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There are some terrific ideas in these posts. Having studied the first version of Marco’s image (thanks Jo), I am very confident that most of the diffuse background is real. I know that Marco produces superb images built from raw data recorded under very dark outback skies, and there is a very strong resemblance between his “background” and mine. I also think that I understand what Mike (Placidus) is getting at – process a series of OIII flat fields as light frames calibrating them with the master flat field for the OIII filter. The idea is to test just how good one’s calibration procedures are, determining the extent to which calibration errors can generate artifacts in a naturally flat background. We should all do a test like this from time to time, especially when trying to extract faint nebulosity from the noise. The luminance data of my Sharpless 308 image is based upon about 29 x 15 minute exposures at 2x2 binning through an Astronomik 12 nm OIII filter. A large fraction of the sub frames were contaminated by passing thin clouds (summer in Sydney). I deleted about another dozen sub-frames which I judged too poor because of passing clouds. Thank goodness for narrow band filters and sigma rejection.
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