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Duncan what you are seeing is a reflection from either the sensor upto the MPCC and back down to the sensor or the front window to the back of the MPCC and back down to the sensor there is a formula for figuring out the distance it is.
It is not the primary mirror spot more a image of your secondary holder. if you where to use a refractor it would do exactly the same thing bar the doghnut would be a solid circle. In my shot of M42 i have the same problem its just that i used some careful cloning and healing brushes in photoshop because they annoyed me! bright stars highly focused are just something we cannot avoid!
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