Webb Office Hours Session 35: August 14, 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: The user submitted a help desk ticket about aligning images between HST, ACS, and NIRCam. They have a NIRCam F150W image that they want to align to an older HST image, using the HST image as a reference. However, when doing this, the HST image comes out very blurry. How does the user produce nicely aligned JWST and HST images?
A2: JHAT is designed to help with this issue. We suggested the user try JHAT, with either image as the reference image. The expert in the help desk ticket can help with HST drizzling issues. The user will try JHAT and also provide the help desk expert with the log information to try to solve the issue from the HST drizzle side.
Q3: When resampling images onto the same pixel grid, there seems to be different ways to do it: by editing the resample parameters, or by providing an output WCS. Does the output WCS override the resample parameters?
A3: Yes, those are two ways to accomplish the same thing. The output WCS would contain the same information that would be provided manually in the resample step parameters, so you can choose either method to resample onto the same pixel grid.
Q4: The user would like to put aligned images onto the same pixel grid, but isn't sure of the correct order of operations. Does resampling onto the same pixel grid guarantee aligned images?
A4: JHAT works on the Stage 2 cal.fits files, and essentially is a wrapper around the STScI pipeline tweakreg (alignment) step. So, you should first get a correct alignment with JHAT and then you can run those correctly aligned cal.fits files through the Stage 3 pipeline to generate a combined image, but skip the tweakreg step since JHAT already aligned the images. In Stage 3, you can provide the resample parameters or output WCS in the resample step to resample the images onto a common pixel grid. But the first step would be to resolve the alignment issues using JHAT.
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