“Images of the star on the slit can be deceiving if the star is over exposed as the overspill looks worse than it is.”
Yes – I try and stretch the guide image so that I see only the central core of the star. Without doing this I do see a bloated disc [Incidentally, I am using Hyperion Prism as my frame-and-focus/guiding/ software now… I like it]
With my 35mu slit, Isis reports a resolution of around 15K. This compares with the 23mu slit R of just over 17K – about a 10% loss. Inspection shows that the fine atmospheric lines are not quite as deep but I’m not worried about that..
By the way, do you know how Isis calculates R? It seems to do it from the image/spectrum and not from parameters as such