The response of a detector, which signal depends of the energy of an incoming photon, like a proportional counter, distributes the photon of a certain energy over many pulse height channels according to the gain and energy resolution of the detector. Usually this resolution function is relative complicated and depends on the photon energy. Since the energy acceptance and resolution of a given detector is determined by its design it is convenient to table this function while the photon energy serves as a parameter. This procedure leads directly to a form of a matrix and gives the whole data set the name detector response matrix. Usually, but not necessary, this detector response (file type DRM) is normalized to one. In the context of MIDAS this homogeneous set of data can be handled most efficiently using a bulk data frame (image).
In general the number of counted photons depends,
beside the efficiency of the whole instrument,
on the source, its distance, and the time of its observation.
Therefore this number can vary over many orders of magnitude.
To take advantage of a 
analysis
it is necessary to bin the observed spectrum.
This means, that the detector response matrix also
has to be binned (see subsection 6.2.1).
The information of the binning and an additional
energy calibration of the pulse height channels are stored
in descriptors, which are explained in
Table 6.4.
Table 6.4: Descriptors of a detector response matrix
The energy grid, used to calculate the monochromatic photon flux spectrum, is taken from the two descriptors ENERGY and E_BIN_WIDTH. Descriptor ENERGY_OF_CHAN delivers an energy calibration of the pulse height channels of the detector for plotting an unfolded photon spectrum.
Note:Details about the PSPC detector response matrix of the ROSAT X-ray telescope can be found in section 6.5.