Webb Office Hours Session 32: June 26, 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 am working with MIRI data taken in full array, fast readout mode, looking at star forming regions.
In my MIRI data I have many saturated pixels that I am trying to recover information from. Some have saturated in 2-3 groups, others have saturated between groups 1-2 and some have saturated in the first group. We are trying to estimate the flux using the first group value, even though it is flagged as do not use. What are your recommendations?
A2: MIRI has a set of non-ideal, but known detector effects, and the first few groups in an integration are the most strongly affected. Especially due to the reset effect at the start of MIRI integrations, the first group is flagged as do_not_use. (note: the RSCD step does not do an actual correction, it only flags the first n groups to do_not_use). Typically for MIRI we recommend having a minimum of 5 groups. (See: https://jwst-docs.stsci.edu/jwst-mid-infrared-instrument/miri-observing-strategies/miri-cross-mode-recommended-strategies#MIRICrossMo[…]ro&gsc.tab=0) For very bright sources you may use between 2 and 5 groups, but here photometric accuracy cannot be expected for these very short integrations.
Photometric accuracy should be ok to within a few percent for things where groups 1 & 2 are unsaturated, and this can be handled by the pipeline with the bright_use_group1 flag. When you just have a single unsaturated group (group 1) you'll have to work outside the pipeline to get an estimate of the slope assuming some zeropoint, but the result may only be good to approximately 15-20%. If group 1 is saturated you have no information except a lower bound on the slope.
In the future, it would be best to observe these types of bright sources using a subarray that will have a shorter fram time and won’t saturate in the first few groups, especially if you need photometric accuracy.
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