These are caused by small dust particles or tiny defects on the vacuum window. They show up as diffraction rings because the pin hole projector produces a very collimated optical beam, much different than the F#1.23 beam we have in the Rubin Observatory. Rubin Observatory images will not have such diffraction rings. Also the current vacuum window is a test window. The final cryostat window is the third Camera lens and has been made to a higher optical standard than the test window. It will be installed later this year.
These diffraction rings come in pairs because we have applied a simple illumination correction, made from a calibration image taken of a blank piece of paper. For each image we had to remove and replace the pin hole projector, so these rings do not line up perfectly between the two images.
You can also see many cosmic rays in the images; these are small bright spots or short streaks in the images from secondary electrons or muons. These occur in all astronomical images, and in Rubin Observatory images will be detected and masked. These images were taken with long 600 second exposures, compared to the 15 second exposures planed for our survey, and the longer the exposure the more cosmic rays.
Lastly, there is a circular reflection in these images, coming from the inside of the cryostat. Light from the telescope will be shielded, or baffled, by the full LSST Camera, and should not reach this part of the cryostat.