I have long lenses like the 500 f4 IS, 300 f2.8 IS but haven\'t really applied this for night phitography or astrophotography except when I shoot the moon with a landmark combined with TPE software. My mentor is a seasoned astro photographer and the longest focal length he used was a 15mm fisheye for the Milky Way.
I would think your 300 f2.8 would work well for astrophotography. I used a 200mm f1.8 at f2.8 for this shot of the Andromeda galaxy. It was taken with a 50D at ISO 800. About 25 stacked frames at 30 seconds if I remember correctly. I probably needed to capture more frames and some dark and flat frames too but I was a total amateur at astrophotography (I still am actually).
