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Thanks for the detailed reply Mike n Trish,
AS stated in my first paragraph, 2 weeks ago I shot 4 hrs on the same object mistakenly using the SII filter and got just the star field. The same artifact is there on the SII but of course no nebula. So I guess it isn't dust on the filter but perhaps elsewhere in the optical chain or a reflection somewhere (probably the cover glass on the sensor) as others have complained about. Those that have complained are talking about square shaped flares around very bright stars, not necessarily a reflection projecting elsewhere on the sensor.
I guess I need to learn how to take decent flats and go from there. Thanks again for your input and advice.
Bruce.
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