Hi John,
The thread that you started has prompted me to re-visit the accuracy of my own Alpy calibration. I use BASS so calibration is a manual process. I use 13 lines from the Alpy calibration lamp spectrum and a 4th order polynomial. From my observing session a couple of nights ago the calibration lamp fit had an RMS of about 0.53A. I had selected SAO 25665 as reference star for X Per and b Per observations. SAO 25665 has a very low radial velocity according to SIMBAD, giving the spectra and results that I attach below. The calibration accuracy of the Balmer lines in the SAO 25665 spectrum is pretty good even into the near UV. And like you have found, the accuracy is worst, although still reasonable, for Hα. I wonder what could be the explanation for the dveiation in the Hα accuracy? By the way, I marked on the spectrum the position of the 13 calibration lamp lines that I use – I stack 3 x 40sec calibration lamp exposures.
Cheers
Hugh