The cameras noise reduction (dark frame subtraction) takes an image for the same length of time as the image you just captured, so if you took an image of 120sec the camera will do a noise reduction for the same length of time, 120sec. Nothing moves inside the camera when the dark frame is being done it just simply captures an image whilst the shutter is closed (same as having a lens cap on) Then processes the images before writting the file (.CR2 and/or JPG). Best just to leave the camera alone during this process to prevent any missread information onto the CF card.
Hope this helps a bit..
Havent had a chance to try a few deeper exposures around Orion etc... have to waitfor the weather to clear up, but I am having fun learning about Deepsky Stacker, a neat little program that stacks quite well, both my scorpio and Orion shots were done with it... Lacks a bit of editing power, but thats what Photoshop was invented for
Oh Photoshop CS3 comes out in March.
Does DSLR shutter work on the 400D then? How do you connect from the DSLR to the program, will the supplied USB cable work?