[icegreentea2]

JWST has 4 different instruments on it. While they all share the same focusing mirrors, but otherwise are 4 different measurement devices.

For the red dot observations, I believe this things have been measured by at least 3 of the 4 devices on board - NIRCam (near infrared camera, has very limited spectral capabilities through its filter wheel), NIRSpec (near infrared spectrograph) and MIRI (mid infrared instrument).

I cannot pretend to have the actual expertise, but it does seem vanishingly unlikely that all 3 instruments could create consistent artefacts in the same location.

[wussboy]

Unless there was a flaw in the mirrors they all use. I’m not saying this is so, but the software developer in me would immediately try to figure out what was wrong with the component they shared.

[tux3]

A flaw in the mirrors wouldn't leave the anomaly in a consistent place, it would keep causing problems no matter where you look.

But I'm pretty sure they thought of all of this and many more objections already. It's not like this is a super advanced thread of skepticism that physicists would have overlooked

[consp]

Afaik they did that during calibration. Take known close by objects, compare results, make sure they are the same (up to the capabilities of your ground truth).