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Sorry to hijack this thread, but Kevin, you don't have coma, refractors
rarely show any. If you magnify your screen you will see that the stars in the lower right corner are rectangular and elongated more than in the other
corners. Never seen rectangles before, but likely your camera is not
precisely perpendicular to the light path.
Re focusing; I assume your D7200 has Live View with 10x magnification; if so, choose a star that is not too bright, as it will have a sharper edge making it easier to discern when it is at its smallest size.
raymo
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