Thank you both for sharing your data. They make a good contrasting pair. I will update you as (if) I make progress.
Interestingly Paul, I have yet to see a reference to tellurics in the documentation – not that I have looked for it.
Thanks again to both of you. I think I have enough data now and I even managed a clearish night two nights ago so have some new echelle data to ponder from a standard RV star and the yellow hypergiant Rho Cass.