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Folks thanks for your input. The problem is twofold. Poor calibration - failure to remove hot pixels causes the effect to carry over from one image to another during registration. If the images are aligned by hand resolution is at pixel level, and as I understand it during registration the streaking effect is occurring at sub pixel levels.
If you are a PI user, which uses temperature independent scaling during calibration, there is a temptation to take only a few dark frames. Temperature is not such an issue but a good master frame is required all the same.
Hope this is useful to someone.
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