Hi Robin and Andy
Thanks for comments…
Its nice to know I am up against the absolute limits here (although I understand you could, in principle, see differences in same target, over time, up to around 1.5A)
Robin – the comment on the relation between PSF, slit, and spectrum, is very interesting (although I don’t understand the physics at work here :).. My guiding is not as good as I would like (around 0.8s rms) so I think I may be getting some dither by accident than design :). I calculated heliocentric correction in BASS and it gives a blue shift of 16.8 km/s. This would seem to nearly cancel the absolute radial velocity… so it looks like my offset is definitely an artefact..
I may take a few extra calibrations over the next session just to see how stable it is..