The other night when I slewed to a bright star to check the focus on my Skywatcher 127 Mak I got this image from the ASI174MM camera:
I’m thinking the off-centre ring must be a reflection of the bright star, but the question is what is causing it? My best guess at the moment is that perhaps it is a reflection off the sensor cover that is again reflected by the back of the filter being used at the time, but I guess it could also be a reflection off the front of the filter and/or a reflection back from the secondary. I hope the ring isn’t central because the star isn’t central and that it doesn’t also mean something isn’t square to the optical axis.