To see if I could resolve the astigmatism issue I opened the PST, found the pentaprism out of orthogonality and 'squared it up'. However unfortunately this did not result in any improvement. The next step would be to remove the pentaprism. This was done and a hole bored in the back of the PST 'Black Box' and a helical 1¼" focuser (from Hutech) added plus 1¼" extension piece. At this stage I continued to use the original Ca K blocking filter (BF).