Precision astronomy with imperfect fully depleted CCDs — an introduction and a suggested lexicon

Abstract

This paper summarizes the introductory presentation for a workshop (held Nov 18,19 2013 at Brookhaven National Laboratory) that explored the challenges associated with making precision astronomical measurements using deeply depleted = “thick" =“high-r” CCDs. While thick CCDs do provide definite advantages in terms of increased quantum efficiency at wavelengths 700 nm < l < 1.1 mm and reduced fringing from atmospheric emission lines, these devices also exhibit undesirable features that pose a challenge to precision determination of the positions, fluxes, and shapes of astronomical objects, and for the precision extraction of features in astronomical spectra. For example, the assumptions of a perfectly rectilinear pixel grid and of an intensity independent point spread function become increasingly invalid as we push to higher precision measurements. Many of the effects seen in these devices arise from lateral electrical fields within the detector, that produce charge transport anomalies that have been previously misinterpreted as quantum efficiency variations. Performing simplistic flat-fielding therefore introduces systematic errors in the image processing pipeline. One measurement challenge we face is devising a combination of calibration methods and algorithms that can distinguish genuine quantum efficiency variations from charge transport effects. These device imperfections also confront spectroscopic applications, such as line centroid determination for precision radial velocity studies. Given the scientific benefits of improving both the precision and accuracy of astronomical measurements, we need to identify, characterize, and overcome these various detector artifacts. In retrospect, many of the detector features first identified in thick CCDs also afflict measurements made with more traditional CCD detectors, albeit often at a reduced level since the photocharge is subject to the perturbing influence of lateral electric fields for a shorter time interval. I provide a qualitative overview of the physical effects we think are responsible for the observed device properties, and provide some perspective for the work that lies ahead. Finally, I take this opportunity to make a plea for establishing a clear and consistent vocabulary when describing these various detector features, and make some suggestions for a standard lexicon based on discussions at the workshop. A more refined understanding of the device imperfections we are working to circumvent lies ahead, and this workshop was convened to help us find our way.