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Andrew,
It's easier to see the "Rust circles" if you actually remove the ITF first.
It's not the "fungus" type problem you can have with the Lunt......
I do have some images of the typical burn out circles...
OICURMT,
Unfortunately the blocking filter is made up from a couple of elements...
The upfront element is the ITF which suppresses the IR energy, the final element is the critical part - a very tight (about 8A) narrowband filter.
You need the final narrowband element to make an effective blocking filter.
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