Intrinsic Pixel Size Variation in an LSST Prototype Sensor

TL;DR

This study investigates sensor anomalies in an LSST prototype, revealing intrinsic pixel size variations and flux-dependent gain structures that could impact the telescope's imaging accuracy.

Contribution

It provides the first detailed analysis of intrinsic pixel size variation and gain non-uniformities in an LSST prototype sensor using ultraflat images.

Findings

Detected intrinsic pixel size variation near 0.3%

Observed flux-dependent gain structure at high light levels

Found no spatial gain or linearity structure below 100 ke^-

Abstract

The ambitious science goals of the Large Synoptic Survey Telescope (LSST) have motivated a search for new and unexpected sources of systematic error in the LSST camera. Flat-field images are a rich source of data on sensor anomalies, although such effects are typically dwarfed by shot noise in a single flat field. After combining many ($\sim 500$) such images into `ultraflats' to reduce the impact of shot noise, we perform photon transfer analysis on a pixel-by-pixel basis and observe no spatial structure in pixel linearity or gain at light levels of 100 ke$^-$ and below. At 125 ke$^-$, a columnar structure is observed in the gain map--we attribute this to a flux-dependent charge transfer inefficiency. We also probe small-scale variations in effective pixel size by analyzing pixel-neighbor correlations in ultraflat images, where we observe clear evidence of intrinsic variation in effective pixel size in an LSST prototype sensor near the $\sim .3\%$ level.