How to avoid purple speckles with ColQc?
The speckles are "bright pixels" related to flaws in the CCD imaging chip
itself. I believe the way the camera works is that pixels are like
capacitors that start out charged up. The amount of light that a pixel sees
governs how much of that charge leaks out, and thus what voltage is on the
"capacitors"; these levels are then A/D converted to produce the output
data. All the pixels slowly leak charge, so you can't have an image stored
forever on a CCD. The bright pixels leak charge abnormally fast. Also, the
charge leakage rate is proportional to temperature, so as the camera heats
up, these begin to matter for shorter exposures.
The only way I know to get rid of them (short of drastically cooling the
camera, which does help) is to process the image to eliminate them. You can
take a long exposure of a black image to determine which pixels are bright.
Then post-process the image to substitute for the bad ones the average of
the surrounding pixels. To do this probably needs the raw 8888 mode data,
since the bad pixels are always a single color element.