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A2: If you're not detecting continuum, it's possible you're just in the error levels. The region has low flux and it's very diffuse. Pixel-wise background subtraction is a good idea when you have diffuse extended emission and you're trying to eliminate any other noise errors (especially if it's the same depth as your science data). The spectrum looks reasonable in that you're getting down into the detector nosie. MIRI dark current is variable with time, which makes it complicated because you can't have a dark reference file to subtract that would give you an exact zero. One of the pipeline steps looks at a central region in the middle of the detector and computes the median effective residual dark, and subtacts subtracts that off. It assumes the delta dark current is constant across the detector. Within some fraction of the pixel uncertainty, it may not be exactly zero on average, so that could explain what you're seeing here. If it was multiple sigma below zero, it would be more concerning, but this spectrum seems reasonable. 

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