Every other frame FHWM all over the place?

So I noticed this last night. Very odd behavior.

I was shooting short 60 second frames of M11 and I noticed that with the Lum filter the FHWM was showing a distinct pattern, every other frame was ‘good’ the ones in between were not so good!?

Bah flickr cuts off the screen I will edit in a better view shortly. In the meantime you can click the image for a full view.

SGP IH by Tom Whit, on Flickr

SGP IH by Tom Whit, on Flickr

Here is what PixInsight things of the frames!

PI graph by Tom Whit, on Flickr

Never mind… mystery solved.

Evidently the sequence had two runs on the lum channel one at 120 sec and another at 60 seconds…

Kind of embarrising !