Thanks for your comments. There may well be another reason for the effect I’ve seen but I’m struggling to identify what it is.
I think it would be difficult to detect such an effect photometrically because of focus variation between filters.
My first thought was that it was the problem Christian identified with parallactic angle as I cannot set my LISA to that orientation because the cameras which project from both ends of the LISA foul the pier when near the meridian. However I would have expected the effect of that to be worse at the blue end.
Thanks for the reference Andrew, I’ll digest that tomorrow and keep thinking.