300mm with a doubler, maybe. I also here a blind works wonders or a remote shutter release. Here are the lengths I have read about

Set blind up and hang around in it for four days without leaving so your scent is not as prevelant, wait three more days for raptor to get used to your BO and then maybe if you are lucky you will oly have to wait a couple of more days to get those really powerful shots.

Put camera in tree or on rock near nest or perch. Wait for rapter to get used to it. set up a remote movement sensor shutter release and pray.

Get lucky as hell and then lie about it on an internet forum.

You choose which one those folks on the other board are doing. This is a rough thing to do. I looked into it and realized it took much more patience than I have. I don't have to stalk and wait out a flower of a mountain. Those damn rocks keep getting away from me though.