I think it is the shadow or a reflection of the camera itself within the telescope. It's visible in multiple places.

https://sites.astro.caltech.edu/palomar/about/telescopes/ima...

exactly. I see it as well in multiple other spots as well. eg: 1. 05 49 04.008 +01 09 47.27 FoV: 1.3° X 2.2° 2. 05 44 35.074 +01 52 52.36 FoV: 3.4° X 5.9° 3. 06 37 17.795 -47 18 20.97 FoV: 1.4° X 2.4°

many more that i didnt list.