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/ab

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