Spectrograph Raw CCD images are exceptional but not perfect. Due to the digital nature of the data many of the imperfections can be compensated for or calibrated out of the final image through digital image processing. Composition of a Raw CCD Image. Electrons are generated from the actual source photons.
Thermal signal is reduced by cooling of the CCD to low temperature.
The rate at which photons are received is not constant. The rate at which electrons are produced in the semiconductor substrate due to thermal effects is not constant. The Sensitivity Variations can be removed by proper flat fielding. The dark exposure is a measure of the Bias Signal and Thermal Signal and may simply be subtracted from the Raw Image. M27 Planetary Nebula.