Focusing: Set your camera to manual focus and focus out into infinity. Your camera won’t be able to use Auto-focus in these dark conditions. Take a test shot to make sure your stars are sharp.
Exposure Time and ISO: Now let’s calculate the exposure. Take one 30 second exposure at ISO 1600. If it looks properly exposed, just take that 30 seconds and multiply it by 16. If you are using ISO 3200, multiply the shutter speed you used by 32, and so on.