Originally posted by timw4mail Focus on a mirror is always somewhat tricky, though.
No more so, I've found, than with any other long tele lens. The problem with some mirrors is they have a short focus throw, which makes focus very touchy. Fortunately the Tamron 500mm mirror has almost 360 degrees of focus throw and the Sigma 600mm mirror has almost as much throw as the Tamron. But even with this added range, both the Tamron and the Sigma can still be quite touchy.
When I'm focusing with any of my long focal length lenses, whether mirror or refractor, I always take multiple shots of the subject, refocusing between shots. Theory being that, if I take several shots, chances are one of them will be very close to correct focus. Of course, this technique works best for stationary subjects -- or close to stationary, like the moon. Trying to keep up with birds, on the other hand, is a real adventure.