I know this will be met with some suspicion, but I have one of those 500mm f8 long lenses, and it is actually quite good. (Mine is the Tou/Five Star label, but there are many others.) It's big, but it's very light and does come with the tripod mount. It's a preset, but it has an easy way to open it up for focusing before closing it down to shoot. Yes, f8 is slow, but when you're shooting at long distances, f8-11 gives you some workable depth of field. BTW, these tend to be T-mount lenses, but T > K mount adapters are very cheap. You have to do everything in manual, though. And when I say it's quite good, what I mean is that it produces better images than cropping / enlarging pics shot w/ the DAL 55-300.
Here's a pic w/ the FiveStar 500mm. Handheld on a dreary day...
1/800 at f11 ISO 1000