Stubbe Hviid, OSIRIS Operations Manager, has sent us a response to a comment made on this post:
"In the last pic, I'm wondering what the tiny dots are (multiple white
and one black) : due to Osiris?, to data processing?, to objects above
clouds? (X,Y) for the black at (760pxl/2048, 456pxl/1083) and for one
white at (753/2048, 172/1083). At least the block dot is also present
on the first pic."
Stubbe says that the white dots are caused by cosmic ray events. When the images were taken, the spacecraft was in open space (of course). This means that the spacecraft experiences the full effect of the solar wind and high energy particles. These high energy particles are detected by the CCD as light flashes. On the surface of the Earth, we normally do not see these cosmic ray events because the atmosphere shields us.
The white dots due to the cosmic ray events are completely normal. For example, the coronagraph of ESA's SOHO satellite regularly experiences a bakeout. With respect to SOHO bakeout, Stubbe added:
It is the same effect but a different kind of detector. OSIRIS also has the capability to 'bake out' the CCD (also called annealing). As a rather extreme example of this, here's an image we took of Comet Machholz back in 2005, during which Rosetta was caught in a Coronal Mass Ejection solar flare with a radiation level about 100 000 times higher than normal.