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Hey Barry,
Your post gave me an idea that worked a real treat. I simply moved the shim from running all the way under the front mounting ring, to only having it insert maybe 10mm on the side closest to the SCT. This slightly tilts the front ring up and to the left - which moved the second OTA's optical path much closer to the first OTA's optical centre. After making this adjustment to the position of the shim then I simply separated the two OTAs by pushing them apart gently by placing my left fist between the OTAs and clenching it; whilst under this slight load I then tightened the front mounting ring's bolts. I had to adjust the shim thickness three times, but using this technique the mis-alignment is now only about 1/15th of the original delta. I have gone from around a 3 arc minute misalignment to about 10 arc second.
With a bit more thickening of the shim I reckon I could get it under 5 arc seconds (say one or two star diameters) so it's really much improved!
In BYE when I take an image with the Refractor whilst the SCT is dead centre on a star the has gone from being a bit outside the outer framing circle to now well within the much smaller inner framing circle.
You folks really helped - many thanks all!
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