At the dawn of CCD astronomy, before flat panels became common, a way of doing flats was to take pictures of the sky just after sunset or before sunrise, at the time when stars were invisible but the sun was below the horizon.
In September 2023, I was interactively taking pictures of comet Nishimura, in the early morning, just before sunrise. The sky was already pretty bright, the sun being close to clear the horizon, and I was in the “flat” condition described above. Working interactively, I was watching each picture, as they came. I noticed a blemish in the top right side of my pictures, telling me I had some dirt of sort somewhere in the imaging train.
Of course the first reflex was to rotate filters, to see if the dirt was on a particular filter (in which case it would go away with the next filter), or somewhere else in the imaging train. The blemish remained invariant after filter rotation. By the look at it, I knew it was not on any of the mirrors, but much closer to the sensor (if you see donuts, it is probably not on the mirrors). That left the camera or the corrector.
With a rotator I could have determined if the corrector or the camera was affected (just rotating the camera and see if the blemish rotates with it or not). As mentioned in another post, there is no rotator on Moana at this point.
The following day, I asked P. at the observatory to dismantle the camera and corrector. The camera was clean. The Paracor corrector however seemed to have a speckle of paint in it, on one of the doublet, inside the corrector. Nothing that can be cleaned without a factory return.
I redid a full series of Flats, using the flat panel. The new flats take care of the problem for now, further helped by the fact the default is in a corner.
Of course I am not too happy with the performance of the Televue Paracorr falling apart after only a year at the observatory.