I've never really done a lot of astral-photography. I enjoy watching others work through the problems and seeing the results. Do you do a lot of slow shutter shooting when doing the astral stuff.
If you have the interest in digital look into a used D3s, it's not EXPEED4, but it's good. The number of megapixels isn't nearly as important as the size of them and how the software in the camera handles the information. FX is the size of 35mm and the pixels collect a ton of information. There is a little different look to digital than film but the quality is every bit that Kodachrome64 was. Also, digital handles very high ISOs much better than film does when it is pushed.
The only down side to current digital cameras is the cost. I remember when a camera body only needed to be a light tight box that held film flat. I was shooting F3HPs when I finally made the change. It's so much more than that now. If you are even thinking of digital, the D750 is EXPEED4 and a lot cheaper than a D4s. And, you can find them factory refurbished now and then.