) and the response matrix (
) from a
response file (for FITS file format see George et al 1992
) in a compressed format that only stores non-zero
elements. XSPEC also includes an option to use an auxiliary response file
(George et al 1992), which contains an array 
that XSPEC
multiplies into
as follows:
Conventionally, the response is in units of cm
.