Focus Results Poor on one side of pier, fine on the other

[Peter Hannah]

First, I use Planewave focus (PWI) rather than FocusMax but this seemed the most appropriate forum group to use.

For the past several nights I have been targeting Barnard 72 (the S nebula). I am finding that the images taken with the scope looking East are out of focus, but those taken after the GEM flip - looking West - are fine.
In the attached images, Maxim measures a typical FWHM of 9 arcsec for stars in the out of focus image on the left, compared to 3.2 arcsec for the in-focus image on the right. So this is not a subtle effect, we're talking about tripling the size of the stars. These results have been repeated over several nights.

The rotator rolls everything over 180 degrees during a GEM flip of course, but that has never caused a problem before. I wondered if something might be loose in the image train but it's not likely.
Because this hasn't happened with any other targets, I wondered if it could be the thousands of stars in this image confusing the PWI software, but I can't see why this would just happen on one side of the pier.

I just wondered if anyone has had a similar experience, or if there are any other ideas what's causing this.

Equipment:
Planewave CDK14 on 10Micron GM2000 mount
Hedrick focuser
FLI Proline P09000 camera and FW-5-7 filter wheel with 50mm square filters
Pyxis 3-inch rotator

Software: ACP Expert using PWI3 focus.