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First off check what your exposure settings and frame rates are. The darker it gets the longer your exposure needs to be. The longest you will be able to go for is 1/25 sec. For an average night of seeing (twinkling) set your frame rate to 10 frames per sec. If the night is really good you might get away with 5 frames per sec. If your exposure is maxed at 1/25 and your fps are at 10 then start to slide your gain up. It will start to become grainy, that is normal. If you are capturing the images of jupiter, keep it to under 120sec and then stack them in registax 4. You will find that most of the grain will start to disappear.
btw the image is getting darker because you are taking a set amount of light reflected from jupiter and spreading it over a larger area of ccd chip.
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