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This seems to clearly be an issue with the Deep Sky filters.
I've now taken flats of all my filters and only the Deep Sky ones show the middle spot.
The Astronomik Type2c Luminance filter (attached below image #2) is fine, the Astronomik 6nm OIII filter (attached below image #3) is fine, but the Deep Sky Red (post above) shows a spot, as does the green (attached below image #1) and blue.
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