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The objectives on the newer model PST's can't rust/oxidise since the filter that was mounted between the doublet objective elements on the original design has been shifted to the rear of the scope (at the bottom of the eyepiece holder). Rear mounted filters can still oxidise and have been known to oxidise on some of the Coronado scopes - I don't know if this is still a problem. Lunt claims to avoid the possibility of blocking filter oxidisation through a proprietary manufacturing process.
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