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I don't know enough about optics to tell if these particular defects are definitely caused by the mirror coating. I do know enough about the DSLR camera dng files and PS to tell that these are not hot pixels. In this DSLR I have updated the hot pixel list in camera and this information is passed to the software which automatically replaces the hot pixel . These images are from single unstretched raw DNG files in PS. The second image is from a Tak F8 refractor with the same camera for the same time and does not show the same amount of defects.
There are other types of sensor noise generated by the heat of long exposure and which you can see in the dark frame taken at the same time, if you take the time to look at the middle image.
However the Centaurus A image defects are more numerous and different to the sensor noise and so are not removed by the dark frame removal process in DSS or the automatic hot pixel list removal also used on camera raw DNG files imported into DSS.
It is good to imagine that a mirror coating would last for 25-30 years used night after night with no deterioration visible in a 20mp prime focus image or visible pinholes, but I doubt that is the case. Internal condensation must play a role. The question is whether deterioration causes a problem in a final image. Anglo Australian recoat their huge mirror every year for a reason I suppose.
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