a weird hop (Halifax)

Received more test images. The Hα are getting washed out by the moonlight but I'm still experimenting.

Yesterday received the L, HA, and OIII images with the offset 50,0. So left, east, in RA only by 50% from the centre of the Rosette Nebula. They worked fine.

Tonight I received a set with offset 135,0, once again from the same reference star HIP 31130. They are fine, not perfect, but further to the east, with some overlap.

Then I received the image with offset 50,85. Should have been immediately north of yesterday's image with a 15% overlap. Nope. It is well away with a huge gap!

I don't get it.

Reported it to the hu-mon.

I wonder if the percentage is according to the long edge...