The signals from CCD detectors are subject to an uncertainty due to the charge-to-voltage conversion on the chip.
This is called readout noise. Most modern CCDs are run to not under-sample the noise signal, so the noise level is a couple to a few counts in each pixel. Older chips with higher readout noise levels are still in use.
ECHOMOP expects the value for readout noise to be specified in the data frame itself (for CCD frames). You should check that the correct value has been obtained.
Some CCDs use multiple on-chip amplifiers to provide a more rapid readout facility. Each of these will have its own readout noise value. The best procedure for processing such frames is to split the image in to two sub-frames (or four for quad-amplifier chips). Each sub-frame should be separaetly processed in the normal way.
Martin Clayton