Webb Office Hours Session 47: March 12, 2026
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: We are looking at NIRSpec S1600A1 Fixed Slit spectrum of a flux calibrator and we are seeing offsets between the MAST data and a convolved calspec model. For example the F070LP G140M spectrum, where we see both a continuum offset between the data and model, as well as a difference in the line depths. What might be causing this?
A2: For the continuum differences, the default MAST processing of S1600A1 fixed slit data will attempt to perform a background subtraction. For many of the flux calibrators, these have been observed by nodding the source along the slit and subtracting the nods from each other pixel-by-pixel to remove any background light in the slit. For S1600A1 the slit is relatively short, and doing a nod subtraction (as is done by default in MAST when nods are available), will over-subtract some source flux, whereas the flux calibration is performed assuming that there is no source over-subtraction. This can mean that the MAST products for nodded data in S1600A1 could have a lower reported flux than the calspec model. To check to see if this is the source of the continuum differences you could reprocess your data, skipping background subtraction (set bkg_subtract.skip = True in calwebb_spec2)and compare with the model to see if you get a better agreement. Or if you are processing data using the JWST Pipeline notebook for NIRSpec Fixed Slits you can set bg_pixel = False in the set up of the notebook.
For the difference in line shape between the MAST data and the model, this could potentially be an issue with the convolution LSF or an issue with the model, where some strong lines can be challenging to model. For questions regarding the reliability of given calspec model lines, we would recommend submitting a ticket to the JWST help desk to get advice from the relevant experts.
Q3: We are seeing flux differences between NIRSpec Fixed Slit S200A1 data taken in PRISM and F070LP G140M for faint sources. The PRISM data agree with NIRCam observations, but the F070LP spectra are systematically lower in flux. Are there any obvious reasons for this difference?
A3: One possible source of flux differences for a given faint source 1/f noise or picture frame effects, that can cause pedestal level offsets in the background of the detector and can look like flux offsets between exposures. This however changes from exposure to exposure, so it shouldn't cause systematic flux offsets between dispersers.
Another possibility is that currently the F070LP G140M/H calibrations were using observations made during commissioning, and since then there have been updates to the telescope pointing (SIAF) for the S200A1. That could have resulted in some changes to the flux calibration for these filter/grating combinations, and may also explain the flux differences. New observations for F070LP flux calibrations have been taken recently and will be used to update the flux calibration for S200A1 observations with the F070LP filter. Other filter/grating combinations had previously been updated using post-commissioning observations, and should not be similarly affected.
For more questions about this potential issue please reach out to the JWST help desk.
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