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I made up an image that illustrates how it works. It overlays the images taken 180 degrees apart. One with white stars, the other with yellow stars.
The red dot is the centre of rotation of the camera which is also the RA Axis. It just happens to be close to the SCP in this case.
So to establish the centre of rotation one could start by placing BQ Oct such that when the mount is rotated, its image does not move. It could be any star but best to choose one near the pole or at least near the right altitude so you don't need to put the mount far out of alignment.
The green circles show where BQ Oct and HD90105 would be located if the mount is polar aligned. They are centred on the red dot. The radius of each circle is simply the distance of each from the SCP.
Hopefully, it is apparent from the graphic that no matter the orientation of the mount, if BQ Oct and HD90105 are placed on their green circle then the centre of rotation of both image and RA axis are aligned to the SCP.
Now the trick is to find some software that allows a custom overlay on the image. PHD2 allows bookmarks and SharpCap has a movable, rotatable reticule. Any other possibilities?
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