Design and evaluation of a white-LED ring for uniform CCD flat-field and contamination monitoring in HARPS3

Abstract

The High Accuracy Radial velocity Planet Searcher 3 (HARPS3) is being developed for the Terra Hunting Experiment, a 10-year programme of nightly observations of solar-like stars to search for long-period, low-mass exoplanets. Reaching extreme-precision radial velocities (EPRV) at the 10 cm s−1 level requires every instrumental error source to be characterised, including the detector. Non-uniform pixels and surface defects can imprint signatures on the extracted spectrum that mimic or distort spectral lines; illuminating the detector with a spatially uniform source lets any residual structure in the flat-field frames be attributed to the detector and treated during data reduction. We present a white-LED flat-field source for the HARPS3 CCD, mounted between the camera objective and the detector so that it illuminates every pixel, including the inter-order regions. Building on the HARPS and HARPS-N LED rings, the design replaces blue/green LEDs with recessed white LEDs and adds an interchangeable Perspex diffuser disc. Laboratory measurements identify the most uniform configuration. With a diffuser, the ring reaches 4.2% RMS flatness across the CCD active area, meeting the < 5% requirement and roughly a factor of two better than HARPS-N. We show that the resulting flats resolve the lithographic block-stitching boundaries of the CCD, and we describe how the source will be used as a standard calibration product: as a flat-field reference, a long-term detector health monitor, and the input to the ESPRESSO data-reduction approach for correcting the block-stitching radial-velocity systematic error.