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Thanks gents. I've noticed that it's not related to smearing / larger FWHMs in poor seeing or windy conditions, but rather to do with the total ADU count fluctuating between exposures.
Some nights the guide image and ADU count will be rock steady, and others (such as last night)will fluctuate wildly - e.g. with a baseline of 30,000 I've seen it sometimes drop down to 500 and as high as 60,000 in the tracking panel. If it "locks on" during the 60,000 sample, then guiding is screwed.
This is a very weird bug. I would imagine that the simplest fix would be for the software to reset the target total ADU count when it attempts to re-acquire the guide star (or if it's failed for a certain amount of time).
The good news - I managed to capture 8 hrs worth of data last night. The bad news - I had to wake up every 40 mins or so during the night to reset the guiding. Very sleepy today!
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