From a physical stability aspect, there's no issue as such... you should be supporting the lens with one hand anyway when taking a shot. Otherwise it should all be quite solid.
Obviously from an image stability aspect, you will be needing a tripod for the best results... though it depends upon the lens and it's light gathering ability.
There's the often quoted shutter-speed vs focal length rule for handholding lenses... total 500mm = 1/500 sec, not written in stone (some people will need faster, some a bit less) but a good guide. The dslr crop factor needs to be taken into accout as well
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