So the Vivitar CAT goes on the shelf. I'm not sorry that I bought it since it had figured so prominently in my dreams and aspirations some 20 years ago. I wish it had worked out though as a practical tool rather than just a historical footnote. At the risk of being unoriginal, if you need a really long lens for the type of photography that you do then don't settle for yesterday's solution or today's kludge. A current generation 300 - 600mm lens of moderate to high speed is the solution.
Image Stabilized lenses are unsurpassed, in quality as well as price.
With lenses, as with almost everything else in life, you get what you pay for. Remarkably, when used with a Canon 2X extender this lens still provides remarkable resolution and quality. For anyone interested in the more interesting lenses of yesterday I can recommend the classic third party lenses web site. Lots of fond memories here for those who've been involved in photography for a while.