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Nigel,
Sounds like there may be an "issue" with your PST....
The prism/ focuser can cause problems with contrast/ clarity but not normally as "major dark patches" - this however could be due to a failed ITF filter in the eyepiece holder.
I'm assuming your PST has the "blue" objective.
A good PST makes a good PST mod...a poor PST will cost you money for a mod but will not perform well....
Drop me a PM with your email details and I'll send you a copy of the PST mod write-up.
I would not recommend moving forward with a mod until you can ascertain what is the current problem.
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