Radial velocities and the CCF
This page gives the science meaning behind the numbers, so you know which columns are “the answer” and what the rest are for. No math — just the chain from photons to a velocity.
From a spectrum to a velocity
A star’s spectrum is full of narrow dark absorption lines from atoms in its atmosphere. As the star moves toward or away from us, those lines shift slightly in wavelength (the Doppler effect). The radial velocity (RV) is that shift converted to a speed along our line of sight, in m/s.
This matters because a planet makes its host star wobble in a small orbit. Measuring the star’s RV over time reveals that periodic motion — and so reveals the planet. The spectrographs whose archives this package serves are built to measure RV to better than a meter per second.
How the pipeline measures the shift
You do not measure the line shift line by line. The standard approach is the CCF (cross-correlation function): the observed spectrum is cross-correlated against a template (a “mask”) of known stellar lines, producing a single average line profile. The center of that profile gives the Doppler shift, hence the RV; its depth and width also diagnose the measurement.
You never compute any of this yourself. The L2 data level stores the finished CCF results.
Which columns are the answer
Every archive’s L2 table answers the same handful of questions, though the column names differ between archives:
The RV itself — the number you plot against time.
Its error bar — needed to weight points in a fit.
The precise time — a Julian Date, often the right x-axis for an RV time series.
Quality indicators — the photon-noise precision floor (dvrms) and the extracted signal-to-noise (extsnr).
Question |
NEID |
EXPRES |
|---|---|---|
Radial velocity |
|
|
RV uncertainty |
|
|
Time of the RV |
Each archive’s page lists its full set of RV columns. For the column tour, see Understanding your results.