"Perfect" spectra for ESO's HARPS spectrograph

Abstract

HARPERFECT is the first application of "spectral perfectionism" to a high-resolution echelle spectrograph, reconstructing 1D spectra from 2D detector data via a forward model built on a calibration matrix consisting of point-spread function (PSF) shape, wavelength, and order trace, which were built entirely from laser frequency comb (LFC) observations. HARPS cannot resolve individual LFC lines, so each of 21 148 identified lines directly samples the PSF. We modelled these with a bivariate Gaussian, determining the PSF shape on the detector, calibrating wavelength, and tracing orders more accurately than the standard HARPS pipeline. Applied to 52.5 hours of HARPS data on quasar HE0515-4414, HARPERFECT gives S/N comparable to, though slightly lower than, the standard pipeline. Its value lies in an exact resolution matrix and independent samples, properties standard extraction cannot guarantee for a non-separable PSF - a step toward better measurements of fundamental constants and exoplanet atmospheres.