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Q4: When we collapse the cube in Channel 2 we find a bright bar-like structure on a dither pointing (see figure, right panel).
A4: It looks like this is aligned with the IFU slices given the orientation of this observation; in the data on MAST I see a fainter bar but one that is present in nearly all of the pointings. This isn't very noticeable in the data cubes themselves, but pops out as a systematic difference when median combining across all wavelengths. I think what may be going on is that we're seeing systematics due to small (~0.04 DN/s) variations in the detector dark current pattern across the detector.
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Q5: I have a program with MRS and simultaneous imaging. What is the box to the to left of the image? What are the structures/dark patch I am seeing on the MIRI imager FOV? The red dots are the source catalogue outputs. The program did not have an associated background.
A5: The top left "box" is the region of the MIRI Lyot Coronagraph, . The dark region looks like the result of subtracting the background and there is a positive source that is having that impact on the data. Will be interesting to look at the data. The filter used is F2100W, which is background dominated, and at that wavelength it is recommended to subtract a good background. Looking at the data, it seems that there is a single undithered background observation- this has a different overall flux level than the science image and also contains a very bright target that is saturating the detector. When the background is subtracted this thus produces a systematic negative residual over the entire field of view (since the background pointing was brighter than the science pointing), and also leaves artifacts from the sources in the background. These are averaged over to a degree due to the dithering of the science frame, but around the saturated source the residuals are still significant.
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