Hi Trevor, James Ling on CN has been working with Valery (who made the Chromacorrs) getting one set up for is huge Istar refractor and it makes for very good reading. From my reading, it improves things on axis. My searching for one came up empty when I had the Istar 150mm F10. I found that a Baader SemiApo filter did improve things quite a bit but not anywhere near true apo performance.
http://www.cloudynights.com/ubbthrea...l/fpart/1/vc/1