Transverse electric fields' effects in the Dark Energy Camera CCDs

Spurious electric fields transverse to the surface of thick CCDs displace the photo-generated charges, effectively modifying the pixel area and producing noticeable signals in astrometric and photometric measurements. We use data from the science verification period of the Dark Energy Survey (DES) to characterize these effects in the Dark Energy Camera (DECam) CCDs, where the transverse fields manifest as concentric rings (impurity gradients or "tree rings") and bright stripes near the boundaries of the detectors ("edge distortions") with relative amplitudes of about 1% and 10%, respectively. Using flat-field images, we derive templates in the five DES photometric bands (grizY) for the tree rings and the edge distortions as a function of their position on each DECam detector. Comparison of the astrometric and photometric residuals confirms their nature as pixel-size variations. The templates are directly incorporated into the derivation of photometric and astrometric residuals. The results presented in these proceedings are a partial report of analysis performed before the workshop "Precision Astronomy with Fully depleted CDDs" at Brookhaven National Laboratory. Additional work is underway, and the final results and analysis will be published elsewhere (Plazas, Bernstein & Sheldon 2014, in prep.).