Hi Robin,
Thanks for your comments, tips and wealth of information. And sorry for being a bit tardy with an acknowledgement.
I have been busy developing a utility to blend images of stellar and calibration lamp spectra. Using this I found that I couldn’t get consistent polynomial fits simultaneously to both stellar absorption lines and calibration emission lines, especially if the ‘scope had been moved in between. This pointed to a flexure problem. I was dismayed to find I hadn’t tightened all six of the Alpy thumbscrews properly. I should have checked this much earlier!
Now to look again at focusing for the blue end of the calibration line spectrum.
Best wishes,
John