On-sky measurements of the transverse electric fields' effects in the Dark Energy Camera CCDs

摘要

Photo-generated charge in thick, back-illuminated, fully-depleted CCDs istransported by electric fields from the silicon substrate to the collectingwell at the front gate of the CCDs. However, electric fields transverse to thesurface of the CCD -with diverse origins such as doping gradients, guard ringsaround the imaging area of the sensor, and physical stresses on the siliconlattice- displace this charge, effectively modifying the pixel area andproducing noticeable signals in astrometric and photometric measurements. Weuse data from the science verification period of the Dark Energy Survey (DES)to characterize these effects in the Dark Energy Camera (DECam) CCDs. Thetransverse fields mainly manifest as concentric rings ("tree rings") and brightstripes near the boundaries of the detectors ("edge distortions") with relativeamplitudes of about 1% and 10% in the flat-field images, respectively. Theirnature as pixel size variations is confirmed by comparing their photometric andastrometric signatures. Using flat-field images from DECam, we derive templatesin the five DES photometric bands (grizY) for the tree rings and the edgedistortions as a function of their position in each DECam detector. Thesetemplates can be directly incorporated into the derivation of photometric andastrometric solutions, helping to meet the DES photometric and astrometricrequirements.