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Trial and error basically. The good thing is once its done it seems to hold quite well.
To make it easy I defocus so the stars are like small donuts and use my CCD in "live-mode" making adjustments. The goal is to get a concentic out of focus star at the centre of the field. Once this is done you can refocus so that stars are almost at focus and refine the collimation. Finally, I'll check in focus the centre and 2 opposite corners of the image for the shape of star images. I'm yet to get the collimation spot on - I suspect the CCD is very slightly out of square - but I'm pretty happy with edge to edge sharpness none-the-less.
Terry
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