Many thanks for your feedback, much appreciated.
beachcomber - ICNR is "in camera noise reduction". It's the "dark frame" that is taken automatically by the camera when you do a long exposure, and it's subtracted by the in-camera processing before it gives you the image. It's to remove hot pixels and noise.
You can also take "manual" dark frames (cover the lens), and use software to subtract them later.
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