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Martin, one piece of advice I received when I owned am LX90 8" and LX200 10" - Don't shine a torch down the tube. It will freak you out. The light from the torch exaggerates every miniscule speck of dust and it looks worse than it really is. As others have said, perhaps inquire with the owner to establish what the stain could be, but I'll also bet you'll never notice any degradation in viewing.
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