Yes, I knew that the Nikon's best ISO is around 200, so 1600 may not give you a signal boost further away from the noise floor. More likely it will raise the noise floor higher than the gain in signal. But I'm one to give things a try just so I can say I tried it and the experts were right again! Realistically, the only thing it can do for you is to shorten your exposure time if your guiding becomes problematic at 300 seconds.I have read that Canon's do well at high iso's. If I can get good performance at 300 seconds at iso 800, what would be the advantage of trying iso 1600?