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I've seen a variation of the rotating column of air effect when using a dew heater as well as a cap on my SCT. The air warmed by the heater seems to cling to the inside of the dew cap and slowly spiral to the top and out. This is clearly seen in the out of focus star pattern. In this case though, probably because the rotation is so slow, it seems to have no discernable effect on the in focus image.
Regards,
Chris
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