Best exposure practices for night photography?

[BHuij]

Hey all—

I'm getting into this astrophotography thing, and I have a few questions. First off, I am using a Canon 5D Mk II with a handful of prime lenses with nice apertures. So I'm well-equipped for this task.

My main question is - how much noise reduction should I let the camera do?

In my experience, when shooting RAW, I can get better high-ISO noise reduction results out of Lightroom than the camera can with the built-in "High-ISO Noise Reduction" feature. However, knowing the basics of how the "Long Exposure Noise Reduction" works, I think that one is worth using. Anyone have a different opinion? I would basically like to know what will give me the least noisy finished product: Leaving on the Long Exposure Noise Reduction, or turning it off? Would the answer to this question be different when exposing for 30 seconds vs, say, 15 minutes?

Another question: How long do you think it is safe to leave my shutter open and exposing, without developing long-term problems (hot pixels or worse)?

Thanks for the input everyone!