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Hi Nigel
First off, does the dark patch you mention change shape and / or position when you rotate the etalon tuning ring. By the sound of it the dark patch may be may mean a defect in the coatings. The internal prism focusing system of the PST can move out of alignment so it is worth the time to remove the cover of the black body and check everything is as it should be inside.
I did the PST conversion last year after reading several online tutorials and generous instructions from a fellow IISer, Merlin 66. I purchased an 90mm Baader ERF from TeleskopShop in Germany and explained to them that it was for an PST conversion and had no problem with buying it. Luckily the exchange rate was a bit better at the time but I now wish that I had gotten the next size up, 110mm.
Remember to keep the f ratio at f10 or a bit longer is ok too.
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