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In Stellarium, there are two green squares in the constellation Sextans (close to the star Alpha Sextantis). The app doesn't provide any information as to what these are.

enter image description here

I was curious and so I decided to contact the Stellarium team, who thankfully replied:

These are optical artifacts in the source images we used for the background sky. We hope to clean these up at some point.

Can someone please explain to me what optical artifacts in the source images means?

Does Stellarium use real images (source images) of the sky and build on top of them the graphical interface?

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    $\begingroup$ Other examples of optical artifacts are diffraction spikes; things caused by the way the instrument is set up that are not physical but often are unavoidable in some form or another. I don’t actually know anything about stellarium, but based on what they said it sounds like it means that the image they used to make that patch of the sky had some kind of artifact (maybe a cosmic ray hit the CCD at the wrong time) and maybe the squares are to cover them up from being too distracting. All of this is hypothetical, I don’t actually know for this case $\endgroup$
    – Justin T
    Jan 31, 2023 at 2:48
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    $\begingroup$ For someone willing to do some digging, MAST could be a good resource to use to try to find candidates for the source images and identify the exact kind of artifact $\endgroup$
    – Justin T
    Jan 31, 2023 at 2:52
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    $\begingroup$ different but somewhat relate: What am I looking at here at Google Sky? and Dozens of Oblong Green Objects in Google Sky and Google Sky Rectangle with No Stars $\endgroup$
    – uhoh
    Feb 1, 2023 at 0:49

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Artifacts are anything that is not your physical signal that you look for, but is instead a result of the observation process, or the observation instrument. I.e. it is an artifact, a remnant of the 'camera'. Examples of those have been given in the above comments, a nice example I've seen can also be found on Universe today.

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