I've been trying to find a good, simple, explaination of focusing for IR, and can't. And Brucie demonstarted how hard ir is to write one. Google it, experiment, and don't expect it to be great at first. Like lots of things in life really....

There is one excption, if you use a mirror lens, which focuses visible and IR at the same point. 500mm is a useful general purpose lens isn't it?