Thanks for the quick response Andrew – yes, not the same, but similar air mass. The two in the graph are close together and image sequences were taken one after the other over 25mins or so. Altitude range 51-52 deg. I’m ALPY novice so I suspect something basically wrong about my set-up or what I’ve done to process, the errors seem to be too big compared to what’s been achieved by Robin. The annoying thing is that the corrected HD27819 seemed so close to its Miles library ref.