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Dave if you or I were imaging through a refractor I would have said it was chromatic aberation. But SCT and Newts shouldn't suffer from that. So my thinking was that if I was getting blue on a limb it either had to be in my barlow or something was acting as a diffraction crystal. The only thing I could think of was a miscollimation. I dont know if I was right , but the blue limb is largely gone now.
Matt if you are interested I have an excel spread sheet that I made up that does all the calculations for you.
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