Photometry, Centroid and Point-Spread Function Measurements in the LSST Camera Focal Plane Using Artificial Stars

TL;DR

This study characterizes the LSSTCam's pixel response using artificial stars, showing small anomalies that meet survey requirements and identifying features that could impact PSF measurements, guiding future on-sky operations.

Contribution

It provides a detailed laboratory analysis of LSSTCam CCD anomalies and their effects on photometry and PSF measurements, informing operational strategies.

Findings

Residual effects on PSF size and shape are below 0.1%.

CCD mid-line distortions can be mitigated by masking.

Tree-ring effects influence centroids and PSFs, confirmed by flat-field studies.

Abstract

The Vera C. Rubin Observatory's LSST Camera (LSSTCam) pixel response has been characterized using laboratory measurements with a grid of artificial stars. We quantify the contributions to photometry, centroid, point-spread function size, and shape measurement errors due to small anomalies in the LSSTCam CCDs. The main sources of those anomalies are quantum efficiency variations and pixel area variations induced by the amplifier segmentation boundaries and "tree-rings" - circular variations in silicon doping concentration. This laboratory study using artificial stars projected on the sensors shows overall small effects. The residual effects on point-spread function (PSF) size and shape are below $0.1\%$, meeting the ten-year LSST survey science requirements. However, the CCD mid-line presents distortions that can have a moderate impact on PSF measurements. This feature can be avoided by masking the affected regions. Effects of tree-rings are observed on centroids and PSFs of the artificial stars and the nature of the effect is confirmed by a study of the flat-field response. Nevertheless, further studies of the full-focal plane with stellar data should more completely probe variations and might reveal new features, e.g. wavelength-dependent effects. The results of this study can be used as a guide for the on-sky operation of LSSTCam.