Mapping charge transport effects in thick CCDs with a dithered array of 40,000 stars

TL;DR

This study uses a dense array of simulated stars to map and analyze charge transport effects causing astrometric distortions at the edges of thick CCDs, crucial for improving weak lensing measurements.

Contribution

It introduces a novel bench-top method with a dense star array to characterize pixel-level charge transport effects in thick CCDs.

Findings

Identified non-uniform electric field effects causing edge astrometric errors.

Quantified the magnitude and onset of edge distortions.

Demonstrated the importance of correction for precise astronomical measurements.

Abstract

We characterize the astrometric distortion at the edges of thick, fully-depleted CCDs in the lab using a bench-top simulation of LSST observing. By illuminating an array of forty thousand pinholes (30mu m diameter) at the object plane of a f/1.2 optical reimager, thousands of PSFs can be imaged over a 4Kx4K pixel CCD. Each high purity silicon pixel, 10mu m square by 100mu m deep, can then be individually characterized through a series of sub-pixel dithers in the X/Y plane. The unique character [response, position, shape] of each pixel as a function of flux, wavelength, back side bias, etc. can be investigated. We measure the magnitude and onset of astrometric error at the edges of the detector as a test of the experimental setup, using a LSST prototype CCD. We show that this astrometric error at the edge is sourced from non-uniformities in the electric field lines that define pixel boundaries. This edge distortion must be corrected in order to optimize the science output of weak gravitational lensing and large scale structure measurements for the LSST.