Detection of differential chromatic refraction with LSST camera data

Abstract The Earth’s atmosphere acts like a prism, deflecting blue light towards the zenith more than red light. Airmass-dependent changes in astrometry can therefore be used to infer additional information about the wavelength distribution of light within a photometric passband. The Legacy Survey of Space and Time (LSST) will observe each point in the southern sky several hundred times, with observations spread across a range of airmasses. Using commissioning data from the LSST camera released in LSST Data Preview 2, we report the detection of this differential chromatic refraction (DCR) effect in observations of galaxies and quasars in the COSMOS field. We show how information from DCR can complement the six-band colour information from LSST and help break degeneracies when selecting extragalactic samples and estimating their photometric redshifts. DCR will be a valuable resource for scientific studies and care should be taken not to remove useful DCR information from future LSST data releases.