Pixel-Based Non-Linearity Correction for the WFC3 IR Detector

TL;DR

This paper presents a new pixel-based non-linearity correction for the WFC3/IR detector, derived from in-flight calibration data, improving photometric accuracy especially near detector saturation.

Contribution

It introduces a novel in-flight derived pixel-based correction method for WFC3/IR, replacing the previous ground-based averaged quadrant correction, and provides an updated reference file for archival data reprocessing.

Findings

Improved photometry accuracy with the new correction.

Largest improvements near detector full well limit.

Reprocessing of all WFC3/IR data with the new correction.

Abstract

The current non-linearity correction for the Wide Field Camera 3 Infrared (WFC3/IR) channel is based on ground-based data acquired during WFC3's Thermal Vacuum 3 (TV3) testing campaign. In the current reference file, the correction coefficients derived for each pixel are averaged over each of the four detector quadrants. In this work, we compute a new pixel-based non-linearity correction using in-flight calibration observations with the internal tungsten lamp flats acquired between 2011 and 2013. We derive the new correction coefficients by fitting a third-order polynomial to the accumulated signal ``up-the-ramp" for each pixel. Approximately 2\% of IR detector pixels are flagged as bad, and a solution cannot be computed. For these, we use the quadrant averages of the new correction coefficients. An accompanying report (\cite{huynh2025}) provides detailed testing results using both internal flats and external science targets acquired in a variety of observing modes. The report highlights improvements in photometry derived from ``\_flt.fits" data products calibrated using the new reference file, with the largest improvement for pixels with fluence levels approaching the full well limit of $\sim$80,000 $e^- $. A new NLINFILE reference file was delivered in October 2025 and will be used to reprocess all WFC3/IR imaging and grism observations in the MAST archive.