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Webb Office
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Hours Session 1: February 8,
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2024
Q&A's:
- Q: I have stripes in my MIRI MRS Ch2 data (seen below), but this isn't listed as a caveat in the known issues table in JDox. Do we know what causes them and how I might fix them?
A:This may be the cruciform artifact, prominent in Ch1. See JDox article: https://jwst-docs.stsci.edu/jwst-calibration-pipeline-caveats/jwst-miri-mrs-pipeline-caveats#JWSTMIRIMRSPipelineCaveats-CrossartifactThe cruciform artifact is corrected in the straylight routine in general, but as the pattern has not been observed to affect Ch2 very noticeably, the correction for Ch2 may be imperfect. Try skipping the straylight step when reprocessing with the jwst level2 pipeline, to see how it changes the products (strun calwebb_spec2 --steps.straylight.skip=True). See: https://jwst-pipeline.readthedocs.io/en/latest/jwst/straylight/main.html
Furthermore, if the pattern seen in Ch1 without straylight correction is similar to that observed here it suggests that the Ch2 correction may need to be updated.
- Q: I’m working on Webb data for a galaxy, looking at diffuse HII lines. We obtained MIRI IFU data back in October. There are two background subtraction methods: pixel-based and master background subtraction. Looking at the available MRS notebooks, they mainly suggest using the master background. Is it ever advisable to use the pixel based one? What’s the criteria to choose one over the other? Could you use a combination of both?
A: You should only use one of the background subtraction techniques. Otherwise you will subtract the background multiple times. The master background subtraction works better in the general case when you have a background image that is not as deep as the science image. Pixel-wise background subtraction is good for faint, diffuse background, where you have a background image as deep as the science image. Q: What causes the black dots in the MIRI MRS channel 2 image below?
A: These are bad pixels. The bad pixel mask was updated a couple weeks ago. Try reprocessing again and these will hopefully be eliminated. If the problem persists, we recommend that you submit a Help Desk question to get additional help on this topic.- Q: I have a question about spectral extraction in cubeviz. After specifying circular apertures, why does the extracted spectrum have units of flux density (MJy/steradian)? The JWebbinar 28 python notebook says to multiply by a factor in the header. Do I also need to multiply by the area of the aperture region?
A. You only need to multiply by the factor in the header keyword. Don’t multiply by the aperture area. The area of each pixel in the extraction region is already included in the extracted spectrum. - Q:I have a question about the moment map plugin for Cubeviz: should the reference wavelength be entered as observed wavelength or rest frame wavelength?
A: Observed wavelength. That Cubeviz plugin does not know the source redshift. The prompt that Moment Maps that says “Rest wavelength of the line of interest” may be misleading and needs to be changed to “Observed wavelength of line of interest.” - Q: Is STScI adding a capability to calculate the ratio of moment maps across MRS channels?
A: Currently, Cubeviz doesn’t construct ratios of moment maps. We suggest calculating each moment map separately and exporting each to a FITS file. Then you can read them into your notebook to work with them further. - Q: I reprocessed 3 x 2 MIRI MRS mosaic data. I tried both master background and pixel-based background subtraction. Both have small bright features at certain wavelengths. What are these features?
A: This is similar to the issue in Q2. The features are due to the MRS bad pixels changing with time. The pixel mask reference file has been recently updated. If you reprocess it now the expectation is that features would go away. If that does not work, please reach out to the Help Desk to get additional help on this topic.
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