That would be a great fix, making it so that there's only 1 frame of dark frame subtraction at the end of a long exposure. It doesn't exist already, does it? I wait 20 min after a 20 min exposure now... I tried just turning off the dark frame subtraction, but the pixel artifacts are pretty bad then.
Many programs support dark frame subtraction, so you just have to make your own dark frames. I suggest you google around a bit on this topic, it's very common in astrophotography.
Note that this does require that you work in raw (CRW or DNG) to get the best results. The jpeg process would spread the noise around a lot in unpredictable ways.
Hmm, it would be possible to modify CHDK to do dark frame subtraction on the RAW buffer using pre-existing dark frames stored on the camera. These could be looked up by exposure time and sensor temperature. This would be slow, but it would be constant time so for exposures that run many seconds it would be a pretty big win.
