Thanks Øivind, and correctly assumed that it was 600mm f/4. It was a rather cold evening at dusk, the moon was bright and clear (seemingly very little atmospheric distortion).
It traveled (it can be argued what traveled in respect to what) pretty fast when looking through the viewfinder, so I tried to use an appropriately short shutter time. This effect would as you suggest be more pronounced as the focal length increases. I do not have a tracker, so there is a limit to things.
I do have both 1.4x and 1.7x TCs, so I guess I could experiment a bit further. But at 1400mm it would already be a stretch I suppose. Not sure about the quality of the combined optics either.
At 1000mm as in this case, the moon is far from filling the viewfinder (I only have FX cameras), so heavily cropping was necessary.