# Photon-weighted barycentric correction and its importance for precise radial velocities

## TL;DR

Accurate barycentric correction in radial velocity measurements requires integrating over the entire exposure due to non-linear time dependence, as neglecting this can cause errors exceeding 1 m/s.

## Contribution

This paper highlights the significance of photon-weighted barycentric correction and proposes methods to mitigate systematic errors in precise radial velocity measurements.

## Key findings

- Second-order errors can reach over 1 m/s in some scenarios.
- Recording the exposure flux curve improves correction accuracy.
- Adding a constant flux correction reduces errors when flux data is unavailable.

## Abstract

When applying the barycentric correction to a precise radial velocity measurement, it is common practice to calculate its value only at the photon-weighted midpoint time of the observation instead of integrating over the entire exposure. However, since the barycentric correction does not change linearly with time, this leads to systematic errors in the derived radial velocities. The typical magnitude of this second-order effect is of order 10 cm s$^{-1}$, but it depends on several parameters, e.g. the latitude of the observatory, the position of the target on the sky, and the exposure time. We show that there are realistic observing scenarios, where the errors can amount to more than 1 ms$^{-1}$. We therefore recommend that instruments operating in this regime always record and store the exposure meter flux curve (or a similar measure) to be used as photon-weights for the barycentric correction. In existing data, if the flux curve is no longer available, we argue that second-order errors in the barycentric correction can be mitigated by adding a correction term assuming constant flux.