I got a shutter release cable for my D90 to do some long exposures at night during the stars. Since i'm shooting with a crop frame camera, it's obviously a lot more prone to noise. I did a 5 minute exposure the other night and even at ISO 200 I got a bunch of noise. Are there any other tips for reducing noise? There was a little ambient light from the city behind me which could have made the grain show up more than it normally would have but if anyone has any reccomendations let me know.