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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.

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above: Cropped subsections from a HD Frame grab from the video at about 00:55. Recorded with Python using this script, then extracting frames using this script in Blender.

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You're right to think that this is a calibration technique. In use here are laser combs that use self-interference of short pulses that are reflected multiple times inside their own cavity to generate the spacing that you see.

The spacing then is mainly limited by the inverse of the pulse length, and thus by current laser technology. Sharpness of individual lines is usually increased by putting a Fabry-Pérot interferometer in sequence after the laser. For additional precision and estimation of laser stability, one uses often some secondary lamp with theoretically or laboratorially well-known lines (Th-Ar for example).

The technology demonstration for ESPRESSO was done with its precursor, HARPS.
Also A site with more details, the lowest poster looks pretty good in explaining the basics, I find.

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  • $\begingroup$ That's great, thanks! I guesstimate about 15,000 peaks (roughly three per Angstrom!) and so of the order of 20 to 50 ps period (50 to 20 GHz). That's an incredibly fast repetition rate, even for a mode-locked laser! $\endgroup$ – uhoh Dec 8 '17 at 15:04
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    $\begingroup$ I think it is usually quoted that one has an inter-strip distance of 0.5-1.0 Å, so your guesstimate seems reasonable. Well, only the best is best enough for astronomy ;) $\endgroup$ – AtmosphericPrisonEscape Dec 8 '17 at 16:32

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