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snippet of SIMBAD sky map

I was randomly looking through the SIMBAD sky map when I came across this artefact. It looks like the projection of a telescopes secondary mirror mount to me but I am wondering how that would end up in the map.

Other positions where I found similar artifacts are here and here.

(The blue disk has an apparent radius of around 50 arcmin.)

update: my suspicion about the nature of the artifact is confirmed in this answer to What have I encountered here?

Still the question remains; how did such an artifact end up in the SIMBAD sky map?

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    $\begingroup$ Possible duplicate astronomy.stackexchange.com/questions/48645/… $\endgroup$
    – GrapefruitIsAwesome
    Commented Sep 6, 2022 at 19:49
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    $\begingroup$ Does this answer your question? What have I encountered here? $\endgroup$
    – Prallax
    Commented Sep 6, 2022 at 21:12
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    $\begingroup$ potential close voters: The OP already mentions what they think it is and doesn't ask what it is. What they ask is "how did it end up in the SIMBAD sky map?" Instead of closing and preventing answers, I've moved the linked question into the answer itself in order to highlight the difference between them. I don't think the OP's question is asked or really answered at the linked question. $\endgroup$
    – uhoh
    Commented Sep 6, 2022 at 22:57

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The artifact comes from the DSS, the Digital Sky Survey. SIMBAD is a star catalog, and it displays sky maps from Aladin. Aladin sources the data from different sky surveys, and DSS is the one with this particular artifact.

There is a very bright star just outside the field of view. You provided three examples, and two of them have bright stars nearby, while the middle one does not seem to. Here is a different zoomed-out view of your main example: enter image description here

You can see the bright star between 4° and 5° from the artifact. For this pointing, "Mellinger color" is another sky survey option. It does not show the same artifact.

It would take a lot of digging to find out exactly how they put together the data for the DSS, which used photographic plates. I can provide a guess:

These kind of artifacts get worse the closer you are to the bright object. They must have used some kind of screening algorithm to select the right frames close to the bright star to remove these artifacts. But perhaps the algorithm was only applied within 4° of bright stars, leaving your examples as rare outliers. It could be significant (in terms of the telescope light scattering) that both the artifacts lie directly to the east of the bright star.

I am at a loss to explain the middle example with no bright star 4°-5° away. It is at declination -42°, so the Moon and planets should never reach this far south. One possibility could be that there was a brief light source inside the dome during the exposure.

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