300 is hand holdable, although it won't be perfectly sharp, even with IS.
You are correct in the L/DO/normal lens quality heirachy.
If you're doing astro imaging, you should have your lens/camera mounted on a sturdy tripod, etc, so IS should be turned off when doing this imho.
I'd usually recommend a stand alone telephone, say the 300mm f4 IS L series, but a zoom is also handy...maybe the 100-400 might be OK, but I don't think it's IS enabled from memory, and it's a big and heavy lens, probably too much to realistically hand hold imho.
Also - 300mm isn't very long to be honest. You really want to be working with a minimum focal length of 500mm, but that's big moolah...
Dave
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