Josh,
I have that lens but can't comment on it's use for astronomy. I used it at the total solar eclipse but managed to bump the focus as I took off the solar filter
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You don't say what other uses you have for it but I use mine for birding. For that use it is better than the 100-400 zoom because it is sharper, lighter, cheaper and you will be at 400 nearly the whole time anyway (I also have the 70-200 and a 1.4x extender if I need wider field). Although it isn't IS and it is difficult to hand hold you can do so if you stay at 1/600 sec or quicker (about 1/1000 is good). I'm not in the same class as 'Reverend' but I occasionally fluke a good shot and take a few OK shots. I recently tried it on a tripod with the extender to photograph ducks etc on a lake and found I never quite got the focus right.
Looking forward to seeing some shots!
David