What is the black disk in blue spectrum near the center?
Absorbtion only in 0,665µm?
by voyager1682002 moderator
It looks like just a speck of dust
There are more examples here:
Take a look at "The Disk Detective FAQ" no. 5 " What do artifacts look like, and where can I find examples?"
by mboschmd moderator
These are old, original silver emulsion photos with scratches, spots, artifacts of all kinds. Confined to DSS2. SDSS used digital detectors