QUOTE (jasedm @ May 2 2012, 07:29 PM)
Can anyone explain this image?
The top half is fairly 'clean' but the bottom half is speckled with cosmic-ray hits and sundry noise - I've not seen this before, how does it occur in the same image??
I forget the exact technical reason, but in essence the CCD readout speed overwhelmed the speed the downstream components can handle (say if the spacecraft recorder configured telemetry rate was set lower) so the lower part of the image was forced to sit on the CCD for a while longer. This increases dark current background (brightening seen visible) as well as chances of cosmic ray hits. BTW, this complicates dark current calculations for calibration quite a lot.
Chances of this happening depend on whether both cameras are active simultaneously, binning and compression mode, spacecraft telemetry pickup rates (the speed at which ISS sends packets to the spacecraft recorders, which also depends on whether other instruments are actively collecting as well).