Charge Diffusion Variations in Pan-STARRS\,1 CCDs

TL;DR

This paper investigates charge diffusion variations in Pan-STARRS1 CCDs, revealing that observed spatial photometric and morphological variations are caused by vertical charge transport differences rather than lateral electric fields, unlike other detectors.

Contribution

It provides the first characterization of charge diffusion effects in Pan-STARRS1 deep-depletion CCDs, highlighting a different mechanism from other wide-field cameras.

Findings

Identified systematic spatial variations similar to 'tree rings' in Pan-STARRS1 CCDs.

Demonstrated that variations are due to changes in vertical charge transport, not lateral electric fields.

Compared charge diffusion effects with other instruments, showing a different underlying cause.

Abstract

Thick back-illuminated deep-depletion CCDs have superior quantum efficiency over previous generations of thinned and traditional thick CCDs. As a result, they are being used for wide-field imaging cameras in several major projects. We use observations from the Pan-STARRS $3\pi$ survey to characterize the behavior of the deep-depletion devices used in the Pan-STARRS1 Gigapixel Camera. We have identified systematic spatial variations in the photometric measurements and stellar profiles which are similar in pattern to the so-called "tree rings" identified in devices used by other wide-field cameras (e.g., DECam and Hypersuprime Camera). The tree-ring features identified in these other cameras result from lateral electric fields which displace the electrons as they are transported in the silicon to the pixel location. In contrast, we show that the photometric and morphological modifications observed in the GPC1 detectors are caused by variations in the vertical charge transportation rate and resulting charge diffusion variations.