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I was wondering what focal ratio you were working with. Despite your great long subs and low ISO, you aren't getting enough signal and your shot is suffering from a lot of noise. Consequently you're having to lift up the levels to get some structure in the nebulosity. I'd try 800 or 1600 ISO and you'll be amazed at the detail you will get without having to move the levels around in your histogram too much.
Just as a reference, I shoot at 1600 ISO 30 seconds unguided at f5. If I translate that to your f 9, I would have to increase the exposure to about 2 minutes. At 400 iso, I would have to give it about 8mins. So with your iso and focal ratio, 4 minutes is not enough. You have a great rig so upping your ISO will be an easy fix because everything else is working beautifully.
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