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. He reported the camera was clean. He also reported the corrector seemed to have a speckle of paint in it.
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.
Update:
I have now picked up the telescope and transported it back home. I am in the process of cleaning and evaluating each element. So far it seems to me there is no problem whatsoever with the corrector, and the blemish was due to some oiling inside the camera, on the protecting window.
Having cleaned each element I am in the process of rebuilding the telescope and will take some new flats (after collimation) to get to the bottom of this
Preliminary conclusion: having somebody else fixing your telescope remotely is difficult, and getting diagnostics right may be very tricky.