I was out with a club a few weeks back and one of the guys was using a sony and a 500mm mirror lens and the results werent actually too bad not really cutting sharp and a bit dull but as long as you were prepared for this then you could save a small fortune also they are reasonably compact compared to a prime lens . have you considered the sigma 100-400 the non os models can be picked up second hand for not a lot of cash and they have the tripod collar for ballance on the mono pod / tripod . hope this helps
