Regardless of whether try shoot with full frame or crop sensor. Important point in this process is properly caliculate the exposure. That shooting would take as long as possible, but not more than the stars in moving from their seats. (I say more in the sense that over a period of time to be picked off more light. This is conceived formula: 600 units divided by the focal length of the lens. For example if you take pictures with a 50 mm lens. 600 divide by 50 = 12, shutter speed 12 seconds, it is for a full -frame camera. If shooting with crop sensor. Let’s say Nikon, such a matrix through ratio of 1.5, is the lens focal length is multiplied by 1.5 and divide by 600 obtained result. If it is the same 50 mm lens is 50 x 1.5 = 75, 600 units from 75 = 8… Shutter speed of 8 seconds.