The count rates observed from a given astronomical radiation source have no simple relationship to the flux from that source, such as might be incident at the top of the Earth's atmosphere. This lack of a simple relationship is due in no small part to the complex properties of radiation detectors.
These detector properties can be divided into
*those that merely attenuate the beam, including
*#residual atmosphere between source and detector,
*#absorption in the detector window when present,
*#quantum efficiency of the detecting medium,
*those that redistribute the beam in detected energy, such as
*#fluorescent photon escape phenomena,
*#inherent energy resolution of the detector.