The CBC News Technology and Science Q&A This ESPRESSO machine doesn't make coffee but scans the skies for habitable planets includes a nice description of ESPRESSO (Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observations) and links to the YouTube video Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observations.
In the spectrogram shown at about
00:55 in the video I've noticed that each band is accompanied by a finely space array of points. I am assuming that they are some kind of calibration, but not sure how they are produced.
Since the orders are changing, I can't really figure out if these are equally spaced in wavelength or in frequency.
How are these densely space calibration "lines" produced?
You can see the full spectrogram in this question.