Oops, I got my maths wrong... turns out the 383L+ requires 17 mm back focus, 22 mm for the filter wheel, and 19 mm for the Geoptik = total of 58 mm.... i.e. 14 mm too long for Canon EOS mounts.
After a bit more reading, I see that SBIG makes a special 36 mm filter wheel for the ST-8300 that has enough back focus for Canon EOS and Nikon lenses (FW8-8300 with their proprietary adapter). Astrodon also happens to make the full LRGB/Ha/OIII/SII filter set in 36 mm sizes too.
Hmmm tough choice - 36 mm filters with the ability to use my Canon L lens collection, or 1.25" filters and save a lot of money?
Either way, I think I'll go with the SBIG ST-8300M instead to at least have the option of using my Canon lenses.
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