Webb Office Hours Session 41: November 13, 2025
Q&A's:
Q1: Where are the Webb Office Hours procedures and guidelines?
A1: Webb Office Hours. Type your question into the WebEx chat. We will asynchronously copy questions from the chat to this main page and work through them as a group. If you have images to share please give WebEx permission to share your screen (you may need to log out and log back in again to enable this feature.)
Q2: I have some MIRI MRS data that have different levels of fringing apparent in some channels. For example, one target has dramatic fringing mostly at channel edges (for ex., near 17-18 um), while a second has some moderate texture there, and then a third has minimal fringing at the same wavelengths. From what I could find, the standard JWST pipeline performs a “residual-fringe” correction (dictionary key “RF_FLUX”) post stage 3 processing. I ensured that this residual fringe correction was applied to all three targets I mentioned earlier. How can I further improve the fringing, especially for the first target ? I have images that I can share if this improves clarity.
For the target (HD 143006) where there's so much fringing, it's hard to see spectral lines (there are some moderate "wiggles"). The other target has very little. Why do some targets have more texture, while others don't? How do I alleviate that?
A2: It seems like this is a source that's been extracted in some moderate size aperture from the cube. The issue you run into with fringing is that the exact fringing signal depends on where the source falls in the FOV (i.e., where exactly the source falls in the pixels). There's a 1D residual fringe correction in the pipeline that looks at the uncorrected spectrum (it looks through frequencies at the known fringe frequencies) and tries to remove those in a Fourier sense. But first, it has to clip out things that aren't fringes – so emission and absorption lines can affect how well it does. We are working on making improvements to the 1D residual fringe correction to allow it to have a few more knobs that can be tuned for finding/rejecting emission features and fitting the spectra. It's not ready for prime-time yet, but it might be able to help with this case. It seems like what's happening is for one target, it finds the fringe frequencies pretty well, but in the other cases it isn't working well - we can try tweaking how it's finding and masking lines to see if it does a better job. These test cases would be useful to test the routines we're developing right now. Please open a help desk ticket and send us the spectra, and we'll continue investigating there.
Q3.1: I have a couple other stellar targets (a binary, a triplet). On MAST, when I search for a triplet target (WL 20), I specifically want one object in the triplet but when I download the spectra I get the largest/brightest target spectra (and not the fainter one). How do I specify that I want just one of the objects versus just the brightest one?
A3.1: You 'll need to run the pipeline for yourself. The way the pipeline is designed, it looks in the cube and finds the brightest object in the field to extract the source there (if it's a point source). If it's an extended source, it combines everything in the FOV to generate a combined spectrum. You could start with the data cubes or higher up in the pipeline stream to do your own custom 1D extraction. There's a couple ways to do that, but one of the ways is to modify Stage 3 of the pipeline. There are keywords you can pass to the 1D spectral extraction step to ignore previous directives and specify the source location for the extraction. You may run into some issues in that it tries to do aperture photometry around it (i.e., subtracting the sky from some annulus) so you may end up having to do more custom processing to get that source extracted properly. But if nothing else, you should be able to put a box around the source of sufficient size to extract it well enough. There's an MRS pipeline notebook that should help.
Q3.2: What part of stage 3 allows you to do that?
A3.2: In the pipeline notebook, look at the cell where it sets up the dictionary for Stage 3. Specifically, look for the extract_1d step dictionary entries. Comment out the line for the ifu_autocen parameter, and instead use the option center_xy. Your total flux might be off by a few percent, but it should give you a reasonable starting point.
Q4: My group just got new Cycle 4 data for a galaxy and extended emission nebulae and we're working on the calibration of the MIRI MRS data. We have the full MRS spectrum overall for S, M and Long bands. We take one of the pipeline cubes and collapse it on the wavelength axis. We see noise artifacts that change in shape and structure (they show up as dark spots/bands in the image). Sometimes the dark bands are along the edges of the pointings when constructing the moisaic, or just along one side. What might be causing that?
A4: Since this isn't something you see in individual frames of the cube, it sounds like what you're picking out is very subtle systematics that are affecting things at all wavelengths. The first guess is that it has something to do with the straylight subtraction. The MRS PSF without a straylight subtraction shows horizontal streaks. So, there's a correction in the pipeline that applies a model to estimate the straylight component and subtracts it out. But if it's slightly oversubtracting, that would leave a subtle systematic residual that you'd see if you're medianing over wavelengths. If you turn off the stray light step, the dark residuals would go away, but you'd have much worse residuals overall and it may not be helpful. The first step is to figure out where the bands are coming from. If they are due to the straylight subtraction, that's one thing, but it could be coming from somewhere else. Try remaking the cubes with the straylight subtraction off – it'll create a lot of positive artifacts, but if the negative artifacts (dark spots/bands) are gone, that will indicate it's the straylight step causing issues. We can adjust based on what happens in that test. Please submit a help desk ticket and let us know what happens.
Q5: For the background subtraction, I've been using pixel-wise subtraction with a dedicated sky. For master background subtraction, does that by default choose some sort of region to extract the 1D spectrum from within the science observations? Or is there a way to use the dedicated sky observation?
A5: Master background will use a dedicated sky observation, it doesn't use information from inside the cube. The 1D spectral extraction will do some kind of aperture background subtraction, but that's the only time information from inside the cube is used for background subtraction.
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