A good point. Buil seems to have done a reasonable job of selecting evenly spaced lines for the fit just using the lamp though and the fit residuals are much smaller than the errors seen. The use of a 4th order fit though does make me slightly nervous that we might be “fitting elephants” .
lamp lines used (EDIT 2018-11-02 Note these are the fit values, not the published wavelengths )
| 3946.331 |
| 4158.342 |
| 4510.673 |
| 4544.769 |
| 4657.878 |
| 4765.023 |
| 4965.325 |
| 5400.742 |
| 5852.471 |
| 6266.287 |
| 6506.282 |
| 7147.661 |
| 7383.626 |