robust fitting is still not robust enough

[cylammarco]

The two ends of the polynomial fit can run away (see only bottom plot):

from each pixel, 0 to 1000, it is showing the histogram of the distribution of the deviation of the wavelength solution from the median value at that pixel.

the spectrograph is only usable from 4020 A onwards (see top x-axis), meaning, we have some data off by less than 10A at the usable wavelength while SPRAT has a resoution of 9.2A. From 4400 to 8000 A, the deviation is less than 1A, which is better than 0.1 pixel i.e. robust fit is good for almost all of it, except it can go run-away at the extreme ends when they are unconstrained.

[cylammarco]

During the final peak/line fitting (after RANSAC) the polyfit is not constrained (e.g. by wavelength range) which can cause really weird solutions.

Should be straightforward using one of scipy's minimisation routines that allows parameter bounds.