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The ISO almost doesn't matter. If you take a 10 min exposure, you get the same total amount of light hitting the camera sensor, no matter what the ISO setting is. However, if the total light on a pixel is very low (<0.5 on the 0-256 scale), then quantization will round it off to zero, so doubling the ISO would help to actually record a signal. The same sort of thing happens if you have big faint areas that only differ by a fraction; you won't distinguish the detail. If on the other hand you have something very bright which gives lots of readings from 128 to 256, then doubling the ISO will blow all these out to the same value of 256.
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