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 proportional to 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^2.