Hi John
“Last night’s specrum of Pi Andromeda (HD 00369) was still about 3 A out at H alpha but very accurate from H beta to epsilon. “
I presume this was using the mix of lamp lines and Balmer lines for calibration?. If so, the Balmer lines from H Beta down will always be in exactly right the right place because ISIS used them for calibration.
Similarly if you just used the lamp lines for calibration and produce a spectrum of the lamp, I suspect you will find that the lamp lines are in exactly the right place (within the accuracy of the fit ~+-0.5A)
The problem arises when we use lamp lines to calibrate star spectra. There is a shift somewhere between the star spectrum and lamp spectrum. Based on the latest result this does not seem to be a stability problem.
There are differences between the optical paths for lamp and star which could produce small systematic shifts. We may have found the limit of absolute accuracy of using the internal lamp for calibration. (note this is a systematic error, not a lack of precision/repeatability so not a problem when studying changes in wavelength eg in radial velocity measurements of binary stars.)
To take this further I would say we would first need to compare results from other ALPYs to determine if it is a limitation of the instrument or if this particular example is worse for some reason. Since I am seeing signs of similar size errors in star lines with my ALPY when using just the internal lamp, I suspect it could be a limit in absolute accuracy. In which case there is no need to be overly concerned but longer term there may be some modifications that could be made to the ALPY or the calibration technique to improve the calibration accuracy further.
Using a mix of lamp and star for calibration confuses the issue. I would say The definitive test would be for ALPY owners to take a spectrum of a star with known reliable RV, Calibrate just using the internal lamp and look for errors in the star lines.
This could be a good project for the workshop
Cheers
Robin