Hi Peter,
Think you use ISIS – to see which images are shifting the most you might try a run ticking “wavelength registration” under the tab [2.General], then when you click next to go to tab [3.Calibration] tell ISIS to use the zero order i.e. x-coord=0, whatever pixel that corresponds to in next box and tick emission. If you tell ISIS not to delete the intermediate files under the Settings tab, you’ll see how much drift there is in each image wrt the first and maybe be able to select the best to stack.
Hope that’s a help.
john s